When one takes in his hand a bare human skull (Fig. 469), which during life was never exposed to the light of day, and views it thoughtfully from all angles, as Hamlet viewed poor Yorick’s skull, it invites contemplation. Only the uninitiated and thoughtless shudder and turn away. The moralist is reminded of the inevitable end of every human life; the artist sees in it a complex of continuous curving lines that spell grace and beauty; while the comparative anatomist realizes that he has before him the culmination of countless adaptations, the age-long grist of a tireless evolutionary mill.
The skull of higher vertebrates is a double structure, as shown by its embryology and morphology, as well as by its physiology, and its biological study should be undertaken from these three aspects.
Embryologically it is made up of two sets of bones of diverse origin, an outer and an inner set supplementing each other. In the course of development these two kinds of bony elements join together into a unified whole and are no longer distinguishable as of two kinds.
Morphologically one skull of the double structure, the neurocranium, surrounds the brain end of the neural tube, while the other, the splanchnocranium, similarly encircles the anterior end of the splanchnic tube, or the digestive tract.
Physiologically, the two fundamental functions of support and protection are both provided for by the skull, so that it may truly be said to serve at least a double purpose.
The Development of the Brain Case
A complete sketch of the rise and union of the two embryonic skulls that become one in adult life may, for clearness of description, be divided into a series of arbitrary stages that pass continuously from one into the other. These stages are: (1) notochordal; (2) underpinning; (3) fusion; (4) upgrowth; (5) roofing-over; (6) shingling; (7) ossification; (8) union; and (9) bone-complex. They are fairly well represented in the adult condition of various vertebrate types, extinct as well as living, and although they have their counterpart in the developmental stages of the human skull, the parallel is by no means exact or complete.
Beginning with the notochordal stage (Fig. 470) one observes that just as the brain lags behind the formation of the nerve cord, so the first evidences of a future skull do not appear until after the notochord is well established. Before any skeletal elements except the notochord are present, the brain is surrounded by a thin, tough, membranous sac which is later replaced by the dura mater and by the skull itself. The anterior tip of the notochord lies just behind the hypophysis and beneath the midbrain. There are also present three pairs of conspicuous sense organs, arranged along the sides of the brain like devices on the instrument board in an automobile. These sense organs, the olfactory pits, eyes, and ears, are as yet without skeletal support.
At this early stage the anterior enlargement of the nerve tube, that is destined to become the brain, extends horizontally in front of and beyond the end of the notochord without any skeletal platform to support it. This urgent need is soon met, however, in the underpinning stage by the appearance of two pairs of independent flat cartilages that form not only a sort of floor upon which the rapidly developing brain may rest, but also furnish something solid to which the important jaw muscles can be attached. One pair of these cartilages, the parachordalia, is placed under the posterior part of the brain on either side of the tip end of the notochord, while the position of the other pair, the prechordalia or trabeculae, is more anterior (Fig. 471). At this time the sense organs also acquire the protection of delicate cartilaginous sensory capsules, the olfactory, optic, and otic capsules associated with the nasal pits, eyes, and internal ears respectively.
The four primitive cartilaginous floor-boards are at first quite independent, not only of each other, but also of the end of the intruding notochord, and of the six sensory capsules. Marginal growth, however, speedily results in their coming into contact and eventually coalescing as a single continuous plate which encloses at its posterior end the tip of the notochord and joins with four of the six capsules. This constitutes the fusion stage (Fig. 472).
The manner of “fusion” with the sense organ capsules is different in the case of each pair, due to the fundamental difference in the kinds of stimuli that the various future sense organs are destined to receive. Thus, the otic capsules of the inner ears, which are attuned to respond to vibratory contact with sound waves that can be transmitted easily through an enveloping protective case, are entirely surrounded by and embedded in skeletal cartilage except for the passage-way left for the auditory nerve.
The eyeball capsules, on the other hand, which need to be able to rotate freely within their future orbital cavities in order to be directed towards light coming from any direction, do not fuse at all with the rest of the skull but retain their independence in the form of the outer tough sclerotic coat of the eyeballs.
Lastly, the capsules of the olfactory pits fuse solidly on their posterior and inner surfaces with the skull itself, although perforated by the brushlike olfactory nerves from behind. On their outer surfaces they remain open like cups for the reception of odorous gases, since these chemical stimuli, in order to produce a reaction, must come into direct contact with the nerve endings within the cups.
In the center of the platform there is at first an opening, the hypophyseal fenestra, around the developing pituitary body. Later the cartilage grows down and around the ventral side of the pituitary to complete the floor of the brain case.
The platform thus assembled by fusion serves not only for support but also as protection for the brain on its under side. The protective function is soon extended to include the sides of the brain by the growth upward of the platform at its edges and in between the sense organ capsules (Fig. 473). This is the upgrowth stage in which the roofless skull somewhat resembles a trough or a deep spoon in the bowl of which the brain lies.
Growth at the margin of the developing cartilaginous skull case continues until the edges meet above and fuse together in the roofing-over stage, thus completing, at least in primitive vertebrates, a protective skeletal envelope around the brain. The skull of the dogfish, for example (Fig. 474), is a continuous cartilaginous casket surrounding the brain, with no sutures to demark its separate components. It is pierced by various small foramina through which the cranial nerves find exit and blood vessels pass in and out, while at its posterior end there is a large conspicuous opening, the foramen magnum, through which the nerve cord extends. In bony vertebrates it frequently happens that the cartilaginous roof is limited to a narrow band extending over the extreme posterior part of the brain to form a ring around the foramen magnum (Fig. 475). As a result a different sort of roof is developed for most of the skull.
Thus far only the formation of the inner skull has been touched upon. The stages that follow concern the origin of the outer investing skull, and the final modification and union of the two skulls into one.
After the formation of the inner cartilaginous envelope just described, or even before that process is complete, the inner skull becomes partially overlaid by certain definite bony elements which are not previously marked out in cartilage but instead are formed directly as bone out of dermal membrane. This may be called the shingling stage. Ordinarily the bony “shingles” do not fuse together so completely as to lose their identity, but rather join, each one to its neighbors, by means of clearly defined immovable joints, or sutures. Together these bones constitute the outer skull.
In the ostracoderms, now regarded as close to the ancestral stock of the vertebrates, the head was quite completely covered by numerous scale-like bones which were not particularly different, except in their enlarged size, from other scales that were found in the trunk and tail regions. The outer skull, therefore, makes its initial appearance as an exoskeletal armor of separate scaly plates, loosely shingled over the inner cartilaginous brainbox. The fully formed skulls of Acipenser (Fig. 476) and other cartilaginous ganoids (Chondrostei) are in the shingling stage.
Centers of ossification, forming the ossification stage, soon appear in the inner cartilaginous skull, particularly around the foramina for the exit of the delicate nerves where protection is especially needed. What was at first a continuous cartilaginous encasement for the brain thus becomes gradually replaced by definite separate bones, which increase rapidly at their margins, thus allowing the entire structure to accommodate itself to the enlarging brain within. Finally, the new replacing bones of the inner skull, like the scaly investing bones of the outer skull, join together in sutures and fusions. Skulls in this stage of development occur principally among the bony ganoids of which there were abundant species in Devonian times. The bowfin, or Amia (Fig. 477), is a modern representative. The process of ossification of the inner skull, initiated in the ganoids, is more completely carried out in amphibians and reptiles. Fossil skulls of lower vertebrates leave much to be desired by way of supplementing our embryological knowledge of the course of events for the reason that usually the important cartilaginous parts have not been preserved.
Following the ossification of the cartilaginous inner skull, the outer dermal skull bones sink deeper in from their former external scalelike position, and, becoming overlaid with skin, are grafted inseparably to the bones of the inner skull. This is the union stage. A single skull is now all that is visible, for there is no way, except by tracing the mode of embryonic origin, to distinguish the investing bones of the outer skull from the replacing bones of the inner skull, since both present the same appearance as to texture. These embryonic stages of the union of two skulls in vertebrates are paralleled by the adult skulls of various amphibians and reptiles. The two skulls of the union stage are not brought in all cases into such close contact with each other that their dual character is obliterated. In the turtle (Fig. 481) for example, the doubleness of the skull is evident, although the inner part immediately around the brain and the vaulted roof above it do not strictly correspond to the inner and outer skulls described by the embryologist. The large outer roof of the turtle’s skull provides adequate surface for the attachment of the head muscles, particularly those of mastication, rather than space to accommodate the brain, which is disappointingly small.
The final phase in the embryonic development of the cranium is the bone-complex stage. It results from the fusion of neighboring bones into complexes that thereafter pass for single bones. Thus, the sphenoid bone in the adult human skull is a combination of at least ten different embryonic bones, namely, the basi- and presphenoids, which are represented throughout life in most vertebrates as single bones although arising respectively from two centers of ossification; and the paired orbitosphenoids, alisphenoids, pterygoids, and the median lamellae.
Since a light, strong brain case is desirable for purposes of flight, the process of bone-complex formation in the avian skull has gone to so great an extreme that most of the sutures become obliterated in the adult bird, giving the cranium the appearance of an almost continuous bone.
In the mammalian skull, as previously intimated, all of the stages described in the preceding paragraphs are not repeated. For instance, the outer, or investing, skull begins its rapid development before the inner cartilaginous skull has been completed, in consequence of which the upgrowth and roofing-over stages are largely omitted, being rendered unnecessary since the investing bones prove adequate in forming a roof over the brain. There remain, nevertheless, unmistakable evidences of the dual origin of the vertebrate cranium even in man.
The description of the skull so far given applies solely to the neurocranium, which invests the end of the neural tube, that is, the brain. The other morphological half of the skull, namely, the splanchnocranium, surrounds the anterior end of the digestive tube in the form of a series of cartilaginous or bony arches which strengthen the walls of the anterior portion of the tube that have been weakened by the respiratory gill slits.
Among the lower water-dwelling vertebrates the splanchnocranium is relatively large and may extend posteriorly farther than the neurocranium, since it furnishes support for the respiratory muscles of the gills. Higher up in the evolutionary scale it becomes more and more reduced. The converse is true of the neurocranium which increases in relative importance with the increasing size of the brain.
The primitive arrangement of the splanchnocranium may best be understood by reference to the skull of Squalus, in which the distinction between the neurocranium and the splanchnocranium is still clearly defined. As will be seen there are present seven cartilaginous arches encircling the anterior part of the alimentary canal, one behind the other, like horseshoes with the open ends up. Each arch is composed of a number of separate elements that articulate with one another with the result that to some extent the arch as a whole may be extended or contracted accordion-fashion, as occasion demands.
The first or mandibular arch, which is the most anterior one, is made up of two pairs of separate elements that delimit the mouth. The original horseshoe arch bends back and breaks on either side to form the articular joints of the jaws, as shown diagrammatically in Figure 478. The dorsal elements, or pterygo quadrate cartilages (sometimes called the palatoquadrates), become the upper jaw while the ventral pair, or Meckel’s cartilages, form the lower jaw.
In the spiny dogfish as well as most other elasmobranch fishes, the second or hyoid arch, emancipated like the mandibular arch from bearing gills, serves as a support for the primary tongue and also as a suspensory apparatus, furnishing the only points of articulation between the neurocranium and the splanchnocranium. The tongue support includes a median basihyal which runs across the anterior part of the tongue and to each end of which is attached a ceratohyai. From the posterior, or embryonic dorsal, end of each ceratohyai, a hyomandibular runs medially to attach to the side of the otic region of the neurocranium. The posterior end of the upper jaw is attached at the junction of hyomandibular and ceratohyai.
The five posterior arches are gill arches that, somewhat in the manner of ribs, extend protectively around the anterior end of the digestive tube (Fig. 479). Between these pairs of arches are gill slits, which allow water entering the mouth to pass out on either side after bathing the vascular gills that hang suspended from the arches. The gill arches diminish in size posteriorly and in numbers in bony fishes, the loss always coming at the posterior end of the series. In a few of the more primitive elasmobranchs there are present six and even seven pairs of gill arches, but the usual number is five.
In higher vertebrates only the mandibular arch becomes overlaid by investing bony elements which are therefore limited to the upper and lower jaw regions of the splanchnocranium. In the bony fishes the gill cover, or operculum, of each side of the body consists of four thin flat bones, the opercular, preopercular, suhopercular, and inter opercular.
In the evolution of the higher vertebrates the need for respiratory gills gives way with the rise of the lungs, and gill arches become relegated to what may be called the anatomical scrap-heap. They are not entirely lost, however, even in man, for certain parts of the mature skeleton are directly derived from the primitive splanchnocranium, inherited from ancestral water-dwellers. Thus the legendary history of remote ancestors is retold in the human embryo. Nowhere is the thrift and resourcefulness of Mother Nature better exemplified than in the disposal of those parts of the splanchnocranium that have outlived their original usefulness, owing to the emergence of vertebrates from life in water to land.
Types of Jaw Suspension
There are four principal types of attachment of the splanchnocranium to the neurocranium, namely: amphistylic, hyostylic, autostylic, and craniostylic (Fig. 480). The first three of these involve the pterygoquadrate, the hyomandibular, or both, while the last-narked is associated with investing bones.
Amphistylic suspension occurs when both the pterygoquadrate and hyomandibular make direct articulation with the neurocranium. The pterygoquadrate usually articulates at two points: (1) a basal process meeting the ventral side of the neurocranium at the level of the posterior end of the embryonic trabecula, and (2) an otic process, at the posterior end of the upper jaw, articulating with the side of the otic capsule. The hyomandibular also articulates with the otic capsule. This type of skull is found in primitive cartilaginous fishes including a few living selachians.
The hyostylic method of suspension, as found in most elasmobranchs and all bony fishes, was described above for Squalus in which the hyomandibular alone acts as the suspensorium of the splanchnocranium. In the bony fishes a quadrate bone develops in the posterior part of the pterygoquadrate while an articular bone forms in the portion of Meckel’s cartilage which articulates with the quadrate. This hyostylic suspension permits protrusion of the jaws in the prehension of food.
In autostylic skulls, which are found in dipnoans, amphibians, reptiles, and birds, the upper jaw articulates with the neurocranium through two or more pairs of processes without intervention ot the hyomandibular. These articulations are usually of the immovable type (synarthroses). The hinge-type articulation between the jaws is between the articular of the lower jaw and the quadrate of the upper jaw, as in the bony fishes. Possibly Holocephali also belong in this category but their entire upper jaw is almost indistinguisliably fused with the neurocranium.
The craniostylic type of skull, found only in the mammals, is a modification of the autostylic type in that the upper jaw articulates directly with other skull elements without any hyomandibular participation. But in the mammals the articular and quadrate bones become transformed into middle ear ossicles (malleus and incus, respectively), while the jaw articulation is taken over for the first time by two pairs of investing bones, the dentaries of the lower jaw and the squamosals of the upper jaw.
Comparative Anatomy of the Skull Bones
According to Gregory there may be as many as 180 bones in the skulls of some of the primitive fishes. In amphibia and reptiles the number ranges from 95 down to 50, with the smaller number in the more recent species. Mammals usually have about 35 skull bones but man has 28 or less, including the middle ear ossicles.
Even though we were to limit ourselves to a consideration of the most important elements of the skull, it would be difficult to follow the evolutionary changes of so many parts at one time. It is preferable, therefore, to give our attention to this complex structure region by region, considering the various parts in the following order:
I. Cartilages or replacing bones of
1. Occipital ring
2. Ventral trough
3. Otic capsule
4. Optic capsule
5. Olfactory capsule
7. Meckel’s cartilage
9. Hyobranchial apparatus
II. Investing bones of
1. Roof of skull
2. Pterygoquadrate covering
3. Ventral surface of skull
4. Meckel’s cartilage covering
Throughout the following discussion it should be kept in mind that frequently the homologies of parts are far from certain. Two bones may even bear the same name in different species and yet not be homologous. Because a bone occupies much the same region in a lower vertebrate as another does in a mammal, the former may have been named after the latter. Yet careful studies of the embryonic developments of these parts may show clearly that they have distinctly different origins and consequently are not homologous. Because most of the parts of the cyclostome skull are of doubtful homology, they will not be included in the present discussion.
At the posterior end of the skull an occipital ring of cartilage is laid down around the foramen magnum, through which the nerve cord is continuous with the brain. In this ring four bones usually develop: a dorsal supraoccipital, two lateral exoccipitals, and a ventral basioccipital (Fig. 481). Modern amphibians, however, have only the exoccipitals. In this region there is usually at least one raised surface for articulation with the first vertebra. Such a surface is called an occipital condyle. In cartilaginous fishes there are two of these condyles, one on either side of the point where the notochord enters the skull, while in bony fishes the entire basioccipital region, concave on its posterior end, projects posteriorly to articulate with the centrum of the first vertebra in a joint similar to that between any two amphicoelous vertebrae. Present in all tetrapods, these condyles may occur on the basioccipital, on the two exoccipitals, or on all three of these bones, but not on the supraoccipital. A tripartite condyle, formed by contributions from the basioccipital and exoccipitals, is found in some stegocephalians, the cotylosaurs, and most therapsids, as well as the turtles (Fig. 481), lizards, and snakes among modern reptiles. From this primitive condition there seem to have been three lines of evolution. The modern amphibians, having lost the basioccipitals, have two separate condyles, on the exoccipital bones. Some extinct reptilian groups, as well as the present-day crocodiles and also the birds, have a single occipital condyle on the basioccipital only, the exoccipital parts becoming reduced until they do not participate in the condylar surface (Fig. 482). Mammals and the higher therapsids have two condyles, on the exoccipitals only, the condylar region of the basioccipital disappearing although the bone itself is not lost as in amphibians (Fig. 509).
In the ventral trough anterior to the basioccipital most mammals develop a row of three unpaired, median bones: basisphenoid, presphenoid, and mesethmoid, with the last-named forming the dorsal part of the nasal septum (Figs. 483 and 500). Extending dorsally into each orbit from the side of the presphenoid is an orbitosphenoid. In tetrapods other than mammals the anterior part of the trough usually remains cartilaginous as presphenoid and mesethmoid bones do not develop.
In the otic capsule three bones are commonly laid down: a dorsal epiotic; a ventral, anterior prootic; and a ventral, posterior opisthotic. In mammals these three usually fuse to form the petromastoid. In birds the prootic forms most of the ear capsule, while the small opisthotic fuses with the exoccipital, and the small epiotic with the supraoccipital. Among reptiles the prootics are usually separate bones while the opisthotics fuse with the exoccipitals except in turtles (Fig. 481). In Amphibia the prootic and sometimes the opisthotic appear as separate bones (Fig. 484).
As mentioned previously the optic capsule does not fuse with the other parts of the chondrocranium. Instead it becomes the sclerotic cartilage, present in the eyeball of most vertebrates except marsupials and placental mammals. In some reptiles and birds a ring of sclerotic bones ossifies out around the cornea (Fig. 485).
In reptiles, birds, and mammals the lateral wall of each olfactory capsule usually gives rise to an ectethmoid bone from which turbinal plates (conchae) extend into the corresponding nasal cavity. In reptiles and birds, in which the olfactory sense is not well developed, these bones are small. In mammals they are much larger, the conchae becoming extensively coiled to fill practically the entire nasal cavity of those species which have a very keen sense of smell (Fig. 681). In man three turbinal plates hang down into each nasal cavity from the lateral wall (Fig. 486). In most mammals the two ectethmoids fuse dorsally with the median mesethmoid, previously mentioned, to form a single ethmoid bone. At the point of junction of the three parts, the ethmoid bone is flattened out into a cribriform plate which forms part of the partition between the cranial cavity above and the nasal cavities below and in front (Fig. 487). This plate is perforated by numerous holes through which the minute branches of the olfactory nerve run from the organ of smell to the olfactory lobes of the brain. Ethmoid is from the Greek and cribriform from the Latin; both mean “sieve-like.”
The pterygoquadrate cartilages, wrhich serve as the upper jaw of cartilaginous fishes, give rise to one or two pairs of bones in most vertebrates. In the quadrate region of all groups below the mammals there is a quadrate bone, with which the lower jaw articulates, while in mammals this region gives rise to the incus, the second in the chain of three small middle ear bones (Fig. 488). A second pair of bones, appearing in some reptiles and in mammals, is laid down in the ascending process of the pterygoquadrate which extends dorsally into the region just in front of the otic capsule (Fig. 489). In reptiles this bone is called the epipterygoid (Fig. 490) while in mammals it is the alisphenoid.
Meckel’s cartilage, the embryonic lower jaw, usually gives rise to an articular bone near the angle of the jaws which, as its name implies, articulates with the quadrate of the upper jaw (Figs. 491 and 492). In mammals the articular becomes the malleus, the outermost of the three middle ear bones (Fig. 488). The remainder of this cartilage usually degenerates, its place being taken by investing bones. In some fishes and amphibians, however, one or more additional pairs of bones may develop in the cartilage.
The hyomandibular becomes ossified as a bone of that name in the hyostylic Osteichthyes (Fig. 491). In the autostylic tetrapods it gives rise to the columella or stapes of the middle ear, transmitting vibrations from the external tympanic membrane to the internal ear in all except mammals. In the latter Class it is the innermost of the chain of three middle ear bones which includes malleus, incus, and stapes (Fig. 488).
The hyobranchial apparatus, which includes the five branchial arches and the hyoid arch except the hyomandibular, gives rise in tetrapods to laryngeal cartilages and the hyoid apparatus. In larval amphibians and adult perennibranchs these structures, in somewhat modified form, are still associated with gills (Fig. 493A). In the adults of strictly land amphibians the apparatus is reduced to a plate of cartilage from which there extend out several pairs of horns (Fig. 493b). Parts of the latter are bony. In lizards (Fig. 493c) and birds an entoglossal process, part of the basihyal, extends into the tongue. In birds only one pair of horns is well developed. In the amphibians, reptiles and birds small laryngeal cartilages of doubtful homologies occur.
In mammals (Fig. 494) the median part of the apparatus is limited to a transverse bar, the body, in the base of the tongue. A pair of long anterior horns, from the hyoid arch, extends up to the ear regions into which the hyomandibulars have migrated as the stapes, as noted above. A pair of short posterior horns, from the first branchial arch, rests against the larynx. The thyroid cartilage of the larynx arises from the second and third branchial arches. Some uncertainty exists about the fate of the fourth and fifth branchial arches although there is evidence that the fourth may give rise to the cartilage of the epiglottis and the fifth to parts of the larynx and anterior tracheal rings.
What becomes of the different elements that make up the splanchnic part of the primitive skull is presented graphically in Figure 495, where the theoretical extent and position of the original arches are drawn as a background for the relics that persist. The same thing is also indicated in Figure 496 and Table VII.
It will be seen that the embryonic skeletal material, which originally had to do with respiration and the support and protection of the anterior end of the digestive tube, has assumed by a complicated series of makeshifts very diverse functions, such as the support of the vocal apparatus and the muscular tongue, and the transmission of sound waves to the internal ear.
Investing bones make up the roof of the skull, a region in which scarcely any cartilage develops in bony fishes and tetrapods. Along the mid-dorsal region, extending from the supraoccipital to the external nares, there is a double row of bones composed of four pairs, namely: interparietals, parietals, frontals, and nasals (Fig. 497). The orbits are usually lateral to the frontals. In front of each orbit may be a more medial prefrontal and a more lateral lacrimal, behind it a more medial postfrontal and a more lateral postorbital.
The interparietals, occurring in many extinct groups, are limited to one present-day Class, the mammals, in which they unite into a single somewhat triangular bone which lies between the supraoccipital and the parietals. Parietals, frontals, and nasals occur in nearly all bony vertebrates, but occasionally at least one pair becomes reduced, by fusion, to a single median bone (Figs. 469 and 498). Lacrimals, prefrontals, and postorbitals are found in lower Osteichthyes, Stegocephalia, cotylosaurs, and therapsids. In addition prefrontals and postorbitals are common among modern reptiles, while lacrimals occur chiefly in crocodiles, birds, and mammals. Postfrontals exist as separate bones only in Sphenodon among living tetrapods (Fig. 490).
Although originally bones of the dorsal surface, the septomaxillaries (Fig. 498) usually migrate into the nasal cavity to roof over the organ of Jacobson, an accessory olfactory organ in the ventral part of this cavity. These bones are found in nearly all tetrapods except turtles, crocodiles, birds, and most higher mammals, groups in which Jacobson’s organ is either poorly developed or absent. In snakes and lizards these parts are of considerable size.
The investing bones covering over the pterygoquadrate form the ventrolateral part of the tetrapod skull and articulate dorsally with the lateral elements of the cranial roof. Beginning in the mid-line at the front of the skull they are: premaxillaries, maxillaries, jugals, quadratojugals, and squamosals. In higher groups these parts are frequently arranged in a single row (Fig. 499) with the squamosal, which covers the dorsal part of the quadrate, slightly more dorsal than the others. Mammals lack quadratojugals. The squamosal, probably homologous with one of the bones of the fish operculum, finally becomes the bone with which the lower jaw articulates in mammals.
The investing bones of the ventral surface of the skulls of lower vertebrates are prevomers, palatines, pterygoids (endopterygoids), transpalatines (ectopterygoids), and an unpaired parasphenoid. The parasphenoid, usually closely applied to the cartilage of the ventral side of the neurocranium, is well developed in most fishes (Fig. 491) and amphibians but fuses with the anterior end of the basisphenoid in reptiles and birds. In mammals the anterior end of the parasphenoid becomes the vomer (Fig. 500), a bone of the ventral part of the nasal septum, while the posterior end may form part of the pterygoid region. Prevomers, found in all tetrapods except mammals, fuse into a single median bone in turtles and most birds. Primarily the palatines, pterygoids, and transpalatines are associated with the median side of the pterygoquadrate.
When the anterior ends of the digestive and respiratory passages become separated by the palate, support for this horizontal partition is provided by four pairs of bones: premaxillaries, maxillaries, palatines, and pterygoids. The premaxillaries and maxillaries, while still retaining their duties as tooth-bearing upper jaw elements, send out horizontal palatine processes which meet in the mid-line to form the anterior part of the hard palate, or secondary roof of the mouth. The posterior part of this region is supported by both palatines and pterygoids in reptiles but by the palatines alone in mammals. In birds these parts remain so narrow that they fail to join into a hard palate.
The investing bones covering over Meckel’s cartilage vary in number from nine pairs in some Stegocephalia down to the one pair of dentaries which make up the entire lower jaw of mammals. In the alligator (Fig. 492) five pairs are present, angulars, supraangulars, splenials, and coronoids in addition to the tooth-bearing dentary. It will be remembered that the articular bone, also present in the alligator’s lower jaw, is of the replacing type. It is probable that in mammals the angular bone, which does not contribute to the formation of the lower jaw, becomes the external tympanic bone which forms the wall of the inner part of the external ear canal and supports the external tympanic membrane, popularly known as the “ear drum.”
Middle Ear Ossicles
The evolutionary history from the cartilaginous jaws of the dogfish to the middle ear ossicles of mammals is quite as remarkable as that of the aortic loops or the urogenital ducts. Primarily in gnathostomes, or vertebrates with jaws, the articular region of the cartilaginous lower jaw forms a movable hinge-joint with the quadrate region of the cartilaginous upper jaw (Fig. 489). A hyomandibular, extending from the angle of the jaws to the otic region, acts as the suspensorium of the jaws.
With the appearance of bone in the Osteichthyes these three key regions ossify as the articular, quadrate, and hyomandibular bones.
In the autostylic tetrapods the hyomandibular, no longer used to attach the jaws to the neurocranium, is reduced in size and enclosed in the middle ear cavity. In amphibians, reptiles, and birds it forms the columella auris which transmits vibrations from the tympanic membrane to the internal ear.
In mammals additional modifications take place. Two investing bones, the dentary of the lower jaw and the squamosal of the upper jaw, provide the joint between these regions. The articular and quadrate, in reduced form, then move into the middle ear cavity to become reacquainted with their old associate the hyomandibular, or at least its homologue. The articular becomes the malleus, or hammer, and the quadrate becomes the incus, or anvil, while the hyomandibular gives rise to the stapes, or stirrup, which is therefore the homologue of part or all of the columella of other tetrapods. There is thus established a chain of three bones (Fig. 488): the malleus, the “handle” of which lies against the external tympanic membrane, articulates with the incus which in turn articulates with the stapes. The last-named fits into the “oval window,” an opening leading into the cavity of the internal ear. It is noteworthy that in mammals these parts are arranged in exactly the same order as their selachian homologues, namely, articular of lower jaw, quadrate of upper jaw, hyomandibular, and otic capsule.
In the primitive Osteichthyes, Stegocephalia, and cotylosaurs (Fig. 497) the investing roof of the skull has five openings: a pair of external nares, a pair of orbits, and a single median small pineal foramen. Behind the orbits the roof is complete and encloses a large temporal space on either side of the brain case. Large jaw muscles occupy much of this space in the intact animal. These skulls, in which the posterior part of the roof is complete, exhibit what is known as the anapsid (an, without; apsid, arch, loop) condition.
In some reptiles two additional pairs of openings, in the posterior part of the roof, are present (Fig. 501). The more dorsal superior temporal fossa is usually bounded by the parietal, squamosal, and postorbital. Ventral to this opening is the inferior temporal fossa which is usually surrounded by postorbital, squamosal, quadratojugal, and jugal. This diapsid type is found in Rhynchocephalia (Fig. 490), Crocodilia, pterosaurs, dinosaurs and related groups believed ancestral to birds. Birds have a modified diapsid type of skull.
In therapsid reptiles only the inferior temporal fossa is present. This synapsid condition is also found, in modified form, in mammals and some of the higher therapsids in which the fossa extends as far dorsally as the parietal. When the postorbital disappears, the fossa becomes continuous with the orbit.
Lizards and possibly some other reptiles have what appears to be only the superior temporal fossa, the parapsid condition, although there is still considerable doubt concerning the relationships of the parts of the skull in these animals.
Turtles (Fig. 499) are anapsid but they are believed to have acquired this condition secondarily as a result of the filling in of the fossae by enlargement of the individual bones.
The skull of cyclostomes (Fig. 502) is quite aberrant from that of other vertebrates. In the embryonic “ammocoetes stage” of Petromyzon, the parachordalia and trabeculae of the brain-case floor are evident and normal, as well as the otic capsules that surround the ears, but a cartilaginous envelope does not entirely surround the brain, since the roof of the brain case is completed in fibrous connective tissue, while the absence of jaws entirely changes the character of the anterior part of the skull. The skeletal structures of the large tongue may possibly be homologous with the lower jaws of fishes. At any rate the tongue bears rasping epidermal teeth and is bilateral in origin like the lower jaws. The cartilaginous elements which lie anterior to the tongue and support the jawless mouth seem to be peculiar to cyclostomes. They are obviously necessary adaptations on account of the suctorial and parasitic habits of these animals. The posterior part of the cranium, which ends abruptly with the otic capsules, is without any true occipital region.
Since in higher vertebrates the occipital region is to be regarded as a derivative from the anterior vertebrae of the spinal column, as the relation of the cranial nerves indicates, it could hardly be expected to be developed in animals whose skeletal axis has not yet emerged from the notochordal stage.
The skeletal support for the gills in cyclostomes is a continuous grillwork of cartilage, more external in position than the splanchnocranium of other fishes and not homologous with it.
The typical cartilaginous skull of elasmobranchs furnishes a morphological point of departure for the skulls of all other vertebrates (Fig. 474). In these primitive fishes the neurocranium is a continuous protective brain case. It is the result of the fusion of embryonic cartilaginous elements, as already described, which may be partially calcified in some adults. The splanchnocranium is present in its most perfected form, being nowhere fused or incorporated with the neurocranium. The first arch of the splanchnocranium serves as both upper and lower jaws and is abundantly supplied with integumental teeth. Perhaps the feature most peculiar to the elasmobranch skull, and least copied by subsequent forms, is the development of a snout in the form of rostral cartilages that project anteriorly, thus making the mouth ventral in position. In the elasmobranch Pristis, the extended rostrum is supplied on either lateral margin with sharp teeth, which give it the name of “sawfish.”
The transition from a single cartilaginous skull to a double skull is effected in this group, which characterizes it as a connecting link between the lower fishes and the higher vertebrates. While an inner cartilaginous capsule is retained unossified around the brain in the cartilaginous ganoids (Chondrostei), there is added to the outside of it a bony skull of many parts, derived from dermal scales (Fig. 476). The bones in the skulls of cartilaginous ganoids are all of the investing type, and since they are very numerous, their homology with the individual investing bones in the skulls of higher forms is doubtful. The number of elements in the splanchnocranium is reduced somewhat as compared with that of elasmobranchs.
The skulls of bony ganoids (Holostei) have two sets of bone, investing on the outside and replacing on the inside, the latter ossified from the cartilaginous brain case. Much cartilage still remains, however, after the replacing bones appear.
Bony fishes exhibit a great variety of skulls, all referable more or less directly to the type characteristic of bony ganoids. In the lower teleosts considerable cartilage persists unossified, but in most cases the name “teleost” (tele, entire; ost, bone) is justified (Fig. 491). No other vertebrates have as many bones as teleosts.
The problematical extinct Stegocephalia, some of which had skulls over three feet in length, show a large number of bones, probably investing in character, forming an extensive roof over a cranium of very small capacity. For the most part these bones fit closely together, allowing only for restricted orbital and nasal cavities, and suggesting the skull of ganoids rather than that of the amphibian type. The fact that cartilages are missing in fossils leaves much to be desired in working out homologies. Between the parietal plates on the top of the skull is an interparietal foramen, the window through which a third epiphyseal eye probably looked upward. There was an abundant occasion, for these lowly monsters to look upward, at least in an evolutionary sense.
Modern amphibians are characterized by much less cartilage in the skull and also by fewer bones than are found in skulls of the bony fishes. It is quite reasonable to suppose that they represent a separate, divergent, evolutionary line, distinct from those of other tetrapods and of teleosts, although all may be referred to a common origin in primitive bony fishes.
The restricted cranium itself, that houses the brain, is somewhat tubular and elongated, extending far anterior between the eyes without any interorbital septum. The conspicuous width of the skull, as well as the extensive horizontal mouth-opening, is largely due to outrigger bones which form the upper jaw and connect with the widened ethmoidal and otic regions. Of the few replacing bones the exoccipitals, bearing the two condyles and the prootics which make up the major part of the otic capsules, are the most constant. There are no basi- or supraoccipitals, but the place of the latter is taken by a band of connective tissue, or a narrow cartilage, the tectum synoticum, which joins the two otic capsules together by a dorsal bar (Fig. 475).
Urodeles show a greater parsimony of inner cartilaginous skull than the anurans, since the embryonic trabeculae never meet and fuse, thus leaving this region of the skull, as in the adult Necturus, without a cartilaginous floor. The quadrate and pterygoid are continuous, while the quadratojugal fuses with the squamosal. The parasphenoid of urodeles is a large flat investing bone, sometimes bearing teeth, covering nearly the entire floor of the skull, whereas in anurans it is reduced to a peculiar T-shaped structure with the shaft pointing anteriorly (Fig. 503). The parietals and frontals also, which are separate in urodeles, become incorporated into parieto-frontals in anurans, and still other hyphenated bones mark the progressive simplification of the amphibian skull.
Little of the original cartilage remains in the skull of the legless caecilians (gymnophiona), but there is more fusion of the investing bones than in caudates, making a compact skull adapted to the subterranean life of these curious aberrant animals.
Cartilage largely disappears from the adult reptilian skull, persisting in Sphenodon and in lizards to a greater extent than in other modern reptiles. Investing bones are more numerous than in amphibians while replacing bones are many and usually independent or unfused.
Except in snakes, the interorbital septum separates the two orbits of the eyes, in consequence of which the brain does not extend as far anteriorly as in amphibians. Between the pterygoid and the maxillary an additional bone, the transverse or ectopterygoid, is added, greatly strengthening the jaw. Premaxillaries, which are usually united in adult reptiles, are embryonically double, and the same is true of the parietals. Meckel’s cartilage is surrounded by a full complement of investing bones (Fig. 492).
In turtles a false roof to the skull, formed by expanded post-frontals, parietals, and squamosals, reminds one of the stegocephals, for the entire chelonial skull is exceptionally large in comparison to the size of the enclosed brain (Fig. 481). A single median vomer and the parietals, which remain separate, are present, but several bones that might be expected, such as the nasals, ectopterygoids, and lacrimals, are absent in turtles.
The jaws of snakes are a distinctive feature, since they are capable of great distension, enabling these animals without the aid of hands to swallow their relatively large prey whole. The two parts of the lower jaw are not only loosely joined to the cranium on either side by the movable quadrates, but are also anteriorly united at their distal ends by an elastic ligament, permitting their lateral separation to a considerable extent. Each half of the upper jaw region consists of a movable chain of four bones, namely: quadrate, pterygoid, palatine, and maxillary. The quadrate is attached to the posterior part of the brain case in a hinge-joint, while the maxillary is attached anteriorly in the same fashion. The pterygoid and palatine do not form a rigid hard palate but are moved as the ventral free end of the quadrate is swung back and forth. Through them the movements of the quadrate are imparted to the maxillary. At the same time the lower jaw, which articulates with the ventral end of the quadrate, is also moved back and forth. With each backward movement of the jaws the teeth, which are tipped posteriorly, hook into the prey and draw it down the throat.
In poisonous snakes each fang is set in a socket in the corresponding maxillary. As the snake prepares to strike, it opens its mouth and moves the chain of upper jaw bones forward, thus swinging the fang out into striking position (Fig. 504). By moving the upper jaw elements posteriorly the snake returns its fangs to the resting position within the mouth cavity.
In crocodiles and lizards the two parts of the lower jaw are joined in front by sutures, but in turtles they are fused into one solid mandible. The formation of an extensive hard palate, in which the maxillae, palatines, and pterygoids all take part, in the case of alligators and crocodiles causes the posterior opening of the choana on either side to be pushed back so far down the throat that these animals are able to drown their prey without being drowned themselves. This feat is accomplished by the aid of a curtain-like velum that shuts off the posterior openings of the nostrils (choanae) from the surrounding water, thus enabling the reptile to breathe through the choanal passage-way with the tip of the snout out of water, while the mouth is kept submerged until the struggling prey within, held fast by the conical teeth, is drowned.
In birds the most characteristic feature of the skull, like that of the entire skeleton, is the elimination of everv bit of bone that can be spared (Fig. 505). Compared with reptiles, from which birds arose, a typical avian skull is more spherical and roomier. The relatively enormous size of the eyes, together with the well-developed interorbital septum, tends to restrict the brain to the posterior part, while the ear-capsules on either side are more deeply embedded than in reptilian skulls.
Notwithstanding the fact that many typical bones, both replacing and investing, are distinguishable in the embryo, they become welded together in the adult by the early closure of the sutures into an almost continuous capsule of bone, which is remarkably strong and protective in spite of its lightness and the paucity of bony material employed.
The zygomatic arch is reduced to a slender bar, the postfrontals disappear, and the premaxillaries, maxillaries, palatines, and pterygoids are so narrowed that they fail to join into a hard palate. The lightness of the skull is furthermore enhanced by the pneumaticity of the bones, characteristic of the entire skeleton, and also by the substitution of a horny beak in place of heavy teeth set in the jaws. Only in the interorbital septum and in the ethmoid region does cartilage persist.
Remains of the investing parasphenoid fuse with the replacing basisphenoid to form a unique, forward-projecting sphenoidal rostrum along the floor of the cranium. The single occipital condyle on the basioccipital bone is similar to that in reptiles, allowing great freedom of movement to the bird’s head. With the rotation forward of the condyle, along with the shift in position of the foramen magnum, these two landmarks appear at the ventral base of the skull instead of at the posterior aspect of it. This change, which is associated with the semi-erect posture of birds, modifies the manner in which the skull is carried by the vertebral column.
The quadrate has a freely movable articulation with the skull, so that the lower jaw has a double joint with the skull as in the lizards and snakes. Many birds have a movable joint between the upper jaw and the skull which enables them to raise the upper jaw without at the same time carrying the cranium with it (Fig. 506), an unusual performance for a vertebrate. The curious hyoid apparatus of the woodpeckers exhibits another peculiar modification of the bird’s skull in connection with the mechanical withdrawal of the extended tongue.
The orbits of the eyes are close together, separated only by a thin septum without any intervening part of the brain between them. It is difficult to imagine how monstrous a human skull would be if, like that of a bird, each orbit were as capacious as the entire cranial cavity.
Mammals, with special reference to man
The skull bones of the primitive monotremes are highly specialized on account of their beaklike mouth parts and the early fusion of bones which obliterates the outlines of the component parts.
It may be said of the mammalian skull in general that the splanchnocranium becomes more completely incorporated with the neurocranium than in any other class of vertebrates. With the process of cranial expansion to accommodate the increasing size of the brain, there is a shortening of the jaws, while the retreating face bones become more and more overshadowed by the upward-bulging cranium until, in primates at least, they are ventral to the cranium in position rather than anterior to it. This evolutionary tendency is made clear when one compares the skull of a deer with that of a man (Fig. 507).
Although there is considerable fusion of bones in the mammalian skull, the sutures usually remain quite distinct, except in old age when they are apt to become obliterated. When of the zig-zag type, formed by the dove-tailing of the bones together edge to edge, the sutures make an interlocking joint almost as firm as solid bone.
In man, with the assumption of erect posture associated with the bipedal habit, the foramen magnum and surfaces for articulation with the first vertebra have shifted to a position beneath the skull instead of at the posterior end (Fig. 507). The human orbits have also migrated forward to positions close together on either side of the nasal region where both eyes may be directed forward, thus permitting binocular vision. Yet as far as the actual bones are concerned the human skull is built according to the same general plan as are the skulls of most mammals. In man, however, more than in most other mammals, groups of embryonic bones tend to fuse together to form bone complexes. The total number of separate bones in the adult skull is thus reduced to 22 plus the 3 pairs of middle ear bones already described.
The occipital (Figs. 487 and 500) is laid down as five separate parts. Four of these make up the replacing ring around the foramen magnum composed of the basioccipital, the two exoccipitals, and the ventral part of the supraoccipital. The fifth is the investing dorsal half of the supraoccipital which sometimes remains separate as the inter parietal, a condition commonly found in other mammals. In man it is also known as the Inea bone (Fig. 508) for the reason that it has been often found in the skulls of the aboriginal Incas of Peru. The suture of Mendosa (Fig. 509), between the two parts of the human supraoccipital, ordinarily disappears about the third fetal month. The other parts fuse between the fourth and sixth years. By the twenty-fifth year the occipital unites with the sphenoid bone at their basal regions, their only point of contact. On the exoccipital portions, alongside the foramen magnum, is a pair of occipital condyles which articulate with the atlas.
The sphenoid bone, the base of which is immediately anterior to the basioccipital region, is made up of at least eight embryonic parts. The basisphenoid and the presphenoid form the median body of the bone. The two alisphenoids become the great wings (Fig. 487) while the two orbitosphenoids become the small wings. The two investing pterygoids attach to the regions of junction of the body and the great wings as the pterygoid processes. All of these parts have fused together by the end of the first year. On the inner surface of the basisphenoid portion there is a deep depression, the sella turcica or “Turk’s Saddle,” which is a conspicuous landmark on the inside of the mammalian skull and in which is cradled the downward-projecting hypophysis of the brain. Just anterior to the sella turcica there is a transverse groove in which lies the optic chiasma where fibers of the optic nerves cross from one side to the other. Enclosed within the body of the sphenoid is a pair of sphenoidal air sinuses (Fig. 500), separated from one another by a bony septum. Each opens into the postero-dorsal part of the corresponding nasal cavity.
The ethmoid, a thin delicate bone lying in front of the sphenoid, forms the dorsal part of the nasal septum as well as portions of the lateral walls of the nasal cavities (Figs. 469, 487, and 500). It is composed of three replacing bones, a mesethmoid and two ectethrnoids, which unite about the twelfth to fifteenth month. The mesethmoid is composed of (1) the perpendicular plate, part of the nasal septum, (2) the crista galli, an upward projection into the brain cavity in the same plane as the perpendicular plate, and (3) a portion of the cribriform plate, which lies at right angles to the other two parts and forms a part of the roof of the nasal cavities. The cribriform plate (cribrum, sieve) is so called because it is perforated like the top of a pepper-box for the passage of the brushlike olfactory nerves, growing back from the patches of sensory epithelium in the nasal chambers. The multiplicity of foramina for the olfactory nerves is peculiarly mammalian, although in Ornithorhynchus there is only a single pair of olfactory foramina while the Cetacea, which have apparently entirely lost the sense of smell, have no cribriform perforations. Each ectethmoid contributes to the cribriform plate and makes up part of the wall between a nasal cavity and the corresponding orbit. It is composed of thin, spongy bone enclosing large cavities, ethmoidal cells, which open into the adjacent nasal cavity. These cells are sometimes known collectively as the ethmoidal sinus. Each of the ectethmoids gives rise to two turbinals (conchae) which extend diagonally down from its medial wall (Fig. 486).
The ventral nasal conchae, one of which is on the lateral wall of each nasal cavity ventral to the two ethmoidal conchae, ossify as separate bones in the lateral walls of the cartilaginous nasal capsules. They do not fuse with the other bones of the nasal wall. In many mammals there are elaborate turbinal bones, in some cases in the form of extensive, delicate scrolls almost filling the nasal cavities. They provide additional surface for warming and moistening air as it passes through on its way to the lungs, as well as providing for extra olfactory epithelium.
The vomer, or “ploughshare,” a thin bone forming the postero-ventral part of the nasal septum (Fig. 500), lies between the ethmoid and sphenoid above and the hard palate below. This bone is homologous with the anterior end of the parasphenoid of lower vertebrates and not with their paired prevomers. The anterior part of the nasal septum remains cartilaginous.
The parietals, which form much of the dorsal and lateral walls of the brain case, are ordinarily separated throughout life by a sagittal suture from which has been derived the name sagittal plane, applied to the plane dividing a bilaterally symmetrical animal into right and left halves.
The frontal forms the forehead, anterior part of the cranial roof, roof of the orbits, and part of the roof of the nasal cavities. Until about the eighth year this bone consists of a pair of bones separated by the metopic suture (Fig. 510). In most mammals the two frontals remain separate throughout life. In the median part of the frontal bone is a pair of frontal sinuses (Figs. 500 and 511), separated by a bony septum, which communicate with the nasal cavities immediately ventral to them. Bony outgrowths of the frontal bones give rise to the paired horns of ungulates as described in Chapter X.
The nasals, lying in the anterior part of the roof of the nasal cavity, support the bridge of the nose, the shape of which is determined largely by the size and shape of these bones (Figs. 469, 512, and 513).
Each of the lacrimals (Fig. 511) forms part of the medial wall of the orbit anterior to the ethmoid. The anterior part of the bone bears a vertical lacrimal groove through which the lacrimal (tear) duct leads into the nasal cavity. Mammalian lacrimals may be homologous with the prefrontals of reptiles which they resemble more than they do the so-called “lacrimals” of the latter.
The maxillae, which are the largest of the face bones in man, not only bear teeth and serve as upper jaws, but they also extend inward, forming a part of the hard palate in the roof of the mouth. Each is hollowed out by a large irregular sinus, the antrum of Highmore (Fig. 511), which communicates with the nasal cavity. Into the ventral side of the antrum project the bony capsules which surround the roots of at least the first two molar teeth. If the sinus is particularly large, all five molariform teeth, and even the incisors, may be represented here. Occasionally the actual roots of the teeth may be exposed in the cavity. Like all the other air sinuses of the skull, the antrum is lined with mucous membrane and in the case of man is one of the happy hunting grounds of bacteria.
Each maxilla is composed of a small premaxillary, into which the incisor teeth are set, and a maxillary bearing all of the other teeth. Premaxillaries are rudimentary or wanting in bats and some edentates. The premaxillaries and maxillaries are independent in most mammals, at least during early life. In man, however, the fusion occurs before birth.
The apparent absence of premaxillaries (or inter maxillaries, as they are sometimes designated) was formerly held to be a distinguishing skeletal mark of human kind. It is interesting to recall that the poet-biologist Goethe discovered their presence in man and wrote hilariously in 1784 to his friend Herder: “Nach Anleitung des Evangelii muss ich Dich aufs eiligste mit einem Gluck bekannt machen, das mir zugestossen ist; Ich habe gefundenweder Gold noch Silber, aber was mir unsagliche Freud macht, das os intermaxillare am Menschen.” (According to the gospel method I must let you know in all haste of a lucky thing that has befallen me. I have discovered — not gold nor silver, but what gives me unspeakable joy, the os intermaxillare of man.)
The jugals (zygomatics or malars of human anatomy) form the zygomatic arches, or “cheek bones,” by connecting with processes from the squamosal parts of the temporals and from the maxillaries anteriorly (Figs. 469 and 496). Each contributes to the floor of the orbit.
The palatines (Fig. 509), lying immediately behind the maxillae, form the posterior part of the hard palate as well as sending processes into the walls of the orbits and the nasal cavities.
Each temporal (Fig. 496) consists of four regions, namely, the squama, the petromastoid, the tympanic ring, and the styloid process. The squama, or “scale,” is the somewhat circular thin antero-dorsal part which corresponds with the squamosal of other mammals, in most of which it remains a separate bone. On the squamosal is the mandibular fossa into which the condyle of the lower jaw fits. The petromastoid, or petrosal, postero-medial to the squama, is a thick region containing the internal ear and most of the middle ear cavity writh its three auditory ossicles discussed in section 5. It is formed chiefly by the fusion of the three otic bones—prootic, opisthotic, and epiotic — which are laid down in the original otic capsule. In the posterior part of the petromastoid are the mastoid cells, spaces which communicate, through a common passage-way, with the middle ear cavity (Fig. 514).
Included in the temporal assembly there are not only the otic bones, middle-ear bones, and squamosal, but also a ringlike fragment, the tympanic ring, surrounding the external auditory canal across the inner end of which the tympanic membrane is stretched. This small bony element is probably the homologue of the angular bone from the lower jaw of other vertebrates. Attached to the petromastoid medial to the external auditory canal is the styloid process, a remnant of the hyoid arch, which probably represents the tympanohyal, from the anterior horn of the hyoid arch of many mammals (Fig. 494).
During development flat skull bones grow out from centers of ossification, like spreading ripples from a pebble thrown into a quiet pond, with the result that when the advancing edges of three or more enlarging bones meet, a small unossified area is temporarily caught between them. Such an open area is called a fontanelle, so named by some imaginative pioneer of anatomy because the throbbing of the blood vessels faintly visible at such regions in the infant’s skull suggested a “little fountain.” In man at least six fontanelles, around the parietal bones, are present at birth (Fig. 496): first, a large diamond-shaped frontal fontanelle on the top of the head between the frontals and the parietals, which usually closes about the end of the second year; second, the occipital fontanelle, triangular in shape and lying between the occipital and the parietal bones, that closes at the end of a few months; third, a pair of small sphenoidal fontanelles, which are formed on either side of the skull by the union of the frontal, parietal, temporal, and sphenoidal bones; and fourth, the mastoidal fontanelles, likewise small and paired, that occupy the posterior space between the parietal, temporal, and occipital bones.
Sometimes tiny sutural, or Wormian, bones occur as extra elements between the usual bones. They are found most often in the lambdoidal suture, between supraoccipital and parietals, but may also occur in fontanelles.
In childbirth the fontanelles are doubtless an adaptation of great practical value, enabling the large-headed infant to emerge into the world through a comparatively narrow gateway, since the cranial bones, as yet sutureless, can temporarily slip past each other at the edges without injury to the soft brain within. Long ago Hippocrates pointed out that it is quite possible, if the head has become somewhat misshapen in the process of childbirth, to mold the cranium back into conventional contours without injury, almost as if the brain were clay in the potter’s hands.
Certain primitive races, like the Flathead Indians of North America and the nobility among ancient Peruvians, seized upon this possibility of shaping the plastic skull of the newly-born infant in order to acquire a conspicuous and distinctive form of the head which, if not an improvement on nature, at least had the quality of lending to its possessor a mark that set him apart from ordinary people in a sort of plastic anatomical aristocracy of his own (Fig. 515).
The late Dr. Louis R. Sullivan of the Department of Ethnology in the American Museum of Natural History in New York kindly gave permission to quote the following information concerning head deformation in North America. “The study of physical types is complicated by the widespread deformation of the head. In some cases it is unintentional, being produced by the hard pillows and cradleboards used. In many instances, however, the head has been intentionally deformed by mechanical devices. The two principal types of artificial deformation are the fronto-occipital flattening (Fig. 516) and the Aymara conical distortion (Fig. 517). There are various subtypes of each of these deformations. The custom was previously wide-spread, being recorded in one form and another from Africa, Malay, Philippines, France, Scandinavia, and Asia Minor. It is becoming less common in America at the present time. In some tribes the entire population practiced deformation while in others it was confined to one sex or even to the chiefs alone. Investigation has shown that it does not affect the mental ability of the subject and that it is not transmitted.”
On account of the increasing size of the mammalian brain several of the cranial bones shift from their original position. This is particularly true of the cetacean skull, where the parietal bones are divorced from each other and shoved over to an extreme lateral position (Fig. 518), while the supraoccipital extends forward between them even as far as the frontals, making up most of the roof of the skull. The maxillaries, meanwhile, push back posteriorly and form a considerable part of the cranium, invading the territory ordinarily occupied by the frontals, which, now become squeezed narrowly between the backward-extending maxillae and the forward-pushing supraoccipital.
The mandible, or lower jaw, is formed by the union of two tooth-bearing dentary bones. It bears a pair of condyles which articulate with the mandibular fossae of the temporal bones. Although in most mammals the two sides are separate they fuse into one solid piece in bats, perissodactyls, and primates.
Since long before the time when Samson slew a thousand Philistines with the jawbone of an ass, the mandible has in many ways been an outstanding bone of interest. Except for the tiny middle ear ossicles, it is the only mammalian skull bone that has an independent movement of its own. In man it has gone a long way, in adjustment to changing conditions, from its cartilaginous prototype still surviving in the dogfish. At no time in its history has there been any cessation in function, although upright posture and the resulting liberation of the hands, together with the effeminate modem days of soft cooked food, have made it no longer imperative for the jawbone to serve as a nutcracker and bonecruncher. In some monkeys and apes the lower jaw may make up as much as forty per cent of the total weight of the skull, but in modern man it falls to less than fifteen per cent.
There is no doubt that its golden age of efficiency is past and now it is turned to other uses. Instead of being employed as a means of terrifying enemies by the frightful gnashing of bared teeth, it has become a part of the skeletal framework for the display of smiling friendliness and persuasive diplomacy.
In the infant before the teeth have forced the lower jaw to enlarge, the chin slopes backward (Fig. 519). Long before puberty it pushes forward until the angle it forms with the lower jaw is not far from a right angle. In adult life the chin projects forward, until finally in old age, with the loss of teeth and the resorption of the alveoli, it projects at almost an acute angle. These successive changes which an individual passes through during a lifetime are strikingly paralleled in the evolutionary emergence of the human race, as shown in Figure 520. The chinless jaw of the gorilla is followed by the almost equally chinless jaw of the Heidelberg man and that, in turn, by the right-angled chin of the Spy man from Neanderthal days, to be finally superseded by the projecting chin of modern man, in which there is less standing room for teeth but a greater space within for the attachment of muscles concerned with speech.
In the ventral abdominal region, left unprotected by sternum and ribs, a series of skeletal structures known as Gastralia (“Abdominal ribs” or Parasternum) is found in Stegocephalia, Sphenodon, some lizards, Crocodilia, and Archaeopteryx. Developing in the ventral parts of the myosepta, they do not meet in the mid-line except in Sphenodon and some of the lizards. They may be homologous with the sternal parts of ribs or with the sternum, hence the various synonyms. On the other hand, they may have arisen as the result of local stresses in the abdominal region of some of these heavier, crawling land animals.