Food Capture and Prehension
Before food can travel along the digestive highway, it must be captured and placed inside the entrance of the tube. This process, which may call for expert performance, occupies a large part of the waking hours of most animals, and even in the case of intellectual man is the actuating motive behind much of his daily behavior. It is no concern of plants.
Probably in the majority of cases the capture of food involves some sort of a chase, since the animal as well as its food may be in motion. Herbivores have the advantage of depending upon food that is generally stationary, so they simply need to seek it out. Sedentary feeders, on the other hand, remain in one spot, catching motile food that comes their way. Devices of various kinds, therefore, like ciliary whirlpools or stretching tentacles, are employed by stationary animals to bring food within range. Many aquatic animals that are not sessile also use cilia to sweep microscopic food particles their way. The ciliated fraternity includes protozoans, sponges, anthozoans, bryozoans, rotifers, brachiopods, sessile annelids, brittle-stars, bivalves, pteropods, entomostracans, tunicates, amphioxus, and many larval forms.
Many animals that are anatomically able to go in pursuit of food, succeed better by lying in wait for passing food than by bestirring themselves in open chase. They have their breakfast, so to speak, served to them in bed. Such animals frequently develop camouflaging coloration, or, like spiders, construct elaborate snares and traps for their prey. Mucous threads are employed by certain coelenterates and mollusks to entangle food particles that are then engulfed. With the evolution of bilateral symmetry and increased powers of locomotion, “watchful waiting” goes more and more into the discard and pursuit of daily bread becomes the more universal method.
When food is finally within reaching distance, there are many diverse organs of prehension (Fig. 221), which come into play for seizing it and placing it within the mouth. These adaptations range all the way from the slow pseudopod of an Amoeba to the reaching “boarding house arm” of modern man.
Birds, possessing neither arms nor hands for taking hold of food, have the edge of the mouth opening drawn out into a point, forming a homy beak which is used as a pair of forceps in picking up things.
The prehensile tongue of such diverse animals as toads, anteaters, and cattle, becomes a very effective substitute for a grasping hand, while muscular lips, particularly of herbivores, serve a similar purpose in food prehension.
Some snakes, with no means for killing their prey when it is overtaken, seize it with their backward-projecting teeth and swallow it alive. When once within the mouth it cannot easily be ejected or escape, but is forced to inch its way down the gullet by the propalinal motion of the jaws.
Many animals, as for example swans and giraffes, have an elongated flexible neck as an accessory organ of prehension, to aid in bringing the mouth into the immediate neighborhood of food. The trunk of an elephant, which is a drawn-out nose and upper lip combined, is a unique device for reaching food without the necessity of lowering the heavy head.
Certain annelids and starfishes prehend their food by everting the pharynx, or the stomach, as the case may be, which enwraps the food and may even digest it outside the body.
The Mouth Aperture and Lips
The mouth is the architectural centerpiece of the face (Fig. 222). The shape and extent of the mouth opening varies greatly in different animals, depending largely upon the different kinds of food utilized.
The limits of the oral slit in mammals are set by the fleshy cheeks. An animal without cheeks, like an alligator or a nestling bird, can open up the mouth to a surprising extent.
Amphioxus and cyclostomes keep the mouth always open of necessity, since structurally it cannot be closed. In man the slit of the mouth normally extends from about the region of the first premolar teeth on one side to those on the other side, although there is considerable range of individual variation, as may be commonly observed.
The puffed cheeks and rosebud mouth of infancy are muscular adaptations for sucking, mammalian characteristics which are largely lost in adult life (Fig. 223). The evolution of cheeks in the adult is closely connected with the muscular equipment for mastication, so it comes about that animals with relatively small mouth openings are usually better able to chew their food than those with an expansive opening. Cheeks and chewing go together, for cheeks make possible the retention of food between the grinders. The retaining cheeks of cattle enable them even to chew “up hill” (Fig. 224). The refinement of chewing food, with all its train of anatomical consequences, is a mammalian peculiarity, for it will be recalled that fishes, amphibians, birds, reptiles, and even many of the lower mammals, swallow their food without chewing it.
In the higher vertebrates the lips are two movable folds at the edge of the mouth aperture. They are covered by skin on the outside and moist mucous membrane on the inside. The red part of a lip, an exposed zone of transition between skin and mucous membrane in man, is extremely sensitive to touch because of an abundant supply of nerve endings. The lower lip is more movable than the upper one. Attention to the form and shape of these portals to the digestive tube is shared alike by the comparative anatomist and the poet.
Immediately within the mouth aperture of mammals is the vestibule, or buccal cavity, bounded outwardly by the lips and cheeks, and inwardly by the external face of the teeth and gums. When the mouth is closed and the teeth are in contact, this cavity becomes practically obliterated, but behind the back teeth, and between the closed teeth, there is still direct communication with the larger oral cavity within.
Various glands open inside the buccal cavity. Along the inner surface of the lips are numerous small labial glands that secrete mucus. These glands may be easily identified by rubbing the point of the tongue back and forth against the inner surface of the lips, when they will be felt as tiny bunches. Other mucus-producing glands, the molar glands, open from the cheeks into the buccal cavity opposite the back teeth, while opposite the second upper molar tooth on either side, is the exit of Stenson's duct that drains the large parotid gland (Fig. 225), from which saliva flows. It is not difficult to locate the openings of these important ducts, for if one sticks the tongue into the cheek, and psychologically aids the flow of saliva by looking at a freshly sliced lemon, or something that “makes the mouth water,” a tiny stream of saliva may be felt spurting into the buccal cavity.
Birds, turtles, and monotremes with beaks, have dry cornified buccal cavities nearly devoid of glands. No one ever saw a bird “spit.”
Saliva, containing a digestive enzyme, ptyalin, is produced at the rate of as much as three pints a day, for the most part during the intake of food. Since saliva is not stored, the glands need periods of rest and recuperation between times of accelerated activity. The reader can draw his own conclusions about the physiological results of the gum-chewing habit.
On the inner face of the upper lip in the middle line, demonstrable by the exploring tip of the tongue, is a vertical fold of mucous membrane which tends to hold the lip close against the gums. This is called a labial frenulum. A second one occupies a similar median position with reference to the lower lip.
In some animals, such as the duckbill, Old World monkeys, apes (Fig. 226), gophers, squirrels, and other rodents, the buccal cavitv can be stretched into distinct cheek pouches, which are used for the temporary storage of food when its collection occurs under circumstances of competition such as to make grabbing as much as possible in a minimum of time desirable. Sometimes greedy little children demonstrate their probable rise from animal ancestry by reverting to the cheek-pouch method of excess disposal of food.
Behind the mammalian buccal cavity and merging into it, is the oral cavity. The roof of this cavity, in higher vertebrates generally, is the arching palate which has a skeletal foundation of bone, the hard palate, in the front part of it, and is supplemented behind by a flexible addition of connective tissue, the soft palate.
The hard palate lies within the upper dental arch and is continuous with the gums (gingivae), that are rich in blood vessels but poor in nerves. The soft palate blending with the lateral walls behind the teeth, presents a free posterior border hanging like a curtain, in the region of the fauces, or the gateway leading to the pharynx.
The posterior border of the soft palate in man is still further prolonged in the median line into a soft, pointed, dangling flap called the uvula, that projects downward and backward, and which may easily be seen hanging down in the back part of a wide-open mouth (Fig. 227).
Along the median line of the human hard palate, from a point near the upper median incisor teeth and fading out toward the region of the soft palate, is a faint ridge, the raphe, which indicates that the hard palate is formed by the union of two lateral components. It may be felt, in those individuals who still have it present in the roof of the mouth, by means of the tip of the tongue.
In many instances there may also be similarly demonstrated a series of transverse folds or ridges at right angles to the raphe, the palatine rugae, diminishing in size from the region of the teeth backward. The rugae are more in evidence in human embryos than in adults, although they not infrequently persist throughout life. They are wash-board like in character and find their highest development in such carnivores as cats and dogs (Fig. 228), where no doubt they aid in securing a surer grip upon any struggling victim that has been seized in the jaws.
The surface of the entire palate, particularly of the soft palate and the uvula, is beset with numerous palatine glands, whose secretion of mucus helps to keep the mouth cavity moist.
The sides of the oral cavity posterior to the back teeth blend with the buccal cavity into a common space, while the floor is largely occupied by the bulky tongue, which fills practically the entire cavity when the mouth is closed. When the mouth is opened wide and the tongue is raised and curled back, the frenulum linguae may be seen in the shape of a fold of connective tissue along the midventral region, that tends to hold the tongue down to the floor of the oral cavity (Fig. 229). Occasionally, when the lingual frenulum is overdeveloped in a human infant, such an individual is said to be “tongue-tied,” and a slight surgical operation is necessary before the tongue can acquire the freedom of movement essential for clear articulation in speech.
Extending on either side of the frenulum linguae in man, and parallel to the lower teeth, is a crescentic fold of tissue, called the sublingual ridge. Along this ridge open the several ducts of Rivinus from the sublingual sailvary glands, while at the widest part of the frenulum linguae, near the lower median incisor teeth on either side, are the openings of Wharton's ducts, that drain the subrnaxillary salivary glands. Thus, three sets of salivary glands, the parotid, sublingual, and subrnaxillary, pour their digestive and lubricating secretions of saliva into the buccal and oral cavities.
This differentiation of mouth glands into various mucous and salivary glands common to mammals does not appear among the lower vertebrates. Fishes, which bolt their food without chewing, do not have digestive salivary glands, while mucous glands, the mission of which is to moisten the food in the oral cavity preparatory to swallowing it, are also unnecessary and practically absent.
Among amphibians, living on the border line between submergence in water and life on land, scattered mucous glands, termed intermaxillary glands from their generalized location, make their appearance in some instances, while the protrusible tongue, particularly in frogs and toads, is supplied with lingual glands, secreting a viscous mucus that aids in the capture of insects and other moving prey.
In reptiles the mouth glands are more grouped and localized, so that it is possible to speak of palatine, lingual, sublingual, and labial glands, according to their location. All of these glands produce fluid that moistens the food and renders the act of swallowing easier, although it is doubtful if they aid appreciably in digestion.
Poison glands in the mouth of certain snakes (Fig. 230), are transformed parotid glands, while those of the only lizard known to be poisonous, the “gila monster,” Heloderma, of the southwestern United States, are modified sublingual glands.
Birds, as noted, have a paucity of oral glands.
In the case of mammals, which usually chew their food to some extent, mouth glands of two general sorts are universally developed, mucous and salivary, for the double purpose of lubrication, and of liquefaction and chemical modification. Mucous glands are especially essential for herbivores that consume large quantities of comparatively dry, bulky food. The action of the salivary glands, which is both chemical and mechanical, will be referred to later in the consideration of digestive glands in general.
What passes under the name of “tongue” in the vertebrate series is not always strictly comparable to the “unruly member” in man (or woman), which must be regarded as the outcome of a long sequence of adaptations.
Amphioxus has no tongue at all, and the muscular piston-like tongue of cyclostomes is such an aberrant, highly specialized structure that it gives no safe clue to the true beginnings of this organ among vertebrates.
In fishes, however, a primary tongue makes its definite appearance. It is a non-muscular elevation from the floor of the mouth cavity, consisting of a covering of mucous membrane, stretched over a skeletal support of cartilage or bone, derived from the framework of the gills (Fig. 231). A projecting basihyal cartilage, that lies between the lower jaws of the mandibular arch, is the skeletal basis of this kind of a tongue. Whatever movement it is capable of is due to extrinsic muscles that act upon the skeletal support in such a way as to enable it to change position but not shape, rather than upon intrinsic muscles that modify both shape and position. It is also not protrusible, although motile enough to aid somewhat in forcing back a mouthful of food to be swallowed, and, in some cases, is beset with prehensile teeth.
The lower amphibians, such as the perennibranchiate urodeles, have fishlike tongues of mucous membrane with cartilaginous support. In the higher salamanders the horseshoe-shaped groove between the primitive tongue and the lower jaw becomes elevated, particularly in front, into a glandular field (Fig. 232), in which a glutinous mucus, useful in entangling captured insects, is secreted. This glandular field gradually rises, thus obliterating the original groove around the under edge of the primary tongue, until finally it becomes incorporated with the latter as an anterior projection, forming the so-called secondary tongue.
In the median line at the junction of the primary and secondary tongues, and originally connected with the thyroid gland, there is a tubular down-growth, the thyroglossal duct that persists in mammals as the foramen caecum (Fig. 233).
The secondary tongue soon becomes invaded by intrinsic muscles, which greatly increase the range of its movements, and make changes in its shape possible. Of these muscles the genioglossals act as protractors, and the hyoglossals, as retractors. In the American salamander Eurycea, they become so efficient that the sticky tongue may be shot out a considerable distance and retrieved with incredible speed in the capture of insect prey.
The secondary tongue of most frogs and toads, which is attached far forward on the floor of the mouth cavity, is retroflexed when at rest, so that its point lies backward down the throat. When it is flipped out after an insect (Fig. 234) or a slug, it is “swallowed” upon its return, along with the captured food, and thus restored to its original position. One family of toads, including the genera Pipa and Xenopus, is named Aglossidae, because in these exceptional animals, the tongue is either absent or very poorly developed.
Reptiles embryonically possess a double tongue, like that evolved by amphibians, although with considerable modification. In turtles and alligators it is thick and only slightly protrusible, whereas in snakes and lizards it may become extremely extensible. The little wall lizards, or “geckos,” for example, can easily lick the outside of their transparent eyelids with their tongues, while snakes can protrude their delicate sensitive forked tongues for some distance through a median notch in the edge of the lower jaw, without opening the mouth.
The chameleon, an arboreal African lizard famous for its kaleidoscopic color changes, while grasping the twig of a tree uses its long tongue like a lasso in entangling its elusive prey, in much the same way as the salamander Eurycea from a position on the ground shoots out its tongue. The mechanism in the two cases is somewhat different. In Chameleon the bony framework of the primary tongue acts as a system of extensible levers to supplement the secondary muscular component of the tongue in its protrusion, which is not the case with Eurycea.
In birds the bony framework of the primary tongue, which supports the secondary tongue, is especially well developed. This framework consists typically of a median bone or bones, the copula (Fig. 235), and two pairs of lateral bones, the small hyoids, and the first branchials, all of which are relics of ancestral gill arches. Its movement is facilitated by means of extrinsic muscles attached to these bones, the intrinsic muscles of the secondary tongue being reduced or absent.
A woodpecker, whose horny spearlike tongue can be projected out of the long beak when impaling a grub in the bark of a tree, possesses an elaborate skeletal hyoid apparatus attached at the base of the tongue, and with long posterior horns (first branchials) lying just beneath the skin. When at rest each of these horns extends from the tongue into the neck, then dorsally and forward over the top of the skull, reaching even into the base of the beak (Fig. 236). As the tongue is extended the springy supporting hyoid coils are straightened out through the action of muscles, while the withdrawal of the tongue to its original position within the beak is accomplished by the elasticity of the hyoids which snap back into place like released watch-springs that have been temporarily straightened out.
The mammalian tongue, like that of reptiles, is made up of two parts. The anterior region, somewhat rough and covered with numerous small elevations (papillae) of various shapes, is separated from the posterior part, bumpy in appearance due to masses of lymphoid tissue (lingual tonsils), by a V-shaped groove, the sulcus terminahs (Fig. 233). In the mid-line, at the posteriorly directed apex of the sulcus, is a small invagination, the foramen caecum, the remains of the thyroglossal duct by which the embryonic thyroid gland communicated with the oral cavity.
The papillae on the anterior section of the tongue, usually associated with taste buds, are of four types, namely, filiform, fungiform, foliate, and vallate.
Filiform papillae are tiny threadlike or conical projections that are largely responsible for the velvety appearance of the surface of the tongue. They are not particularly associated with taste buds, although they serve to retain food solutions temporarily. In many mammals the filiform papillae become capped over with corneal material, taking on a mechanical rasp-like character, as shown by the tongues of cats and cattle, who use this device not only in eating but also as a hair brush.
Fungiform papillae (Fig. 237) are elevations from the surface of the mucous membrane that suggest the shape of a mushroom, hence their name. They are beset with taste buds and serve to bring these chemical receptors into contact with food solutions in the mouth cavity. Over the surface of the human tongue there may be as many as three or four hundred of these papillae, but they are always better developed in children than in adults. They are more numerous along the sides of the tongue than elsewhere, and have the appearance of small red spots.
The foliate papillae, which are usually located near the base of the tongue, are tiny ridges bearing taste buds. In man there are only three to eight of these ridges, but in rodents their number and size is greater.
The most elaborated of all the modifications for the display of taste buds are the vallate papillae. These resemble projecting knobs, surrounded by deep grooves, like the moat around a mediaeval castle, that serve to retain dissolved food substances. In the human fetus taste buds are distributed even over the tops of the knobs, but in adults they are confined to the sunken walls of the moats, where they are not only in direct contact with solutions to be tested, but are also protected from mechanical injury to which they would be liable at the surface.
Serous glands, called Von Ebner’s glands, open at the bottom of the moats and aid in keeping them filled with fluid.
Vallate papillae are usually arranged in rows at the back of the tongue. There are two rows in monotremes, moles, bats, hares, pigs, horses, and edentates; three rows in marsupials, squirrels, many insectivores, and apes; four rows in the monkeys, Macacus and Cercopithecus; and a single row, arranged in a V-shaped formation in front of the sulcus, in the dog and man. They are missing in guinea pigs and coneys.
The posterior part of the tongue is derived from the bases of the hyoid and first two branchial arches, while the anterior or secondary tongue arises embryonically from a median and a pair of lateral swellings. In the human embryo of about four weeks of age, the secondary tongue first appears as an elevation from the floor of the mouth cavity just anterior to the landmark of the ductus thyroglossus. This elevation, which is homologous with the “glandular field” of the amphibians, is called the tuberculum impar (Fig. 238). On either side of it are lateral lingual swellings from the inner surfaces of the two sides of the skeletal mandibular arch, which meet at this point. These swellings soon increase until they completely surround the tuberculum impar, eventually forming the bulk of the anterior part of the tongue. In somewhat similar fashion the copula, the region enveloping the basihyal skeletal part that forms the foundation of the primary tongue lying behind the thyroglossal duct, is augmented by additions from the neighboring hyoid and anterior branchial arches, to form the “root” of the tongue, the part lying in the pharyngeal cavity (Fig. 239).
The tongue of mammals serves many purposes and in consequence of the detachment of its anterior portion from skeletal elements is capable of great freedom of movement. It keeps the food between the teeth during the process of chewing, and starts it on its way when it is ready to be swallowed. It is also decidedly prehensile in many herbivores as already mentioned. Cows, for example, can grasp a tuft of grass with the tongue, to be sickled off against the lower incisors. It is a universal toothbrush giving point to the phrase “as clean as a hound’s tooth,” and it also serves as a currycomb for fur-bearers, while animals like cats and dogs that lap up liquids use it as a spoon. Finally, its dorsal surface is thickly beset with sense organs of touch and taste, which stand in readiness to receive the password of admittance from entering food. In the human female it measures about three and a half inches in length - when at rest.
Teeth are primarily devoted to the manipulation of food within the mouth cavity, to purposes of grasping, cutting or grinding, although in some instances they secondarily assume other functions, such as prehension of food, defence, offence, or even as aids in locomotion, as in the case of the walrus which uses its tusks in dragging its slippery body out of arctic water on to ice.
The extreme diversity of teeth, adapted to their many uses, affords the comparative anatomist much insight into the manner of life of different animals, while to the palaeontologist they are preserved tokens which, like hieroglyphics, aid in reconstructing the story of the long-vanished past.
Teeth are the first hard structures of the body to put in an appearance during vertebrate development, even before any part of the bony skeleton. Although they eventually come into intimate secondary relation with the skeleton, they are in reality derivatives of the stomodaeal region of the alimentary tract. Thus these integumentary derivatives, homologous with placoid scales, become morphologically, as well as physiologically, a part of the digestive system.
In structure a typical mammalian tooth (Fig. 240) consists of a crown which projects beyond the gums; roots that are embedded in a socket of the jaw; and the neck, which is the transitional region between the crown and roots. Inside the hollow tooth is the pulp cavity, harboring blood vessels and nerves that gain access through a passage-way usually remaining open at the base. So long as this opening is unobstructed the tooth can continue to grow, as gnawing teeth of rodents do, by means of inside additions of tooth material. In most of the teeth of higher vertebrates, however, the opening of the pulp cavity becomes so constricted that after a certain size is attained growth ceases and, as the tooth wears away by attrition on the outside, there is no restoration.
The solid part of the tooth is three-fold in character. The bulk of it is dentine, or “ivory,” a tissue denser than bone but, like it, permeated by tiny radiating canals, due to the fact that the dentine material is secreted around the branches of embryonic cells, the odontoblasts.
Over the crown, wherever exposed to wear, the dentine is usually protected by a layer of enamel, likewise penetrated by very minute canals in the lower forms but solid and prismatic in structure in higher vertebrates. Although not cellular in itself, enamel is the product of cellular activity and is the hardest, densest, most enduring part of the vertebrate body.
Outside of the dentine around the roots of the tooth, in those cases where the tooth is set in a socket, there is a bonelike substance, cement, that anchors the tooth firmly to the jaw. In ungulates the cement extends over the crown.
The composition of the dentine and enamel in the human tooth is given by Owen as follows:
In the development of teeth six steps may be recognized, namely, dental lamina, enamel organ, dental papilla, crown formation, root formation, eruption.
About the seventh week in the development of the human embryo, certain Malpighian cells of the epidermis along the edge of the jaws, where the future teeth are to be, start into accelerated activity, pushing down into the underlying dermal tissue in the form of the so-called dental lamina (Fig. 241). Along this lamina at intervals wherever a tooth is destined later to appear, groups of these Malpighian cells proliferate into spherical enamel organs, which later lose their connection with the dental lamina. Under each enamel organ a tubercle of mesenchymal cells constituting a dental papilla is formed, and presses the enamel organ into the form of a double-walled cup.
Crown formation follows through the interaction of cells of both papilla and enamel organ. The cells of the dental papilla are odontoblasts which secrete the dentine on their outer surfaces, thus producing the bulk of the tooth. The cells of the enamel organ next to the odontoblasts are ameloblasts which secrete a cap of enamel on the dentine. Meanwhile, capillaries and nerve endings invade the dental papilla and occupy the beginning of the pulp cavity.
Root formation occurs sometime later, beginning just prior to the eruption of the tooth. The cells of the edge of the invaginated cup have continued to extend deeper into the tissue of the jaw but these deepest cells do not secrete enamel. Consequently the root, formed entirely by odontoblasts, is composed solely of dentine. As the root elongates it pushes the completed crown through the enamel organ and more superficial tissues until the crown emerges and becomes almost entirely exposed. This process of eruption is known as “cutting the teeth.” Around the dentine of the embedded roots of each tooth is deposited, through the activity of neighboring mesenchymal cells from the derma, the cement tissue, which aids in fixing the tooth in the jaw.
Permanent teeth of man are formed in much the same manner as described above for the milk teeth. The enamel organ arises, however, from the original dental lamina, on the lingual side of the first tooth germ in the case of teeth which are to fill in places vacated by “first teeth.”
Like the placoid scales of elasmobranch fishes with which they are homologous, teeth are compound structures of diverse origin, arising from ectodermal ameloblasts, mesodermal odontoblasts, and mesenchymal cells.
The horny, rasplike “teeth” of the jawless cyclostomes are entirely ectodermal structures composed of cornified cells and not homologous with the teeth of other vertebrates.
Lower vertebrates generally have an indefinite number of teeth, but in mammals the number becomes definite and limited. A reduction in the number of teeth is a mark of evolutionary advance associated with terrestrial life, less food, more chewing, shorter jaws, and stronger muscles of mastication, whereas an increase in the number of teeth, such as occurs secondarily in dolphins and other toothed whales, may be regarded as a reversion to ancestral conditions in connection with aquatic life, more abundant food, and less need for mastication.
There are some toothless species representing every class of vertebrates. Among fishes may be mentioned the sturgeon, Acipenser, and the seahorses and pipefishes. Coregonus wartrnanni, a whitefish native to Lake Constance in Switzerland, is a toothless member of a large family of toothed fishes (Salmonidae), although this aberrant species has transient embryonic teeth.
Toads, and among urodeles Siren at least, are toothless, while frogs have no teeth on the lower jaw. Among reptiles the entire order of Chelonia, which includes turtles and tortoises, are without teeth, although in Chelonia and Trionyx a reminiscent dental lamina develops temporarily in the embryo, only to fade away as the homy beak becomes ascendant. Several extinct fossil reptiles, for example, Oudenodon, Baptanodon, and Pteranodon, are likewise known to have possessed beaks instead of teeth.
All modern birds are toothless. That this condition was not always the case, however, is shown by the presence of well-developed teeth in Archaeopteryx, and in the Cretaceous birds of Kansas, Ichthyornis and Hesperornis. Embryonic teeth, of which there is ordinarily no trace in birds, have been found in the tern, Sterna.
Among mammals, monotremes are without teeth, also the edentate Myrmecophaga and the pholidote Manis, and the large whales (Mystacoceti).
A curious instance of hereditary toothlessness in man is reported by Thadani from Hyderabad Sind in India, where there is an inbred community in which the males never have any teeth. They are called “Bhudas,” which means “toothless.” This abnormality is accompanied by baldness and extreme sensitivity to heat, and the peculiarity follows the well-known laws of Mendelian inheritance, being a recessive sex-linked character.
All of these widely different toothless mammals, however, furnish embryonic evidence that, with respect to this characteristic, they are degenerate descendants of ancestors with teeth.
Most of the lower vertebrates are polyphyodont, that is, they have a continuous succession of teeth throughout life. This is exemplified particularly in sharks and dogfishes, where the reserve “understudy” teeth may be seen arranged in diminishing rows behind the line in active service at the edge of the jaw. The continuous gradation over the margin of the jaw that separates the serried rows of elasmobranch teeth from the placoid scales of the skin, points unmistakably to a common plan of structure and accounts for vertebrate teeth as modified scales.
Mammals are typically diphyodont, that is, they have a replacement of so-called permanent teeth following the first temporary milk dentition, which allows the young to chew their food at a time when the jaws are too small to accommodate permanent teeth.
Certain marsupial embryos show traces of a still earlier dentition located in the arch between the milk teeth and the lips. Sometimes in exceptional cases mammals produce an additional partial replacement of the “permanent” teeth in late life, making a total of four possible successions, namely, prelacteal, lacteal, definitive, and post-definitive, all of which suggests that typical diphyodontism of mammals has been derived from the polyphyodont condition of lower forms. Bolk has pointed out that in diphyodont dentition the replacement comes from a different rudiment than that which gives rise to the first lacteal dentition, so that it is possible to have representatives of both dentitions present and on duty at the same time, whereas in polyphyodontism of the lower forms the succeeding tooth in each case arises from the same germ as its predecessor, thus preventing the intercalation of one active generation of teeth with those of another succeeding generation.
There is, moreover, a tendency among mammals toward a still further reduction to a monophyodont condition. Marsupials, for instance, retain all their milk teeth except the last premolars, while certain insectivores, like the moles, Scalopus and Condylura, never cut their permanent teeth. The toothed cetaceans (Odontocoeti), and some rodents, as well as the reptile Sphenodon, may also be described as monophyodont. Bats and guinea pigs have so far foreshortened the normal procedure of tooth succession as to shed their lacteal teeth in utero, coming into the world with their definitive teeth already established.
It is related of Mirabeau, the great orator of the French Revolution, that he was born with teeth already cut, which if true must have been hard on his nurse. Such an abnormality is said to occur as rarely as once out of 15,000 times.
Ordinarily the eruption of milk teeth in man is accomplished in about two years, although it is not unusual for the second milk-molars to come a half year later. The appearance of the permanent dentition begins with the eruption, during the sixth year, of the first molars, just posterior to the last milk teeth. The next year the milk teeth begin to drop out and ordinarily all of them are lost by the end of the twelfth year although an individual may carry some representatives of the lacteal dentition until much later in life. As rapidly as the milk teeth are lost, permanent ones take their places. After the completion of this gradual replacement, two additional molar teeth erupt behind each sixth-year molar, with the last permanent tooth, “wisdom tooth,” being fully formed by the twentieth year in most cases.
Milk teeth differ from permanent teeth by their smaller size, whiter color, and by their shape, being more constricted in the neck region and having a greater spread of roots in the case of the back teeth.
While the teeth in fishes and other aquatic animals occur attached to various skeletal foundations within the mouth cavity, such as the vomer, palatine, pterygoid, parasphenoid, and even on the tongue, on the hyoid and gill arches, in reptiles and mammals they are usually confined to the jaws, although in some snakes, and in Sphenodon, they occur also in the roof of the mouth on the vomer and palatine bones.
Teeth of the upper jaw are interspaced with reference to those of the lower jaw. In man the large median upper incisors bite against not only the median but also the lateral incisors of the lower jaw, and every other tooth of the upper jaw, except the last molars, bites against the corresponding tooth of the lower jaw and also the tooth behind it.
The manner in which teeth are attached to their skeletal support is dependent upon the degree to which the roots are developed.
The simplest type of attachment, termed acrodont (Fig. 242), occurs in teeth essentially without roots that are held to the edge of the jaw or other skeletal foundation either by fibrous membrane, or ankylosed directly to the bone in shallow pits. Such teeth, which are broken off easily, are polyphyodont. In some cases they are hinged on by a ligamentous base and may be folded down when not in use, as in the pike and hake among fishes, as well as in many kinds of snakes. Fishes as well as amphibians are generally acrodont.
An improvement over the acrodont method is seen in certain urodeles (e.g. Necturus) and lizards, where not only the base but one side of the tooth is involved in attachment to a shelflike ledge along the inner margin of the jaw (Figs. 242 and 243). By this method, which is called pleurodont, the blood and nerve supply enters at the side, as in acrodont teeth, instead of at the tip of the root.
The highest and most efficient type of tooth has well-developed roots set in bony sockets in the jaw, a method of attachment known as thecodont (Fig. 242), by which the capillaries and nerves enter the pulp cavity through the open tips of the hollow roots.
Some reptiles are thecodont, alligators and crocodiles particularly, but this type of tooth attachment is more characteristic of mammals, in some of which the teeth have progressed much beyond the primitive grasping function, and consequently require a stronger anchorage than is afforded by either the acrodont or pleurodont methods.
The incisor teeth of gnawing rodents are so deeply set in bony sockets of the jaws that they become very effective tools, as for example the incisors of the gopher Geomys (Fig. 244). The beaver Castor, in its engineering operations, can cut down large trees with such teeth.
Various types of movement for teeth set in jaws are made possible by the muscles of mastication. The commonest type is vertical, or orthal, movement, which consists in lifting up the lower jaw. Just as in a nutcracker, the farther back toward the angle of the jaw the work is done, the more powerful is the effect.
In carnivores the back teeth cut past each other like the blades of a pair of scissors. Ungulates which chew the cud with a sidewise motion have a lateral method of jaw movement, while horses, elephants, rodents, and some other herbivorous animals practice a “fore and aft” movement. In all of these movements the effective use of the jaw involves the teeth on one side at a time, those of the opposite side being temporarily not in contact.
Snakes with their sharp backward-projecting prehensile teeth use the fore and aft movement to advantage in relentlessly passing along struggling prey down the throat. In fact it works so automatically that a snake finds it difficult to eject a mouthful, once started in the proral-palinal mill.
In the higher vertebrates still other modifications in the movement of the jaws may be noted. Dr. Hooton observes that “with the shortening of the canines the human stock developed certain rotary movements of mastication which may be observed in any gum-chewing stenographer.”
According to their degree of differentiation, teeth are described as homodont and heterodont. Teeth when practically all alike are called homodont, but if they are differentiated to serve a variety of uses, such as gripping, tearing, cutting, or crushing, they are known as heterodont.
The teeth of primitive water-dwelling vertebrates are commonly homodont, since aquatic animals do not chew. They are usually pointed or cone-shaped and adapted to serve as prehensile organs. Ordinary vertebrates with homodont teeth gulp their food whole.
In evolutionary history, heterodontism arose along with experimenting upon a variety of foods and with the consequent occasion for chewing. The mammal-like therapsid reptiles and the mammals themselves are heterodont. The back teeth near the hinges of the jaws where the leverage is greatest become modified into grinding premolars and crushing molars, or “cheek teeth,” while the front teeth, notably in the case of rodents, become specialized into cutting chisels, or incisors, to divide the food into morsels of convenient size for the grinding mill of the back teeth. Probably the most ancestral and least changed of all heterodont teeth are the cone-shaped canines, between the incisors and premolars, which resemble the pointed grasping teeth of the homodont type. On either side of the canines, as a point of departure, modification has taken place progressively and in divergent fashion, as indicated by the arrows (Fig. 245), on the one hand toward the flattened, more chisel-like type of the incisors, and on the other, toward that of the flat-topped premolars and molars.
Homodontism in the dolphin and other toothed whales is shown to be secondary by the fact that a fossil ancestral whale, Zeuglodon, was heterodont.
The heterodontism of carnivores (Fig. 246) is characteristically different from that of herbivores. In the former case the sharp edges of certain grinders fit past each other like shears, for cutting up animal food, the grasping canines are prominent, and the back molars tend to become degenerate. It is the fourth premolars on the upper jaws and the first molars on the lower jaws which have developed this special tearing or shearing ability, for which reason they are called carnassial teeth. In the herbivore type of heterodont dentition the more anterior cheek teeth show degeneration, the canines being suppressed, while the posterior grinders near the hinge of the jaw become flattened and enlarged so as to crush seeds, fruits, nuts, and herbage of all sorts successfully.
The molars of ruminants present a flat grinding surface further diversified by crescentic ridges of projecting enamel, alternating with softer dentine. Since the dentine wears away more rapidly than the enamel ridges, the enamel is constantly kept with sharp edges, and at the same time a rasp-like abrasive surface on the grinding teeth is maintained. Such crescentic-surfaced teeth are said to be selenodont (Fig. 247). Similar enamel ridges are present on the molars of elephants, the arrangement of which in transverse lines instead of in crescents makes a washboardlike pattern, described as lophodont (Fig. 248), that is particularly effective in connection with the palinal or from behind forward movement of the jaws.
In man and some other mammals, the grinding surface of the molars is raised slightly into separate rounded tubercles and, being entirely covered with enamel, wears away more evenly. This is described as the bunodont type of teeth.
It is illuminating to know that some of the ancestral elephants, Palaeomastodon for example, were bunodont, while their more specialized descendants of today have become lophodont.
Finally, to add two more “donts” to this descriptive vocabulary of the teeth, the term brachydont applies to teeth with short crowns and comparatively long roots, as in man, while the term hypsodont characterizes teeth with short open roots and long crowns, such as are found in the dentition of the horse, in the tusks (incisors) of elephants, and the canines of boars.
In the case of different species that have heterodont teeth, it is useful to express the degree of their diversity in some convenient and compact form. This is accomplished by means of dental formulae. For example, the permanent dentition of man may be expressed as follows: (22.214.171.124)/(126.96.36.199) in which the figures above the horizontal line indicate in order from left to. right the number of incisors, canines, premolars, and molars on the right side of the upper jaw, while the figures below the line stand for the corresponding teeth in the lower jaw. It is unnecessary of course to indicate the teeth on the left side, which are like those on the right side except in reverse order.
The short-tailed monkeys (Catarrhini) of the Old World have the same dental formula as man, but the long-tailed monkeys (Platyrhini) of the New World have an additional premolar all around, making the formula (188.8.131.52)/(184.108.40.206) with a total of thirty-six.
Some other dental formulae are as follows:
In herbivores the canine teeth are missing or much reduced, leaving a toothless space, the diastema (Fig. 244), between the incisors and the premolars. The canines are relatively so small in the horse that a practical diastema exists, furnishing the space where the bits of the bridle are held.
Origin of the Molars
There are at least two theories to account for the origin of the molar teeth in mammals.
First, the concrescence theory of Rose and others assumes that they are the products of the fusion of separate primitive cone-shaped teeth. The posterior teeth in the jaw of Sphenodon offer evidence in support of this point of view.
The other and more widely accepted explanation is the differentiation theory of Cope and Osborn, which postulates the budding out and growth of additional contact surfaces, or cusps, upon the crown of an originally conical tooth (Fig. 249). This theory is based largely upon evidence presented by the ancestral teeth of fossil mammals. It is quite possible that both theories will be of use, since they are not mutually exclusive, in reaching a satisfactory conclusion in the matter.
The addition of two such cusps gives rise to the tritubercular tooth which is the typical molar of mammals generally from the earliest representatives down to Eocene times. Even today the mole Chrysochloris and certain other insectivores, as well as the opossum Didelphys, and some lemurs, exhibit this ancestral tritubercular type, which is well adapted to the business of crushing insects.
The three cusps of a tritubercular molar are arranged in the form of a triangle. Molars of the lower jaw have a more lateral cusp, the protoconid, medial to which are the two secondary cusps, the paraconid in the anterior position and the more posterior metaconid. The corresponding cusps on molars of the upper jaw are indicated by the termination - us. Thus in a molar of the upper jaw there are a medial protoconus and two more lateral cusps, an anterior paraconus and a posterior metaconus. This means that the orientation of the triangle of cusps on the upper jaw is the mirror image of that on the lower jaw. The seemingly elaborate terminology here employed is indispensable to the student who would make a study of the story of mammalian teeth. The addition of extra cusps on more highly developed molars may bring the total number to a maximum of six. Molars of the lower jaw, for example, elongate somewhat through the development of a posterior extension, the talonid or “heel,” which may bear two or three extra cusps. Because the teeth of the upper jaw alternate with those of the lower jaw in most cases, the protoconus usually strikes against the talonid of the corresponding lower-jaw molar.
Sometimes a pair of teeth develop excessively, forming tusks. These may be either incisors or canines and are more likely to appear in the male than in the female, although both sexes of elephants and walruses have tusks.
The largest known tooth is the tusk of an extinct mammoth, Archidiskodon, that is in the American Museum of Natural History in New York City. It weighs over 250 pounds and is more than sixteen feet in length.
The wild boar with tusks formed from modified canines of the lower jaw, strikes upward, while the male “dugong,” Halicore, or sea-cow of the Red Sea, makes the effective blow from above downward with tusks evolved from the upper incisors. In both sexes of Phacochoerus, the wart hog of Africa, there are four upward curving tusks, which are the transformed canines of both jaws, those of the upper jaw bending sharply to pierce the upper lip.
In general, tusks, as well as the prominent cutting incisors of rodents, retain at their base a large opening into the pulp cavity, thus insuring an abundant blood supply and consequent continued growth to compensate for the wearing away of the crown to which these exposed teeth are subjected. Such teeth in a way may be likened to angora hair in their manner of continuous growth.
The male narwhal, Adonodon has lost all its adult teeth except an upper left one which is prolonged enormously into a formidable twisted pikestaff that may reach seven to nine feet in length. The saw-fish, Pristis, which is not a mammal but a selachian, carries a similar weapon in the form of an elongated snout, or rostrum, with laterally projecting teeth along its sides.
In rodents the chisel-like incisors are faced with enamel only on the anterior surface. Because the enamel is much harder than the dentine that is posterior to it, these incisors wear away more rapidly behind than in front, constantly leaving sharp cutting beveled edges of enamel. When a rodent is so unfortunate as to lose an incisor of either jaw, leaving the incisor of the other jaw with no tooth to wear against, the animal usually meets eventual death by starvation because the surviving tooth, unhindered in growth, often reaches so great a length that the mouth can no longer be properly closed and feeding becomes impossible.
Among poisonous snakes a pair of anterior teeth may develop into fangs, which are teeth that are either grooved or hollow. Whenever a fang is struck into another animal the secretion of the poison gland at the base of the fang is pressed out through the hollow or groove into the wound (Fig. 230).
A so-called egg tooth, composed largely of dentine, is present as a transitory structure in the embryos of snakes and lizards which are imprisoned within an eggshell. It is situated in a median position and projects forward at the tip of the upper jaw. The young reptile uses it like a can-opener to hatch itself out of the imprisoning shell. According to Rose a pair of egg teeth are present at first in the embryo of the viper, Vipera, but only one becomes developed sufficiently to be of service, and this is shed soon after hatching.
There is a corneal egg “tooth” of horny texture on the tip of the beak of many unhatched birds. Although not homologous with the egg tooth of sriakes and lizards, it nevertheless serves the same purpose. It may sometimes be seen still adhering to the tip of the beak of young chicks which have just hatched into the world (Fig. 250b). A similar horny temporary emergency tool is present in Sphenodon, the crocodiles, and turtles, as well as in the monotremes, which are the only mammals that hatch out of an eggshell.
The Trend of Human Teeth
The teeth of ancient man show certain differences from those of man today, which possibly give some suggestion as to the direction of the future evolution of human dentition. The jaws in which the teeth are set are becoming shorter and less prognathous, with the result that the teeth of modern man are more crowded and less regular in eruption. Also decay, or caries, is more common in the teeth of modern civilized man than in the teeth of his prehistoric ancestors, where it was practically unknown. Wiedersheim reports on evidences of decay in teeth after an examination of a large number of skulls from various extensive museum collections, as follows: Eskimos, 2.5 per cent; Indians, 3-10 per cent; Malays, 3-20 per cent; Chinese, 40 per cent; Europeans, 80-100 per cent.
In primitive man the upper incisors came into opposition, edge to edge, with the lower incisors, and were frequently worn flat in consequence, while in modern man there is a tendency for them to shut past each other like the blades of a pair of shears, and thus to maintain a cutting edge.
The “wisdom teeth,” or the third molars, so-named by Hippocrates, the Father of Medicine, are apparently doomed teeth. They are the last to appear and the first to go. Frequently they remain uncut, or do not develop a grinding surface. In prehistoric man, however, they were plainly in evidence, and they are unusually well developed in negroes, mongols, and aboriginal Australians.
The upper lateral incisors and the second molars also show evidences of being degenerate structures, failing to appear in a considerable number of cases.
Davenport writes of human teeth as follows: “At birth the front teeth are always formed with enamel and dentine and so the trouble that is associated with these marvelous organs begins... The teeth are cut in discomfort, they decay and treatment brings pain, they are pulled out with pain... Let us be happy that we have not so many teeth as the sharks.”