The Main Skeletal Axis

Vertebrates are bilaterally symmetrical animals with the long dimension of the body stiffened by a skeletal axis, usually in the form of a backbone lying between two tubes that also run lengthwise the body.

The digestive tube lies below the axis. It is ordinarily much longer than the body itself, and consequently is more or less coiled. The neural tube extends just above the axis and is relatively short and straight like the backbone which encases it. Thus, the skeletal axis is in intimate and fundamental relations with both the main nervous and the digestive systems, having evolved primarily to meet the mechanical necessities arising from a locomotor bilateral type of symmetry.

The Parts of a Vertebra

The skeletal axis is for the most part composed of separate bony elements, or vertebrae, of which there are 32 or 33 in man, so fashioned as to lend the entire backbone a certain degree of flexibility without sacrificing the stiffening quality for which the “backbone” stands.

Most vertebrae consist of a main body, or centrum, bearing one or two arches and one or more pairs of processes (Fig. 426). The centrum first appears as a ring of tissue which develops around the notochord and then fills in to become a disc or cylindrical mass. Fused with the dorsal side of the centrum is a neural arch enclosing a neural canal through which the delicate spinal cord safely threads its way. Usually a neural spine extends from the mid-dorsal part of the arch into the dorsal septum. In the caudal region of many vertebrates there is, in addition, a similar set of structures on the ventral side of each vertebra. They are called haemal arch, haemal canal, and haemal spine because through the canal run the caudal artery and vein (haema, blood).

Projecting in various directions from the centrum and the base of the neural arch are several outgrowths, or processes, which offer convenient surfaces either for the attachment of muscles, or for frictional contact of one vertebra upon another in a sliding joint. On the arch are zygapophyses (zyg, yoke, joined; apophysis, process) so called because they bear the surfaces for articulation between vertebrae. There are usually two pairs of these processes, anteriorly projecting prezygapophyses and posteriorly projecting postzygapophyses. The articular surfaces are on the dorsal sides of the anterior processes and on the ventral sides of the posterior ones (Fig. 427). The arrangement of the articular surfaces is “onward and upward,” that is, the “onward” or anterior process has its articular surface on the “upward” or dorsal side. Thus it is possible to determine the anterior face of an isolated vertebra.

Extending laterally from the sides of the vertebra are transverse processes which are primarily associated with the ribs. Originally on the arch, but occasionally shifting over onto the centrum, is a pair of laterally projecting diapophyses which articulate with the dorsal heads of Y-shaped, dorsal ribs. On the centrum is another pair of these transverse processes, the parapophyses, which articulate with the ventral heads of these Y-shaped ribs. Frequently one or the other of these pairs of transverse processes is weakly developed or absent.

Movement of skeletal parts depends upon the presence of joints with their equipment of muscles, blood vessels, and nerves. In those regions of the backbone, therefore, where movement is most needed, as for instance in the neck, the various articular processes of the vertebrae are found to be most elaborated. Where rigidity and a minimum, or the entire absence, of movement between the vertebrae is desirable, as for example in the sacral region, all processes are much reduced.

The parts which constitute a typical vertebra undergo the widest variation, not only in the different species of vertebrates but even in the different vertebrae making up the backbone of any individual.

A generalized vertebra

Vertebrae form the chief mark that brands an animal as a vertebrate, and the phylogeny of these structures has never been successfully traced back to any invertebrate source. Such a new structural invention that has not been led up to by a series of gradual modifications in ancestral forms the biologist hails as a neomorph.

The tenth, eleventh, and twelfth thoracic vertebrae shown from the right side

Within the vertebrate class the steps by which the vertebrae have taken form have been traced in considerable detail, both by the embryologist and by the palaeontologist.

The Notochord and Its Sheaths

The embryonic formation of the vertebrae is preceded in every backboned animal by a temporary skeletal axis called the notochord. The position of this temporary axis, which lies lengthwise between the neural cord and the digestive tube, is the same as that of the centra of the vertebrae which replace it later.

Differing from other skeletal tissue such as bone and cartilage in which intercellular material may be excessively developed, the cells of the notochord are at first large, thin-walled, and closely packed together. They are enclosed in a thin tough sheath of connective tissue, the elastica externa, mesenchymal in origin. Notochordal cells are devoid of both nerve supply and blood vessels. They are, therefore, nourished vicariously by diffusion from outside cells.

As the notochord grows older the cells within the sheath change. The marginal ones next the primary sheath itself form a definite layer of peripheral notochordal cells, while those in the central part tend to become vacuolated, losing their outlines and fusing together. The peripheral cells are then transformed into a thick, fibrous secondary sheath inside of the primary one, with the result that the notochord as a whole at this stage might be described as a somewhat rigid cylinder tapering at either end, made up of the remains of closely packed turgid tissue surrounded by a dense double sheath.

In amphioxus, cyclostomes, dipnoans, and certain other fishes, the vertebral column evolves no further than the notochordal stage, except for the presence in some instances of vertebral arch elements laced by connective tissue to the notochord. In other vertebrates, after the notochordal stage is passed, an axial skeleton of another and more complicated sort is established upon its ruins. The notochord, therefore, is the oldest part of the vertebrate skeleton, antedating cartilage and bone not only during the development of the individual but also in the long phylogeny of vertebrate types.

The Formation of Vertebral Arches

Although support, in the form of a stiff rod through the long dimension of the bilaterally symmetrical animal, is the earliest function that any skeletal tissue performs in a vertebrate, the supplementary function of protection, particularly of the precious nerve cord lying just above the notochord, begins to become manifest very early.

Along the notochord and on either side of the nerve cord of the primitive lamprey eel, Petromyzon, pairs of small cartilage struts appear, distinctly foreshadowing the future neural arches of the higher vertebrate types (Fig. 428). At first these pairs of strutlike plates do not meet and fuse together to form complete archcs, but in later evolutionary stages this does happen, usually through the insertion of a keystone of cartilage or bone that becomes the dorsal neural spine (Fig. 426).

Diagram of a piece of the notochord of a lamprey eel. Ball-and-socket vertebrae of an adult alligator

The original independence of the neural arch from the centrum is demonstrated not only by the fact that these two parts develop separately but also for the reason that in young mammals, as well as in adult alligators and turtles, distinct sutures mav be seen between arch and centrum (Fig. 429). The same general story is told upside down by the haemal arches (Fig. 426).

The Embryonic Development of Vertebrae

During the consideration of vertebrate development it was noted that segmentally arranged sclerotomes are formed from mesenchyme given off by the medial sides of the epimeres. These sclerotomes are arranged as two rows of blocks, one row running along each side of the notochord (Fig. 430). Each sclerotome differentiates into a more compact posterior half and a less compact anterior half. Ultimately the two parts separate and the posterior half of one screlotome joins with the anterior half of the next sclerotome to form a vertebra. In this manner each vertebra comes to occupy an intersegmental position with the myocomma attached to the middle of the centrum. As the myotonies retain their original segmental positions, each becomes associated with two vertebrae. This relationship is a necessity if the myotomes are to bend the vertebral column.

Diagrammatic frontal sections at level of notochord, showing differentiation of sclerotomes of left side only

Two centers of chondrification (cartilage formation) appear in each half-sclerotome. One of these is just above the notochord, the other just below. The cartilages and associated mesenchyme cells of the more compact posterior half-sclerotome are known as basals, a dorsal basidorsal and a ventral basiventral (Fig. 431). The corresponding regions of the anterior half-sclerotome, or the interbasals, are an interdorsal and an interventral. Thus there are four regions, known as arcualia, in each sclerotome (four pairs in each segment).

The basidorsals and the basiventrals, are responsible for the anterior half of the vertebra

The four pairs of arcualia may spread and fuse in various ways within different types of vertebrates. Some of them may even vanish entirely in the course of the subsequent formation of the vertebrae. In most vertebrates the basidorsals grow upward to form the neural arch while the basiventrals grow downward to form the haemal arch. The development of the centrum region varies greatly in different vertebrate groups.

In cartilaginous fishes the centrum is formed largely by cells from the two pairs of basal arcualia (basidorsals and basiventrals) which break through the elastica externa. Within the fibrous secondary sheath they form a ring of tissue which soon begins to chondrify. The cartilaginous ring, the beginning of the definitive centrum, continues to grow by the addition of new material onto its anterior and posterior surfaces and by filling in toward the center. This centripetal growth slowly constricts and squeezes out the enclosed notochord. Ultimately, in the adult, the circular band of cartilage is thickest in the mid-part of the vertebra so that the notochord is nearly eliminated at that point but only slightly constricted at the intervertebral level (Fig. 432a). The cartilaginous centrum as a whole, therefore, has a concavity on each end occupied by notochordal tissue. Such a biconcave centrum is said to be amphicoelous (amphi, both; coel, cavity). These centra, which develop within the tissue of the notochord, are known as chordal centra in contrast with the perichordal centra of bony fishes and tetrapods which develop from mesenchyme remaining outside the elastica externa. Even in the cartilaginous fishes some perichordal cartilage is eventually added to the original chordal centrum.

The vertebral column of fishes

In the bony fishes the perichordal centra are usually laid down directly as bone without any intermediate cartilaginous stage as in tetrapods. Perhaps the most interesting feature of the vertebral column of these animals, however, is the occurrence of two centra per segment in most of the tail region of Amia and a number of other genera (Fig. 433). The more anterior of the central discs, or the hypocentrum, is formed in part at least from an enlarged basiventral while the more posterior pleurocentrum owes its development to the interdorsal.

Caudal vertebrae of Amia calva, left side view

Apparently the most primitive of the amphibians, some of the stegocephalians, had both hypocentral and pleurocentral elements throughout the entire length of the vertebral column. From the plan found in these animals there seem to have been two evolutionary lines, one in which the hypocentrum is emphasized, the other the pleurocentrum. In modem amphibians the centrum is largely a hypocentrum, while in tetrapods it is mainly a pleurocentrum (Fig. 434).

Diagram illustrating supposed divergence in development of vertebral elements leading from a primitive ancestral tetrapod

The mammalian vertebra, therefore, develops from three of the four pairs of arcualia. Neural and haemal arches arise from basidorsals and basiventrals, respectively, of one pair of sclerotomes, while the centrum is largely a derivative of the interdorsals of the next pair, the interventrals of which disappear (Fig. 431). Between the bony centra are intervertebral discs of fibrous cartilage formed from basiventrals which are not otherwise utilized throughout most of the trunk region. In the core of each of these discs, even in adults, may be found a persisting notochordal remnant called the nucleus pulposus (Fig. 435). If this “nucleus” could indulge in reminiscence what a story of evolution it could tell!

A long section through the centra of two vertebrae and an intervertebral disc in the center

The vertebrae of mammals differ from those of all other vertebrates in having epiphyses which develop as separate intervertebral bony discs one at each end of each centrum. Some time after birth they usually fuse with the main central mass.

Types of Articulation between Centra

All vertebrae articulate end to end. But the ends of vertebrae vary in shape. Hence the type of joint between vertebral centra differs in the various classes and, in some cases, even in different regions of the same animal. The five commonest types of articulation are: (1) amphicoelous; (2) procoelous; (3) opisthocoelous; (4) heterocoelous; and (5) amphiplatyan.

The amphicoelous type is found in most fishes, in primitive amphibians (stegocephalians) and primitive reptiles (cotylosaurs) as well as some living amphibians (e.g., Proteus and Necturus) and reptiles (Sphenodon, some lizards and, to a certain extent, turtles). As mentioned previously the centrum of such a vertebra is concave at both ends (Fig. 432). Owing to the fact that the actual contact is limited to the edges of the two cups, placed rim to rim, little freedom of movement is possible between the vertebrae held in place by considerable connective tissue, hence the limitation of this type mainly to water-dwellers.

In a procoelous vertebra the cavity on the anterior end {pro, before) is retained but the posterior end fills in and becomes convex (Fig. 429). There is, therefore, a simple type of ball-and-socket joint which allows considerable movement because of the reduced amount of connective tissue needed to lace together the vertebrae. Procoelous vertebrae occur in some anurans, chiefly frogs and toads, in the extinct pterosaurs, and in modern reptiles, including Lacertilia, Ophidia, and Crocodilia. The somewhat rare condition of a biconvex vertebra is found in the case of the sacral region of Bufo and Rana and the first tail-vertebra of Crocodilia.

The opisthocoelous condition, the reverse of the procoelous, has the concavity on the posterior end (opisthos, behind). These vertebrae are not characteristic of any major group but are found in widely separated species or Orders among all vertebrate Classes except cyclostomes. Examples are: Lepidosteus, the exception to the amphicoelous condition typical of fishes; some anurans; dinosaurs (cervical vertebrae only) ; penguins, parrots, and a few other carinate birds; and ungulate mammals (cervical only).

Heterocoelous vertebrae are the usual type in birds, especially in the cervical region (Fig. 436). These are also known as saddle-joint vertebrae because of the somewhat remote resemblance of the articulation to the situation of a rider in a saddle. The anterior end of the centrum is rounded off dorso-ventrally so that it appears convex in a section through it in the sagittal plane. Short processes extend anteriorly from the lateral parts of the centrum to give it a concave appearance in frontal sections. At the posterior end this condition is reversed. The elevations, on the dorsal and ventral parts of the centrum in this region, fit over the rounded part of the anterior end of the next centrum, while the rounded region of the posterior end fits between the elevations on the sides of the anterior end. Great freedom of movement between these vertebrae is permitted.

Amphiplatyan vertebrae (Fig. 427), flat on both ends (am phi, both; platy, flat), are typical of mammals.

Ventral view of one cervical vertebra of a swan, and a part of another, showing a saddle joint

History of the Vertebral Column

The total number of vertebrae in the backbone of an animal does not ordinarily increase with age and growth. On the contrary, as the result of fusion, the number frequently decreases in adult life. Frass reports that the tail vertebrae of one species of fossil ichthyosaurs varied from 129 to 160 according to the size and probable age of the specimen examined. There are usually more vertebrae in the lower fishes than in higher forms, and in general deep-sea fishes have a larger number than those inhabiting shallow waters. David Starr Jordan cites the curious fact that certain kinds of fishes living in the southern part of their range have more vertebrae than representatives of the same species in more northern waters.

Of all vertebrates pythons probably hold the record for the largest number of vertebrae, one having been reported with as many as 435.

While the vertebrae composing the backbone are easily referable to one structural plan, no two are exactly alike. The variations that have arisen are closely correlated with the diverse kinds of work which each vertebra, or group of vertebrae, has to do. Since it is desirable, for example, to have the head move in any direction without turning the entire body, the vertebrae of the neck which carry the head have developed joints that permit freer movement than is found elsewhere along the skeletal axis. The sacral vertebrae, on the contrary, whose function, particularly in bipedal animals, is to bear the weight of the body upon the legs, have entirely lost their movable joints and become fused together into a solid efficient unit of support (Fig. 437).

A diagram showing the part the fused sacral vertebrae play as a keystone of the arch that supports the weight of the body

The point of attachment for the legs, as well as the region where the ribs are present, serve as landmarks, dividing the vertebral column of man and the higher animals into five natural groups of vertebrae, namely, cervical, thoracic, lumbar, sacral, and caudal.

The forerunner of the vertebral column is the notochord. The only part of the axial skeleton present in lowest chordates, it persists as a well developed unconstricted structure in cyclostomes, dipnoans, cartilaginous ganoids, and a few other fishes in which no central rings develop about it.

Among the cyclostomes only the lamprey eels have vertebral elements. Throughout most of the trunk region each segment has two pairs of cartilaginous rods attached to the notochordal sheath and extending dorsally alongside the nerve cord (Fig. 428). These elements are probably the homologues of basidorsals and interdorsals. As the rods do not meet dorsal to the nerve cord, the neural canal is roofed over by connective tissue. In the posterior trunk region these structures become reduced and irregular. The absence of these parts in hagfishes is believed to be due to degeneration.

Among fishes the vertebrae posterior to the anal region develop haemal arches on the ventral side of the centra (Fig. 432b). The spinal column, therefore, may be divided into trunk vertebrae, essentially alike, which are anterior to the anus, and postanal caudal vertebrae, possessing the haemal arch and diminishing progressively in size toward the posterior end.

A slightly modified condition exists in the amphibians, with a smaller total number of vertebrae involved. A single cervical vertebra, providing for the beginnings of independent head movements, is inserted next the skull, while between the trunk and caudal vertebrae there is differentiated a single sacral vertebra (lacking in the limbless apodans) to which the hind legs are attached. The anchorage of the hind legs to this solitary inadequate sacral vertebra is one of the reasons why the amphibians are unable to “stand up” and bear their weight upon their hind legs. Amphibians are the first vertebrates with well developed zygapophyses.

Pelvic girdle of a frog, Rana

In adult frogs the caudal vertebrae are fused together into a long urostyle (Fig. 438), which acts as a counterpoise for the teetering body that is swung in the crotch formed by the pelvic girdle. Any jumping animal like a frog needs a shock-absorbing device much more than a creeping or walking animal does, in order to lessen the jar that is communicated to the brain when it lands. This is nicely provided for in the frog by the way in which the unsteady body is swung in the remarkable, iliac crotch of the pelvic girdle, and at the same time is prevented from jack-knifing together by the counter-poising urostyle and the muscles that hold it in place. The frog may not have much of a brain but that is all the more occasion for taking care of what there is of it and not shaking it up unnecessarily.

Among reptiles the differentiation of the vertebrae includes several advances. With the exception of snakes and footless lizards, living reptiles are characterized by having two sacral vertebrae for the attachment of the pelvic girdle and the support of the hind legs, while the trunk vertebrae usually become specialized at either end into cervical and lumbar vertebrae respectively, between which are the rib-bearing thoracic vertebrae. The lumbar vertebrae, being free from complications of ribs, permit a certain amount of twisting in the lumbar region of the vertebral column. This is more pronounced in agile carnivores than in less active herbivores. Not all reptiles, however, are equally diversified with regard to vertebrae. In snakes, for example, there is little differentiation of the column, all vertebrae being much alike and bearing paired ribs. In turtles only the cervical and caudal regions are flexible. The trunk, not differentiated into thoracics and lumbars, and sacral regions are immovably fused with plates along the center of the dorsal shell, or carapace (Fig. 439).

Cross section through the carapace of a turtle

The vertebrae of birds undergo great modification in connection with adaptation to flight. The cervical vertebrae undergo much differentiation, reaching the maximum number for any vertebrate. A flexible neck is a prime necessity for a bird which, lacking hands, must pick up all its food either with a prehensile beak or with the talons of its claws. Even an owl can turn its staring eyes in any direction because its apparently very short neck has so many joints. This cervical flexibility is also of importance because the thoracic vertebrae are more or less fused together. This condition, with accompanying modifications of the ribs and sternum, furnishes a stable attachment for the muscles of flight while at the same time keeping the weight of these structures at a minimum. Similarly, firm support for the legs is provided by fusion of the posterior thoracic, all lumbar, the two sacral, and the anterior caudal vertebrae into a single synsacrum to which the pelvic girdle is attached. As many as 23 vertebrae may contribute to the synsacrum. Beyond this region there are several separate caudal vertebrae followed usually by a pygostyie formed by the fusion of the last few caudals.

In mammals the cervical vertebrae typically number seven, whether the neck is functionally absent, as in the whales where the cervical vertebrae are all fused together, or conspicously present, as in the bizarre, long-necked giraffes.

There are four known exceptions to this rule of seven: the three-toed sloth, Bradypus, has nine cervical vertebrae; the ant-bear, Tamandua, eight; while the two-toed sloth, Choloepus, and the American sea cow, Trichechus, each has six.

The human atlas and axis, ventral view

As in reptiles and birds, the first two cervical vertebrae of mammals are further specialized into the so-called atlas and the axis (Fig. 440). The atlas, according to Vesalius, takes its name from human anatomy, since in man it “bears the weight of the world” upon its shoulders in the form of the head, after the fashion of its classical prototype. Two articular surfaces at the base of the skull in mammals, the occipital condyles, are in frictional contact with two corresponding surfaces on the atlas, thus forming a joint that allows for the nodding movements of the head. The atlas is virtually without the projecting neural spine of a typical vertebra, and, unlike the first vertebra of fishes and amphibians, has no centrum. The embryonic centrum of the atlas (interdorsal of the second trunk sclerotome) fuses onto the anterior end of the centrum of the axis, or second cervical vertebra, to form a large projection, the odontoid process, which extends forward toward the skull and rests on the floor of the atlas ring (Fig. 441). As a result the axis is really outfitted with two centra. The atlas would be open ventrally were it not for the fact that basiventral material, which ordinarily forms only intervertebral discs in the trunk region, is added to the other parts of the vertebra to form a complete bony ring.

Relation of odontoid process to atlas in man, as seen from the anterior side

The articulation between the skull and the atlas is the “yes” joint permitting human beings to nod approval, while the atlas-axis articulation is the “no” joint by means of which disapproval is indicated by shaking the head sidewise. The graceful up and down movements of the head of a playful dolphin involve the whole plunging body as well, for these stiff-necked creatures are unable to say “yes” in the orthodox cervical fashion. Modified cervical vertebrae occur also in the burrowing moles, which have the second, third, and fourth cervicals fused solidly together, to the decided advantage of these subterranean tunneling engineers.

A profile showing the vertebra prominens. Sacrum of a human fetus five months old

The neural spines of the cervical vertebrae in man are frequently more or less forked, a modification more apparent in modem civilized races than among either primitive races or man’s apelike cousins. The seventh cervical vertebra has the longest neural spine, notably so in females. It is unforked and projects backward, forming a protuberance under the skin at the base of the neck when the head is bowed forward. For this reason it is called the vertebra prominens (Fig. 442).

The human coccyx

In the human embryo there are present seven cervical, twelve thoracic, five lumbar, five sacral, and four, five, or sometimes as many as eight caudal vertebrae. In the adult the five sacral vertebrae, together with the embryonic sacral ribs, fuse to form a single sacrum (Fig. 443). Usually all the caudal vertebrae are joined together to make the coccyx, although it is not very exceptional for one or two of the most posterior caudal vertebrae to retain their independence (Fig. 444). Rarely the occurrence of tails several inches in length, containing degenerate caudal vertebrae, have been authentically reported in adult human beings.

The differentiation of vertebrae in the vertebrate series is visualized diagrammatically in Figure 445.

Diagram of differentiation of the vertebrae in representative vertebrates

The Entire Backbone

The units of the vertebral column when taken together are more differentiated at either end than in the middle. In the higher vertebrates particularly notable specialization of the cervical vertebrae is connected with head movement, while the frequent pronounced degeneration of the caudal vertebrae is apparently an expression of disuse.

The backbone taken as a whole has in general three uses, namely, support, protection of the nerve cord, and movement.

Skeleton of a rodent with its arched vertebral column from which various parts are suspended

The junction of support is what may be called the “backbone” function proper. The solid bony axis is arranged lengthwise the body because in this way the greatest number of parts can be conveniently accommodated with a sustaining anchorage. In tetrapods particularly the spine is an arch from which a variety of things are suspended (Fig. 446), whereas in bipedal animals like man, that tip up on end and poise a heavy head on top of the vertebral column, the function of support is more effectually accomplished because of certain adaptive curvatures in the backbone which make the column mechanically more capable of sustaining weight and at the same time more flexible. The human vertebral column has four dorso-ventral curvatures, namely (1) the cervical, concave dorsally; (2) the thoracic, concave ventrally; (3) the lumbar, concave dorsally and forming a long sweeping arch; and (4) the sacral, concave ventrally (Fig. 447). The lumbar curvature does not occur in other mammals. These curvatures, which are due more to modifications of the padlike fibrous cartilages between the separate vertebrae than to any modification in the shape of the centra that are stacked one upon the other, are less pronounced in infants and in primitive races than in adult civilized man. The human cervical curvature usually develops when the child begins to sit up while the lumbar appears when the child starts walking. The delayed appearance of the lumbar curvature, that gives the typical hollow back to a well-formed man, is particularly noticeable. Babies which lack it are flat-backed, like their remote quadrupedal ancestors, and are consequently awkward and uncertain om their feet.

A second function of the backbone as a unit is the protection of the nerve cord, which is an indispensable cable of great complexity, extremely delicate and liable to injury. It is not only ensheathed in its own envelopes, or meninges, but it is also surrounded by protective jackets of fluid and is furthermore encased within a bony conduit formed by the neural arches of the vertebrae. Between the neural arches are spinal nerve foramina, passageways for these nerves (Fig. 427). Even the backbone itself is overlaid with ligaments and buried with its valuable contents so that it is still further protected from outside injury by surrounding muscles and fatty tissues. Finally the whole internal mechanism is effectually sealed up within the tough, resistant, practically germ-proof skin.

Diagrams showing the difference in the curvature of the backbone between an infant and an adult

The third general function of the spinal column is movement, and while this is relatively slight between any two vertebrae, when taken all together it amounts to enough to be noticeably missed by anyone afflicted with a stiff neck or a lame back. There are said to be over 150 different articular surfaces along the length of the human backbone, which explains the reason why professional contortionists, and babies whose vertebrae are still in the making, are able to demonstrate so amazingly the possible flexibility of the vertebral column.

The pliability of the backbone is largely due to the springy compressible intervertebral discs which make up approximately one fourth of the entire length of the vertebral column. These discs are buffers, like the bumpers between the cars of a railroad train that take up the shock of impact. Without them the jolting effort of locomotion would affect the central nervous system quite like riding in a springless ox-cart instead of in a luxurious automobile. A traffic policeman who has been on his feet all day is a measurable bit shorter at night than in the morning because of the packing down of the compressible intervertebral discs of his spinal column. In old age these discs lose much of their elasticity and in second childhood it is no longer feasible to try to imitate a baby that delights in inserting its toes in its mouth.

Movement of the spine in turtles is confined to the flexible cervical and caudal regions, but in snakes it is uniformly possible along the entire length of the body, although the vertebrae are so firmly interlaced and held together by ligaments and muscles that these sinuous animals are unable to make abrupt angular bends.

The caudal part of the backbone exhibits a great variety of movements peculiar to different vertebrates and often quite unlike the movements of the rest of the backbone. Such movements serve a wide range of uses, for example, sculling locomotion in fishes, prehension in long-tailed monkeys, removal of annoying insects in cows and horses, and expression of the emotions in dogs.

Some lizards have a peculiar breaking-plane within the centra of many of the caudal vertebrae that enables them in an emergency to snap the tail off when harassed by a pursuing enemy. The part of the tail thus sacrificed continues for some time to jerk and bob about, thus tending to divert the attention of the pursuer, permitting the persecuted bob-tailed lizard to escape and grow a new tail in safety.