The Serial Homologies of the Vertebral Column.—In tracing the variations of form and relative size to which the centrum of a vertebráis subject, we may commence with the human coccyx; where the pieces, four or five in number, are reduced to their simplest condition. The terminal segment consists of a centrum only: it is a globular nodule of bone, rough posteriorly, but smooth anteriorly, where it forms the apex of the sacro-coccygeal concavity, in which are lodged the pelvic viscera. The two next segments are but little more complicated: two longitudinal ridges, with a shallow intervening groove, indicate some trace of neurapophyses, or laminae, and of a neural canal. The first segment, or that which articulates with the sacrum, is flattened anteriorly, and supports posteriorly two backwardly extending neurapophyses, unconnected by a neural spine, but surmounted by zygapophyses, or surfaces for articulation with the sacrum. Two short diapophyses, of semi-lunar form, project outwards, and somewhat upwards on either side. The human coccygeal vertebrae, then, whose office consists in narrowing by their forward curve the antero-posterior diameter of the inferior outlet of the pelvis, are reduced to little more than the central element; but in other animals, and more especially in those supplied with long tails, the number of their component parts, as well as the absolute number, is much increased. In the human embryo the rudimental tail is longer than in the adult, and the cartilaginous deposits indicate a greater number of caudal vertebras than are persistent in the adult. In the ferret there are sixteen segments, of elongated cylindrical form, somewhat expanded at their extremities, to afford firm and broad surfaces for articulation. Many are supplied with zygapophyses or articulating processes, with strong projecting diapophyses, and with a haemal arch to protect the long caudal artery. In birds, several of the terminal coccygeal bones coalesce into a single plough-share-shaped plate, smooth upon the sides, and compressed in the vertical direction. This coalescence of the inferior or terminal pieces of the coccyx is not uncommonly seen in the human subject. Specimens are numerous in which the coccyx consists of three segments : the terminal one composed of three centra, confluent, but somewhat constricted at the points of union; the middle one marked posteriorly by stunted neurapophyses, and united by cartilage to the first segment, which may be either articulated or firmly ossified to the extremity of the sacrum. The centra of five primitively distinct vertebrae unite about the sixteenth or seventeenth year to form the solid mass, termed the sacrum. It is with some surprise that we read the diffident way in which M. H. Cloquet expresses himself, in the earlier editions of his Traite d'Ana-tomie, upon this now well-established fact. "It is in consequence of this mode of development that many anatomists have considered the sacrum as composed of the re-union of five vertebrae placed one above the other."*
But with the centra there have coalesced the neurapophyses and the neural spines, the zygapophyses, and the mass of bone which forms the outer boundary of the sacral foramina.
The special homologies of the iliac, pubic, and ischiatic bones will be mentioned in the description of the haemal arch. The diapophyses posteriorly, and the stunted pleurapophyses anteriorly, which extend transversely outwards from the centra of the sacral vertebrae, become confluent about the seventeenth year, and nothing but the sacral foramina remain to represent the inter-diapophysial spaces behind, and inter-pleurapophysial spaces in front.
These points may be readily understood by the examination of the foetal pelvis, where the different vertebral segments remain distinct. In birds, whose wings require a fixed point of support whilst the body is elevated in flight, the confluence of the trunk vertebrae occupies a much larger region than in man. It extends from the pelvic through the lumbar region, and involves even the lower dorsal vertebrae with the pleurapophyses, or ribs.
The centra of the lumbar and of the dorsal vertebrae agree in all essential particulars; the former, broader than the latter, are inseparably united to their pleurapophyses, which form one mass with the diapophyses. The prominent convex appearance which all these vertebras present when viewed anteriorly, and contrasted with the cervical centra, is, in great measure, due to the absence of parapo-physes, or the anterior elements of the transverse processes. In birds, the anterior surfaces of these centra present a prominent exogenous haemal spine, which divides the haemal surface of the vertebral column into two longitudinal halves, occupied by the powerful longus colli muscles. In the tortoise the centra of the succession of vertebrae supporting the shell, or carapace, are all united into a single long bone, as is the case with the sacral and lumbar vertebrae in the skeleton of the bird.
* Traite d'Anatomie descriptive, p. 189, 1831.
In many of the Mammalia the sacral vertebrae remain permanently distinct; such is the case in the beaver (Castor fiber), a creature that employs not only its long and powerful tail, but even the whole posterior half of the trunk, as an organ of propulsion through the water. In the chelonian reptiles, such as the turtles, the sacrum is usually composed of three vertebrae only; in ophidians, such as the snakes, and in fishes, it is impossible to divide the trunk into the regions familiar to us in the skeleton of man: the centra of all the vertebrae resemble one another, and remain permanently distinct. In the former we recognize a succession of the ball-and-socket articulation; in the latter, a series of bi-concave centra, united by means of an intervening jelly-like semifluid substance.
The centra of the cervical vertebras, compressed from above downwards, have a flattened aspect anteriorly, in consequence of the presence of parapo-physes, which spring from the sides of the centrum. The inferior margin is prolonged downwards, as an exogenous haemal spine, or tongue, over the centrum of the succeeding vertebra, in most of the cervical segments in man. The parapophysis, which exists in no other human trunk-vertebras, is strongest in the upper cervical region, and gradually becomes thinner and narrower as it approaches the thorax. In the seventh cervical vertebra it is often a mere line of bone ; occasionally it is absent. Very frequently the diapophysis and parapophysis of the seventh cervical vertebra are surmounted by a long pleurapophysis, which has been correctly recognised as a cervical rib. It does not remain distinct after the time when the vertebral segments are completed.
In consequence of man's erect posture, the centra of the four cranial vertebras are brought from the vertical to the horizontal position, at a right angle to the spinal column, that the orbits, as well as the aperture of the mouth, may be directed forwards. In the foetus, at the full time, these centra may be clearly made out; the basi-occipital is united by cartilage to the basi-sphenoid, and the basi-sphenoid to the pre-sphenoid. The latter sends down an exogenous spine, which articulates with the vomer, or the centrum of the fourth cranial vertebra.
As in the pelvis, the hasmal surface of the vertebrae is smooth and concave, to receive the blood-vessels and the viscera; so in the cranium the neural surfaces of the basi-occipital, the basi-sphenoid, and the pre-sphenoid, are excavated and smooth, to support the under surface of the encephalon. The vomer is a narrow, compressed bone, which requires a particular description.
The basi-occipital in the cod presents posteriorly the same concave surface which is seen in the vertebrae of the trunk ; so also in reptiles the ball-and-socket joint of the spinal column is repeated in the articulation of a posteriorly-convex basi-occipital with the concave surface of the atlas.
The transition from the single convex occipital articulating surface in reptiles, to the double condyles in the cranium of man, is seen in the head of the crocodile and turtle. The single convex occipital condyle of the former is formed almost entirely of the centrum; two other processes from the ex-occipitals complete the smooth rounded surface on either side. In the turtle a much larger portion of the occipital condyle is formed by the exoccipitals. In man the basi-occipital is bent forwards ; the surface, which should articulate with the atlas, is thin, and forms the anterior margin of the foramen magnum. An oblong articulating convex surface, developed upon each of the exoccipitals, is received into a corresponding concavity in the lateral masses of the atlas, whose compressed centrum is unconnected with the cranium save by ligament.