In the higher vertebrates pads of fibrocartilage—the intervertebral discs—are commonly interposed between successive centra, these or parts of them often ossify, especially in the trunk and tail, and are then known as inter centra.
| 1. neural spine. | 6. prezygapophysis. |
| 2. transverse process. | 7. postzygapophysis. |
| 3. hypapophysis. | 8. vertebrarterial canal. |
| 4. convex anterior face of the centrum. | 9. neural canal. |
| 5. concave posterior face of the centrum. | 10. inferior lamella of transverse process. |
The vertebrae of the higher forms can generally be arranged in the following five groups, each marked by certain special characteristics:
1. The cervical or neck vertebrae. These connect the skull with the thorax, and are characterised by relatively great freedom of movement. They often bear small ribs, but are distinguished from the succeeding thoracic vertebrae by the fact that their ribs do not reach the sternum. The first cervical vertebra which articulates with the skull is called the atlas, but a study of the nerve exits shows that the first vertebra is not serially homologous throughout the Ichthyopsida, so that it is best to reserve the term atlas for the first vertebra in Sauropsida and Mammalia.
2. The thoracic vertebrae (often called dorsal) bear movably articulated ribs which unite ventrally with the sternum.
3. The lumbar vertebrae are generally large, and are often more movable on one another than are the thoracic vertebrae. They bear no ribs.
4. The sacral vertebrae are characterised by the fact that they are firmly fused together, and are united with the pelvic girdle by means of their transverse processes and rudimentary ribs.
5. The caudal or tail vertebrae succeed the sacral. The anterior ones are often fused with one another and with the sacrals, but they differ from true sacral vertebrae in that there are no rudimentary ribs between their transverse processes and the pelvic girdle. They often bear V-shaped chevron bones.
In fish and snakes the vertebral column is divisible into only two regions, an anterior trunk region, whose vertebrae bear ribs, and a posterior tail region, whose vertebrae are ribless.
2. The Skull.
Before giving a general account of the adult skull it will be well to briefly describe its development.
General development of the Cranium[8].
Shortly after its appearance, the central nervous system becomes surrounded by a membranous mesodermal investment which in the region of the spinal cord is called the skeletogenous layer or perichordal sheath, while in the region of the brain it is called the membranous cranium. Ventral to the central nervous system is the notochord, which extends far into the region of the future cranium, and like the nervous system, is enclosed by the skeletogenous layer. The primitive cartilaginous cranium is formed by histological differentiation within the substance of the membranous cranium and always consists of the following parts:
(a) the parachordals. These are a pair of flat curved plates of cartilage, each of which has its inner edge grooved where it comes in contact with the notochord. The parachordals, together with the notochord, form a continuous plate, which is known as the basilar plate. The basilar plate is the primitive floor below the hind- and mid-brain. In front the parachordals abut upon another pair of cartilaginous bars, the trabeculae, the two pairs of structures being sometimes continuous with one another from the first;
(b) the trabeculae which meet behind and embrace the front end of the notochord. Further forwards they at first diverge from one another, and then converge again, enclosing a space, the pituitary space. After a time they generally fuse with one another in the middle line, and, with the parachordals behind, form an almost continuous basal plate. The trabeculae generally appear before the parachordals. They form the primitive floor below the fore-brain;
(c) the cartilaginous capsules of the three pairs of sense organs. At a very early stage of development involutions of the surface epiblast give rise to the three pairs of special sense organs—the olfactory or nasal organs in front, the optic in the middle, and the auditory behind. The olfactory and auditory organs always become enclosed in definite cartilaginous capsules, the eyes often as in the Salmon, become enclosed in cartilaginous sclerotic capsules, while sometimes, as in mammals, their protecting capsules are fibrous.
Each pair of sense capsules comes into relation with part of the primitive cranium, and greatly modifies it. Thus the auditory or periotic capsules press on the parachordals till they come to be more or less imbedded in them. Perhaps owing to the pressure of the nasal capsules the trabeculae fuse in front, and then grow out into an anterior pair of processes, the cornua trabeculae, and a posterior pair, the antorbital processes, which together almost completely surround the nasal capsules. The sclerotic capsules of the eyes greatly modify the cranium, although they never become completely united with it.
The cartilaginous cranium formed of the basal plate, together with the sense capsules, does not long remain merely as a floor. Its sides grow vertically upwards, forming the exoccipital region of the cranium behind, and the alisphenoidal and orbitosphenoidal regions further forwards. In many forms, such as Elasmobranchs, all these upgrowths meet round the brain, roofing it in and forming an almost complete cartilaginous cranium. But in most vertebrata, while in the occipital region, the cartilaginous cranium is completed dorsally, in the alisphenoidal and orbitosphenoidal regions the cartilage merely forms the lateral walls of the cranium, the greater part of the brain having dorsal to it a wide space, closed by merely membranous tissue in connection with which the large frontal and parietal bones are subsequently formed.
The Skull includes
a. the cranium,
b. the jaws and visceral skeleton.
The cranium can be further subdivided into
(1) an axial portion, the cranium proper or brain case;
(2) the sense capsules. The capsules of the auditory and olfactory sense organs are always present, and as has been already mentioned, in many animals the eye likewise is included in a cartilaginous capsule.
(1) The cranium proper or brain case.
The cranium varies much in form and structure. In lower vertebrates, such as Sharks and Lampreys, it remains entirely cartilaginous and membranous, retaining throughout life much of the character of the embryonic rudiment of the cranium of higher forms. The dogfish's cranium, described on pp. 73 to 76, is a good instance of a cranium of this type. But in the majority of vertebrates the cartilage becomes more or less replaced by cartilage bone, while membrane bones are also largely developed and supplant the cartilage.
The cranium of most vertebrates includes a very large number of bones whose arrangement varies much, but one can distinguish a definite basicranial axis formed of the basi-occipital, basisphenoid, and presphenoid bones, which is a continuation forwards of the axis of the vertebral column. From the basicranial axis a wide arch arises,