2104. If new bony rings originate, they must also take these directions. They accompany the vessels that run in a circle, as the vertebral column accompanies the vascular trunks. These annularly-disposed bony twigs or apophyses constitute in the direction downwards the ribs, in that upwards the vertebral arches. Anterior and posterior to the vertebral column there consequently originates a long canal formed by the bony rings. In the anterior canal lie the galvanic or vegetative organs, in the posterior or upper, the organs of light must be situated "par excellence." The former of these canals is called the thoracic and abdominal cavity, the latter the vertebral canal. The vertebral canal is not the bony or medullary cavity itself, but it has been formed by several bony cysts in the same manner as the thoracic cavity. It consists of the body and the two arches. These are thus posterior (superior) ribs. The vertebral canal has the same signification as the thoracic cavity has; it is only a posterior thoracic cavity. It therefore contains, like the anterior canal, viscera dissimilar in kind to bone; the one including the spinal cord, the other vessels, intestine, and lung.
2105. The bony cysts that have originated through constriction do not all harden into calcareous matter, but they remain as alternating membranous cysts. Between the rings there are permanent cysts. The membranous cysts form the joint, or articular capsule. An articular capsule is a bone which has remained soft.
2106. This change in the ossific process takes place through the attachment of muscles, concerning which we shall treat in the sequel.
2107. The whole osseous system is consequently a symmetrical arrangement of several polar cysts and rings.
2108. The vertebra is not a single ring, but is at once a tolerably compound osseous system. The whole osseous system is nothing but a vertebra repeated.
2109. The number of the vertebræ necessarily conforms to that of the pairs of nerves or ganglia of the spinal cord; for they are indeed only the periphery or envelope of the latter. The number of nerves is, however, adapted to that of the organs, which they have to take care of.
2110. Now, the nervous organs are the senses. There are consequently as many divisions of vertebræ as there are senses. Thus there are vertebræ appertaining respectively to the senses of touch, taste, smell, hearing, and sight. Now as the four latter senses make up the head, but that of touch or feeling is distributed over the whole body, and superintended by the spinal nerves, the vertebræ thus divide into two principal sections, into vertebræ of the head and trunk. The number of cephalic vertebræ is 4; namely, nasal, ocular, lingual, and auditory vertebræ.
2111. To a perfect vertebra belong at least five pieces, namely, the body, in front the two ribs, behind the two arches or spinous processes; every vertebra of the head consists therefore of five pieces also. In those vertebræ which are removed from the respiratory organ, the ribs are smaller, as in the ventral ribs, and anchylosed with the body, as in the cervical vertebræ, where they are represented by the perforated transverse processes, and in the lumbar vertebræ they disappear entirely.
2112. The formation of the cervical vertebræ, where the ribs have been impacted or interposed between the body and spinous processes, is continued into the cranial vertebræ. These are only expanded cervical vertebræ. At the base of the skull four vertebral bodies lie in a series one behind the other; the body of the occipital bone, the two bodies of the sphenoid, and the vomer. Upon the sides of each of these bodies are situated alar processes, which correspond to the transverse processes of the cervical vertebræ, or to the ribs; e. g. the articular heads or condyles of the occipital bone, the alæ majores and minores of the sphenoids, and the two sides or lateral surfaces of the vomer. Behind these are placed the two broad cranial bones, which correspond to the spinous processes; as the occipital ridge or crest, the parietal, frontal and nasal bones. The occipital vertebra consists of the body, the two condyles and the occipital crest. The parietal vertebra consists of the body of the posterior sphenoid, the alæ majores, and the parietal bones. The frontal vertebra consists of the body of the anterior sphenoid, the orbitar wings or alæ, and the two frontal bones. The nasal vertebra consists of the vomer, the ethmoid and the two nasal bones. The occipital vertebra is the auditory vertebra; it incloses the auditory bones and the cerebellum, which gives off the nerves of hearing. The parietal vertebra is the lingual vertebra; the maxillary and lingual nerves passing through its alæ majores. The frontal vertebra is that belonging to the eye; through the orbitar plates or wings the optic nerves pursue their course, and it environs the cerebrum, from which these nerves originate. The nasal vertebra contains the olfactory nerves.
2113. Each cranial sense has thus only one vertebra, and the skull will consequently be formed of four vertebræ, whereof three appertain to the cranium, one unto the face. (See Oken's Ueber die Bedeutung der Schädelknochen, 1807.—Isis, 1817, S. 1204.)