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