7*5 Anatomy of the Rabbit. 



It will be seen also that the form of the brain is more or less depen- 

 dent on the existence at certain places of well-marked flexures 

 (of. Plate II). The first of these, the cephalic flexure, is in the 

 region of the mesencephalon, the anterior portion of the brain being 

 bent downward; the second, or pontine flexure, is at the fourth 

 ventricle; while the third, or cervical flexure, is at the. point where 

 the myelencephalon passes over into the spinal cord. 



The peripheral nervous system embraces two groups of paired 

 and, for the most part, metamerically arranged nerves, namely, the 

 spinal nerves — those arising from the spinal cord and leaving the 

 vertebral column through the intervertebral foramina ; and the 

 cranial or cerebral nerves — those arising from the brain and 

 passing through the foramina of the skull. Of these the spinal 

 nerves (p. 68) are less .modified , both in structure and distribution. 



The cranial nerves, those arising from the brain, and making 

 their exit through the walls o'f the skull, are comparable in some 

 respects to the spinal nerves, but in many ways are different in' 

 THE riiANiAT nature in. addition to being in some cases highly 



NERVES. specialized. Three pairs, respectively, ol- 



factory, optic, and acoustic, or first, 

 second, and eighth of the series are afferent nerves from the 

 special sense organs of smell, sight and hearing, the function of the 

 acoustic nerve including also maintenance of equilibrium. The 

 third, fourth and sixth nerves, respectively, oculomotor, 

 trochlear and abducent, are distributed as somatic motor nerves 

 to the muscles of the eyeball, but also contain fibres of muscle sense. 



Of the remaining cranial nerves the fifth, seventh, ninth and 

 tenth are branchiomeric. Although the connections of these 

 nerves are not fully considered in the dissection as here outlined, 

 their chief characteristic as branchiomeric structures may be 

 indicated. The fifth, or trigeminal nerve, the nerve of the 

 mandibular arch, arises in two parts, one of which, the portio major, 

 is sensory, the other, the portio minor, motor. The portio minor 

 unites with the third or mandibular division of the portio major. 

 Thus, the terminal branches of all three divisions, ophthalmic, 

 maxillary, mandibular, are distributed as somatic sensory nerves 

 to the skin of the head, and, in addition, the mandibular nerve 



