76 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 
(cf. 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 of the skull, are comparable in some 
respects to the spinal nerves, but in many ways are different in 
THE CRANIAL 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 
