CRANIAL NERVES. 165 
color, and form gray rami communicantes for a part of their course to 
the spinal nerves. 
The sympathetic system is best developed in the trunk, but it 
extends forward into the head, where a series of sympathetic ganglia 
(ciliary, sphenopalatine, etc.) is connected with the cranial nerves 
as far forward as the fifth. The sympathetic trunk in this region is 
usually closely connected with other nerves, but occasionally (Vidian 
nerve from the sphenopalatine to the facial ganglion, Jacobson’s 
commissure from the seventh to the ninth, fig. 170) it is distinct. 
A few words may be added to this general account. In the elas- 
mobranchs there is no sympathetic trunk, this first appearing in the 
teleosts. The system is more highly developed in the aquatic than 
in the terrestrial urodeles or in the anura. In the sauropsida the 
trunk is usually double on either side in the neck region, one branch 
running through the vertebrarterial canal of the vertebra. In the 
mammals the cervical part of the trunk is usually closely associated 
with the pneumogastric nerve. In the development certain ganglion 
cells migrate from the developing sympathetic system and pass to various 
parts of the body, being usually closely associated with the glands of 
so-called internal secretion—hypophysis, carotid gland, suprarenals, 
etc. They possess a peculiar affinity for chromic salts and are 
known as chromaffine cells. Little is known of their function. 
The Cranial Nerves. 
The nerves which arise from the brain and pass out through the 
foramina in the skull are known as the cranial nerves. While in a 
general way they resemble the spinal nerves, they have been specialized 
and modified in many respects in correspondence with the specialization 
of the head itself, some consisting of sensory fibres alone, some of only 
motor fibres, while others are mixed, that is, contain both kinds of 
fibres. There can also be recognized somatic and visceral nerves as in 
the trunk, while the somatic sensory fibres may be arranged in dif- 
ferent groups, differing in their connexions inside the brain and in the 
sense organs to which they are distributed. Thus six different kinds of 
fibres may occur in the cranial nerves, as follows: 
1. The somatic motor, which go to the muscles derived from the 
myotomes of the head. 
2. The visceral motor, distributed to the muscles of the gill region 
