CHORDATA. 349 



phological value, nor are they strictly equivalent to the spinal 

 nerves. Some have dorsal and ventral roots, but a much larger 

 number have only one group of roots, either dorsal or ventral. 

 Some are purely sensory nerves, some are motor and some are 

 mixed. How these nerves are related to the segments of which 

 the head is believed to be composed is yet an unsettled question. 



The first or olfactory arises from the olfactory lobe of the fore-brain ; 

 its fibres, which are purely sensory, are distributed to the lining of the 

 nose, the end organ of smell. 



The second or optic nerve arises from the second division of the brain 

 (thalamencephalon), consists of purely sensory fibres, and is distributed to 

 the retina of the eye, the end organ of vision. 



The third, fourth and sixth pairs are purely motor and are distributed 

 to the muscles of the eye. The third and fourth arise from the third 

 division of the brain (mesencephalon). The sixth nerve arises from the 

 medulla, as. do the following: 



The fifth (trigeminal) comes from the anterior portion of the medulla 

 (hind-brain) and, like the spinal nerves, has both dorsal and ventral roots. 

 It is largely sensory, supplying the skin of the face, mouth and tongue. 

 Motor fibres pass to the muscles of the jaw. 



The seventh (facial) is largely motor, is distributed chiefly to the 

 muscles of the face and controls facial expression. 



The eighth or auditory is sensory, and is distributed to the inner ear, 

 the end organ of hearing and of equilibration. 



The ninth or glossopharyngeal is a mixed nerve and is distributed to 

 the muscles and mucous membrane of the pharynx and to the tongue. 



The tenth or vagus arises by numerous roots, has both motor and sen- 

 sory fibres, and is the most widely distributed nerve in the body. Its fibres 

 pass to the posterior visceral arches, lungs, heart, stomach and intestines. 



We find the cranial nerves and their nerve endings concerned chiefly 

 with the higher senses, the muscles of expression, and the sensations and 

 activities involved in the fundamental processes of nutrition. 



362. The Sympathetic System which is always distributed to the 

 visceral organs is made up of a series of connected ganglia in the dorsal 

 part of the body cavity. This system is in connection at various places 

 with the central nervous system. It serves to connect the internal organs 

 more intimately, and is reflex in its action. 



363. Organs of Special Sense. The sense organs repre- 

 sent specialized terminations of the nerve fibres, or special 

 epithelial cells which have become associated with such fibres 

 (Fig. 41). From the very nature of the case they must be 

 external. In the case of higher animals, the more compli- 



