180 GENERAL ZOOLOGY 



the nervus terminalis. It has been found in all vertebrates but 

 is easily overlooked because of its inconspicuous size which 

 accounts for its remaining undiscovered until a relatively recent 

 time. It extends from the brain near the olfactory nerve, to 

 the olfactory epithelium. It is provided with a ganglion and is 

 undoubtedly a sensory nerve, but nothing definite is known of 

 its function. 



The nerve cell bodies of the cranial motor nerves are located in 

 the brain. The nerves themselves are composed of axons from 

 their centrally located cell bodies. There is less uniformity in 

 the location of the cell bodies of the sensory nerves. The cell 

 bodies of the olfactory nerves lie in the olfactory epithelium, 

 which morphologically is at the surface of the body. The olfac- 

 tory nerves are composed of bundles of axons extending from the 

 olfactory epithelium to the brain. The nerve cell bodies of the 

 optic nerve are located in the retina of the eye from which axons 

 pass to the brain. The sensory components of the fifth, seventh, 

 eighth, ninth, and tenth nerves arise from cell bodies in the 

 ganglia of these nerves. From the ganglia dendrons extend to 

 the periphery and axons toward the brain. 



Spinal Nerves. — The spinal cord is connected with peripheral 

 regions of the body, below the level of the head, by paired spinal 

 nerves. These pass from the cord through openings between the 

 vertebrae. The total number of spinal nerves varies in different 

 animals — 10 pairs in the frog, 31 in Man, 42 in the horse, etc. 

 Each spinal nerve is attached to the cord by two roots, a dorsal 

 root provided with a ganglion and a ventral root without a ganglion. 

 The ganglion is composed largely of nerve cell bodies whose axons 

 pass into the cord and whose dendrons pass out to the periphery, 

 as fibers from the dorsal root. The ventral root is com- 

 posed of axons whose cell bodies lie in the cord. At their points 

 of attachment to the cord each root splits up into smaller strands 

 spread fanwise along the cord. The dorsal root, except in some 

 of the lower vertebrates, is purely sensory in function. In the 

 exceptional cases, efferent fibers arising from nerve cells within 

 the cord pass out through the dorsal roots along with the sensory 

 fibers. The ventral root in all cases is purely motor. The two 

 roots unite to form a spinal nerve which almost at once divides 

 into three divisions: (1) a ramus dorsalis to the skin and muscles of 

 the dorsal region of the body; (2) a ramus ventralis to similar 



