410 COLLEGE ZOOLOGY 



either side in each body segment, and pass out between the ver- 

 tebrae. Each nerve has two roots (Fig. 349), a dorsal root (d.r) 

 and a ventral root (v.r). The dorsal root possesses a ganglion 

 (g) containing nerve cells (g.c). Its fibers carry impulses tow- 

 ard the spinal cord from various parts of the body and are 

 therefore sensory. The fibers of the ventral root carry impulses 

 from the spinal cord to the tissues and are therefore motor. The 

 constitution of the nerve cells (neurons) is similar to that of the 

 earthworm (p. 225). The direction of the nervous impulses is 

 indicated by arrows in Figure 349. 



On each side of the spinal cord is a chain of ganglia which is 

 connected at various places with the central nervous system. 

 This is known as the sympathetic nervous system. These ganglia 

 send nerves chiefly to the alimentary tract, circulatory system, 

 and glandular organs. 



Sense-Organs. — Vertebrates possess a number of highly 

 developed sense-organs — nose, eyes, and ears. In addition to 

 these there are many species with sense-cells, single or in groups, 

 scattered over the body. In some of the lower vertebrates these 

 take the form of lateral line organs (p. 427) of doubtful function. 

 Usually sense-organs of taste occur as pits over the tongue and 

 soft palate. 



The sense-organs of smell are located in the nose. The nose 

 consists of a pair of cavities at the anterior end of the body. 

 These cavities are lined with folds of mucous epithelium covered 

 with olfactory sense-cells. 



The two ears of vertebrates arise as cavities of the skin at 

 the sides of the midbrain. They are rather complicated in 

 structure, as indicated in Figure 350. They function as organs 

 of hearing and equilibrium. 



The internal ear is called the membranous labyrinth and is 

 enclosed by cartilage or bone. Within the labyrinth is a fluid 

 called endolymph; and between the labyrinth and the sur- 

 rounding cartilage or bone is a fluid called perilymph. The 

 labyrinth is usually constricted into two chambers, (1) a dorsal 



