504 COLLEGE ZOOLOGY 



dura mater and an inner pia mater. The cord is composed of 

 a central mass of gray matter (Fig. 349, gm) consisting mainly 

 of nerve-cells, and an outer mass of white matter made up chiefly 

 of nerve-fibers. A median fissure occurs both in the dorsal and 

 in the ventral side of the cord, and a central canal (c.c) lies in 

 the gray matter and communicates anteriorly with the cavities 

 of the brain. 



The Spinal Nerves. — The relation of the spinal nerves to 

 the spinal cord and the paths taken by nervous impulses are 

 indicated in Figure 349. There are ten pairs of spinal nerves 

 in the frog (Fig. 423, Spni, Br, Js). Each arises by a dorsal 

 (Fig. 349, d.r) and a ventral (v.r) root (see p. 40S) which spring 

 from the horns of the gray matter of the cord. The two roots 

 unite to form a trunk, which passes out between the arches 

 of adjacent vertebrae. The largest nerves are the brachial 

 (Fig. 423, Br), which are composed of the second and branches 

 from the first and third pairs of spinal nerves, and are dis- 

 tributed to the fore limbs and shoulder, and the sciatics (Js), 

 which arise from plexuses composed of the seventh, eighth, 

 and ninth spinal nerves, and are distributed to the hind 

 limbs. 



The Sympathetic System (Fig. 423, Sgi-10). — This 

 system consists of two principal trunks, which begin in the 

 prootic ganglion and extend posteriorly, one on either side of the 

 vertebral column. Each trunk is provided with ten ganglionic 

 enlargements (Sgi-10) at the points where branches from the 

 spinal nerves unite with it. The nerves of the sympathetic 

 system are distributed to the internal organs which are thus 

 intimately connected. 



Sense-organs. — The principal sense-organs are the eyes, 

 ears, and olfactory organs. There are many smaller structures 

 on the surface of the tongue, and on the floor and roof of the 

 mouth, which probably function as organs of taste. In the skin 

 are also many sensory nerve endings which receive contact, 

 chemical, temperature, and light stimuli. 



