Nervous System. 539 



composed of a series of short bones, called vertebrae, placed end to 

 end. There is a hole in the posterior or dorsal part of each of the 

 vertebrae, running parallel with the direction of the body, the effect of 

 which, taken together, is a tube running the whole length of the spine. 

 In this tube is the nervous trunk, called the spinal cord, which puts 

 forth branch nerves in pairs, one pair for every vertebra. To the front 

 or anterior sides of the vertebrae the ribs are attached. In the lower 

 vertebrates, a pair of muscles goes with each vertebra, one on each 

 side, and to each muscle a nerve is sent from the spinal cord. This is 

 the case with the fishes. 



FIG. 262. Nervous System of 

 Frog, underside. 

 L- Olfactory lobes. 

 01 .Olfactory nerves. 

 a-Eye. 



A. Is placed on the crossing of 

 the Optic Nerves. 



Vg. Ganglion of the 5th nerve. 



XQ. Ganglion of Vagus or 10th 

 nerve. 



8p 1, Sp 4, &c. Spinal nerves. 



K. Great Sympathetic nerves. 



tig 1, tig 7, tig 10, &c. The ten 

 ganglions of the sympathetic nerve. 



C. Spinal cord. 



Br Brachial nerves (to the 



Is. Ischial nerves (to the legs). 

 (AfterEcker.) 



After the development of 

 limbs, the relationship be- 

 tween the vertebrae and the 

 muscles became changed, the 

 arrangement of the muscles 

 gradually becoming such as 

 best to accommodate the lo- 

 comotive requirements of the 

 animal. The relation of the 

 nerves to the vertebrae, how- 

 ever, remains the same, and 

 in all there are as many pairs 

 of spinal nerves as there are 

 vertebrae. In man, the num- 

 ber of each is thirty or thir- 

 ty-one. The nerves are dis- 

 tributed to all the muscles of 

 the body and limbs, and to 

 every part of the skin. The 

 nerve trunks leaving the 



FlG 262 spinal cord have two roots in 



the spinal cord called the posterior and anterior, so named from the 



