102 ELEMENTARY BIOLOGY. [CHAP. 



3. The spinal nerves. 



Pin a frog down upon its back and remove the 

 ventral integument and body-wall, together with one- 

 half of the floor of the mouth. Carefully dissect 

 away the viscera and dorsal aorta and remove the 

 centra of the vertebrae, cutting well down to the level 

 of the roots of the spinal nerves. Wash, and exa- 

 mine under water, 



a. The spinal cord ; in form cylindrical; enlarged 

 at the points of origin of the nerves for the 

 limbs (brachial and lumbo-sacral swellings). 



It. The spinal nerves arising from it. Ten on each 

 side ; the first one passing out between the 

 first and second vertebras, the last through the 

 head of the urostyle (cf. Sect. G. 2. d. e). Each 

 leaves the neural canal behind that vertebra 

 with which it corresponds. 



a. Their origin; each arising by two roots (an- 

 terior and posterior) as is most easily seen in 

 the 7th, 8th and pth nerves, in which they are 

 much the longer. Sometimes the fibres of the 

 anterior roots are bound up into two or more 

 fasciculi. 



(3. The direction of their roots : outwards in the 

 anterior nerves; obliquely backwards in the 

 4th, 5th and 6th; almost directly backwards, 

 and running for a considerable distance in 

 the neural canal, in the 7th, 8th, gth and 

 loth. 



y. The ganglia of their dorsal roots (cf. i. r. /?); 

 very conspicuous in the 7th, 8th, and pth; 

 lying, for the most part, outside the spinal 



