32 



HIGH SCHOOL ZOOLOGY. 



87. Like the hrain, the spinal cord is also a tulic, its cavity, 

 the central canal, being, however, more uniform, and its walls 

 of similar thickness throughout, except that the side walls are 

 more developed than either the roofer the floor (Fig. 12). The 

 metameiy of the nervous system is much better seen here than in 

 the brain, for at regular intervals corresponding to the vertebrae, 



a pair of spinal nerves is at- 



tached to the cord. Each 

 nerve originates by two 

 roots, which from their 

 ])laces of origin are known 

 as dorsal and ventral ; the 

 dorsal roots have a knot or 

 ganglion upon them, and 

 they contain only efferent 

 fibres, the ventral on the 

 other hand are formed of 

 afferent fibres : both kinds 

 "<^ of filjres are, however, soon 

 associated in the mixed 

 nerves of the body. 



38. Of the two elements of 



nerve tissue distinguished in 

 Fi-. V2 -Section through Spinal Cord and „ „„ ,, ,, , j, , 



sunoundin;,' ].eirts, in youn^' Catfish. X40. 8 •«> i^'ie nerve cellb, aiSO caueu 



Nc, notoehord, with its sheath ; n, the neural ganglion cells, are most abund- 

 aixh; d, dorsal, v, ventral nerve-root; g, Ran- . _„, „ j 4.i,p povitipts of the 

 1,'lion of dorsal root ; d', vi, dorsal and ventral ^^^^ rouna tne ca\lties OI tne 

 mixed nerves ; s, sympathetic {ranglion ; ao, brain and spinal cord, but are 



also found in smaller centres 



or ganglia, while the fibres are found both in the nerves and in the 



centres. The function of the fibres is to transmit impulses, and this is 



effected by the axis cylinder, while the function of the cells is to store 



up or modify the impulses that arrive through the afferent nerves, and 



to originate iliose which are discharged thi-ough the efferent nerves. 



39. It is easy to undei'stand the way in which the spinal 

 nerves are distributed in the body. Each pair sui)plies chiefly 



