THE NERVOUS SYSTEM 



223 



Ventral 



Dorsal 

 root 



sciatic nerve were cut— the portion carrying efferent fibers 

 (^1, Fig. Ill)— one would remain just as sensitive as ever to hot 

 iron or tickHng, 



but he could not iz.«t^nT>i^-rm.{ } ^-^ ^ — --m- 



move his legs, no 

 matter how hard 

 he tried ; and cer- 

 tainly they would 

 not move of them- 

 selves, for the re- 

 flex arc would be 

 broken, as in the 

 first case. 



182. The brain. 

 The brain of man 

 has the same gen- 

 eral structure as 

 the brain of other 

 backboned ani- 

 mals (see Fig. 

 112).^ The brain 

 is the front end 

 of the main nerv- 

 ous axis, and 

 contains blood 

 vessels and con- 

 nective tissue in 

 addition to many 

 millions of neu- 

 rons. The cortex of the cerebrum (see Fig. 113) consists of 

 nerve cells, and in mammals it is very much wrinkled. The ex- 

 tent of the wrinkhng is related to the number of cells and to 

 the complexity of their connections. 



1 Excepting the whale and the elephant, man has the largest brain ; and while 

 the brain of man is about one fiftieth of the whole body, in the elephant it 

 represents only one five-hundredth and in the whale but one ten-thousandth. 



Fig. 114. Diagram of the spinal cord 



A, left half of cross section, showing impulses entering the 

 dorsal root and outgoing impulses passing out by the ven- 

 tral root. B, the neurons connected with the gray matter 

 of the cord give off branches passing up and down the cord 

 and transmitting nervous disturbances by way of the col- 

 laterals. In the gray matter of the cord, branches of af- 

 ferent neurons carry impulses up and down and pass them 

 on, by way of the collaterals, to efferent neurons and to 



the brain 



