REFLEX ACTION OF THE SPINAL CORP. 



409 



body, as if to escape the cause of irritation. If the irritation applied 

 to the foot be of slight intensity, the corresponding leg only will 

 move ; but if it be more severe in character, motions will often be 

 produced in the posterior extremity of the opposite side, and even 

 in the two fore legs, at the same time. These motions, it is import- 

 ant to observe, are never spontaneous. The decapitated frog remains 

 perfectly quiescent if left to himself. It is only when some cause 

 of irritation is applied externally, that movements occur as above 

 described. 



It will be seen that the character of these phenomena indicates 

 the active operation of some part of the nervous system, and par- 

 ticularly of some ganglionic centre. The irritation is applied to 

 the skin of the foot, and the muscles of the leg contract in conse- 

 quence ; showing evidently the intermediate action of a nervous 

 connection between the two. 



The effect in question is due to the activity of the spinal cord, 

 operating as a nervous centre. In order that the movements may 

 take place as above, it is essential that both the integument and the 

 muscles should be in communication with the spinal cord by nerv- 

 ous filaments, and that the cord itself be in a state of integrity. If 

 the sciatic nerve be divided in the upper part of the thigh, irritation 

 of the skin below is no longer followed by any muscular contrac- 

 tion. If either the anterior or posterior roots of the nerve be 

 divided, the same want of action results ; and finally, if, the nerve 

 and its roots remaining entire, the spinal cord itself be broken up 

 by a needle introduced into the spinal 

 canal, the integument may then be 

 irritated or mutilated to any extent, 

 without exciting the least muscular 

 contraction. It is evident, therefore, 

 that the spinal cord acts, in this case, 

 as a nervous centre, through which 

 the irritation applied to the skin is 

 communicated to the muscles. The 

 irritation first passes upward, as shown 

 in the accompanying diagram (Fig. 

 136), along the sensitive fibres of the 

 posterior root (a) to the gray matter 

 of the cord, and is then reflected back, 

 along the motor fibres of the anterior 

 root (b), until it finally reaches the muscles, and produces a contrac 



Fig. 136. 



Diagram of SPINAL COKD IN VKK- 

 TICAI, SECTION, showing reflex action. 

 n. Posterior root of spinal nerve. 6. 

 Anterior root of spinal nerve. 



