888 THE NERVOUS SYSTEM. 



the remaining nervous connections are left untouched, the result is an 

 immediate and total paralysis of voluntary movement in the muscles 

 to which that nerve is distributed. At the same time, the power of 

 sensibility is undiminished, and the animal is still capable of feeling 

 the contact of foreign bodies or a galvanic current applied to the skin. 

 If the anterior roots of a series of spinal nerves be thus divided, as, 

 for example, those of all the lumbar and sacral nerves on one side, the 

 above effect will be produced for the entire corresponding region of the 

 body, and the whole posterior limb on that side will lose the power of 

 voluntary motion while retaining its sensibility. This is not due to 

 any loss of physiological properties in either the nerve or the muscles, 

 since irritation of the nerve or nerve root, outside the point of section, 

 still produces muscular contraction as before. All these facts prove 

 that the path by which impulses for voluntary motion pass, from the 

 spinal cord to a muscle, is exclusively the anterior root of the spinal 

 nerve. 



On the other hand, if the posterior root be irritated, a sensation is 

 produced, more or less acute, according to the amount and quality of 

 the irritation. This sensation, when of a certain intensity, is accom- 

 panied by movements. But these movements are of a reflex character, 

 and not necessarily confined to the part to which the nerve is distrib- 

 uted ; and if the corresponding anterior root have been divided, this 

 part will remain motionless, while muscular contractions continue to 

 be produced elsewhere. Such movements, accordingly, are not pro- 

 duced directly by irritation of the posterior root, but are caused indi- 

 rectly by the reaction of the nervous centres. The only immediate 

 result of irritation of a posterior nerve root is a sensation, and this root 

 is therefore said to be " sensitive." 



Moreover, if the posterior root be divided, the consequence is a loss 

 of sensation in the corresponding region of the body. This is due 

 simply to the rupture of communication between the integument and 

 the nervous centres ; since irritation of that part of the divided nerve 

 which is still attached to the spinal cord produces a sensation as before. 

 The posterior root of the spinal nerve is, therefore, in this part of the 

 nervous system, the exclusive channel of transmission for sensitive 

 impressions. 



But beyond the situation of the spinal ganglia, the two roots unite 

 in a common trunk. Here, the fibres of the anterior and posterior 

 roots become so intermingled that they can no longer be separately 

 irritated by artificial means. They pass, still associated in this manner, 

 into the branches and subdivisions of the nerve ; and only separate from 

 each other again at its terminal ramifications, where the sensitive fibres 

 are distributed to the integument and the motor fibres to the muscles. 



A spinal nerve, therefore, in its trunk and peripheral branches, con- 

 tains both sensitive and motor fibres, and is consequently a " mixed " 

 nerve. It is both excitable and sensitive, since its artificial irritation 

 causes at the same time sensation and movement ; and if it be divided, 



