402 THE SPINAL CORD. 



nerve, in the living animal, convulsive movements are produced in 

 the parts below, but there is no painful sensation. The anterior 

 root accordingly is said to be excitable, but not sensitive. If the 

 posterior root, on the other hand, be irritated, acute pain is pro- 

 duced, but no convulsive movements. The posterior root is there- 

 fore sensitive, but not excitable. A similar result is obtained by a 

 complete division of the two roots. Division of the anterior root 

 produces paralysis of motion, but no insensibility ; division of the 

 posterior root produces complete loss of sensibility, but no muscular 

 paralysis. 



"We have here, then, a separate localization of sensation and 

 motion in the nervous system ; and it is accordingly easy to under- 

 stand how one may be impaired without injury to the other, or' 

 how both may be simultaneously affected, according to the situation 

 and extent of the anatomical lesion. 



The two roots of a spinal nerve differ from each other, further- 

 more, in their mode of transmitting the nervous impulse. If the 

 posterior root be divided (Fig. 135) at a b, and an irritation applied 



Fig. 135. 



Diagram of SPINAL CORD AND NERVES. The posterior root is seen divided at a, b, the 

 autedur at c, d. 



to the separated extremity (a), no effect will be produced ; but if 

 the irritation be applied to the attached extremity (7;), a painful 

 sensation is immediately the result. The nervous force, therefore, 

 travels in the posterior root from without inward, but cannot pass 

 from within outward. If the anterior root, on the other hand, be 

 divided at c, d, and its attached extremity (d) irritated, no effect 



