THE CEEEBRO-SPINAL SYSTEM OF NERVES. 

 I. SPINAIi NERVES. 



These (thirty-one pairs), which leave the spinal cord through 

 the intervertebral foramina, are mixed nerves — i. e., their main 

 trunks consist of motor and sensory fibers. But before they 

 enter the spinal cord they separate into two groups, which are 



Fig. 424. — Diagram of roots of spinal nerve illustrating effects of section (after Dal- 

 ton). The dark regions indicate the degenerated parts. 



known as the anterior or motor and the posterior or sensory 

 roots, which make connection with the anterior and posterior 

 gray horns respectively. 



These facts have been established by a few simple but im- 

 portant physiological experiments, which will now be briefly 

 described,: 1. Stimulation of the peripheral end of a spinal 

 nerve gives rise to muscular movements ; while stimulation of 

 its central end causes pain. 2. Upon section of the anterior 

 root, stimulation of its central end gives negative results ; but 

 of its peripheral end causes muscular movements. 3. After 

 section of the posterior root, stimulation of the distal end is fol- 

 lowed by no sensory or motor effects ; of its central end, by 

 sensory effects (pain). 



These experiments show clearly that the anterior roots are 

 motor, the posterior sensory, and the main trunk of the nerve 

 made up of mixed motor and sensory fibers. 



