FUNCTIONS OF THE SPINAL NERVE ROOTS 523 



differences in the facility with which afferent stimuli spread through the cord, 

 and that the resistance was reduced to the minimum. The strychnine effect 

 is possibly due to a decrease in the resistance at the synapses, and possibly 

 also to an increase in the irritability of the discharging nerve cells. 



We must also suppose that the centers are particularly sensitive to certain 

 kinds of stimuli, sometimes producing very extensive and violent muscular 

 actions in response to a slight stimulus of a special kind. Such a condition 

 is illustrated in the violent and general muscular spasms occurring when a 

 small particle of food passes into the larynx, violent expiratory spasms ac- 

 companied by contractions of other muscles taking place. 



The time taken in a reflex action for the eye in man has been found to be 

 0.066 to 0.058 of a second, but this estimate includes the entire time from 

 the instant of stimulation to the beginning of the contraction of the muscle. 



Functions of the Spinal Nerve Roots. The anterior spinal nerve 

 roots are efferent in function and the posterior are afferent. The fact is 

 proved in various ways. Division of the anterior roots of one or more nerves 

 is followed by complete loss of motion in the parts supplied by the fibers of 

 such roots, but the sensation of the parts remains perfect. Division of the 

 posterior roots destroys the sensibility of the parts supplied by their fibers, 

 while the power of motion continues unimpaired. Moreover, stimulation of 

 the ends of the distal portions of the divided anterior roots of a nerve excites 

 muscular movements. There are sometimes slight evidences of sensory im- 

 pulses due to recurrent fibers that are distributed through the anterior root 

 to the spinal meninges. Stimulation of the proximal ends of the anterior 

 roots, which are still in connection with the cord, is followed by no appreciable 

 effect. It must be remembered, however, that in the anterior or efferent nerves 

 other fibers besides motor are contained, e.g., vaso-motor, secretory, heat fibers, 

 and when the distal end of a divided nerve is stimulated, the effects are ex- 

 ercised not only upon muscles, but upon glands, blood-vessels, etc. Stimu- 

 lation of the distal portions of the divided posterior roots, on the other hand, 

 produces no muscular movements and no manifestations of pain; for, as al- 

 ready stated, sensory nerves convey impressions only toward the nerve cen- 

 ters. Stimulation of the proximal portions of these roots elicits signs of in- 

 tense suffering. Muscular movements also ensue; but these are the result 

 of the reflex stimulation of the motor neurones of the anterior horn of the cord 

 or are movements in response to the afferent impulses passing to higher centers 

 from the roots stimulated. 



Functions of the Ganglia on Posterior Roots. The cells of the posterior 

 ganglia act as centers for the nutrition of the nerve fibers given off from them. 

 When these are cut, the parts of the nerves so severed degenerate, while the 

 parts which remain in connection with the cells of the ganglia do not. Thus 

 on section of the posterior nerve root beyond the ganglion the peripheral part 

 degenerates and the central does not, and on section of the root between 



