594 THE NERVOUS SYSTEM 



for their protection. Apparently other physiological nerve pathways exist, 

 but it requires a stronger sensory stimulus to arouse nerve impulses along 

 these paths. In explanation we may suppose that the stronger afferent 

 impulses are sufficient to overcome the resistance of increasingly complex 

 paths, that they diffuse through greater and greater extents of the cord. 

 But we may repeat that in the normal state of the cord this diffusion is in an 

 orderly physiological sequence. 



Orderly reflexes can be called forth only by stimulating sensory nerve 

 endings, the first of the essential structures of the reflex arc. If artificial 

 stimuli are applied to a nerve trunk, as the sciatic, unco-ordinated muscular 

 responses occur because the sensory stimuli are diffuse and general and are not 

 specific and local. 



An involvement of multiple pathways may also be accomplished through 

 decreasing the resistance within the cord, as through the use of some drug 

 such as strychnine. In the strychninized frog a slight stimulus brings about 

 multiple and violent reflex spasms. These contractions have lost their order- 

 liness and are unco-ordinated. The entire musculature contracts. It is as 

 though the strychnine removed all differences in the facility with which 

 afferent stimuli spread through the cord, and that the resistance was re- 

 duced 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 

 accompanied 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 



