796 A MANUAL OF PHYSIOLOGY 



the latter. The rapidity with which sensation is restored below 

 the lesion after semisection of the cord in animals is an illustration of 

 this. Another difference, which can be explained in the same way, is 

 that a sharply-marked dissociation of sensations retention of tactile 

 sensibility, for example, with loss of sensibility to pain or to pain and 

 temperature changes either cannot be produced experimentally in 

 animals, or is very difficult to realize. 



The impulses which descend the cord give token of their arrival 

 at the periphery by causing either contraction of voluntary muscles, 

 or contraction of the smooth muscular fibres of arteries, or secretion 

 in glands. They all pass down in the antero-lateral column, but the 

 path of the voluntary impulses in the pyramidal tracts is the best 

 known and most sharply defined. 



2. Modification of Impulses set up elsewhere (Reflex 

 Action). The spinal cord, although it is a conductor of nervous 

 impulses originating elsewhere, is by no means a mere con- 

 ductor. Many of the impulses which fall into the cord are 

 interrupted in its grey matter. Some of the efferent impulses 

 proceeding from the brain are perhaps modified in the cord, 

 and then transmitted to the muscles. Some of the afferent 

 impulses are modified, and then transmitted to the brain ; 

 some are modified, and deflected altogether into an efferent 

 path. These last are the impulses which give rise to reflex 

 effects. A reflex action has sometimes been defined as an action 

 carried out in the absence of consciousness ; not necessarily, 

 however, in the absence of general consciousness, but in the 

 absence of consciousness of the particular act itself. But the 

 term is now more correctly used so as to embrace all kinds of 

 actions which are not directly voluntary, whether the individual is 

 conscious of them or not. For example, when the sole of the foot is 

 tickled, the leg is irresistibly and involuntarily drawn up by reflex 

 contraction of its muscles ; yet the person is perfectly cognizant 

 both of the movement and of the sensation which accompanies 

 the afferent impulse. Many reflex actions usually associated with 

 sensations proceed normally when consciousness is entirely in abey- 

 ance ; during sleep most of the ordinary reflexes can be elicited. 



Anatomical Basis of Reflex Action. Since the essence of reflex 

 action is that the arrival of afferent impulses in the spinal cord or 

 brain causes the discharge of efferent impulses, there must be some 

 connection between the incoming and the outgoing nerve-fibres. 

 When the nervous system is still only a process of an epithelial 

 (sensory) cell joining hands with a muscular cell, the distinction 

 between afferent and efferent fibre does not exist. When develop- 

 ment has gone a step further, and the neuro-muscular process is 

 interrupted by a second epithelial cell transformed into a nerve-cell, 

 the afferent fibre enters one pole and the efferent fibre leaves the 

 other pole of the same cell. In a simple reflex action three events 

 can be distinguished : stimulation of a receiving mechanism, con- 

 duction of the excitation, and the consequent reaction or end-effect. 

 The receiving mechanism or receptor may consist of ordinary sensory 



