THE CENTRAL NERVOUS SYSTEM 



709 



of the cord disorganized, and the reflex tone abolished, the 

 knee-jerk disappears. In addition to the direct stimulation 

 of the muscle on the same side, the tendon-tap may cause 

 also a true reflex knee-jerk on the opposite side, the interval 

 between tap and contraction being about \ second. 



Anatomical Basis of Reflex Action. Since the essence of reflex 

 action is that the arrival of afferent impulses in the spinal cord 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 development 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. With increasing 

 complexity of organization the 

 nervous impulsepassing up the 

 afferent fibre is offered a choice 

 of many routes when it reaches 

 the spinal cord. 



We have already (p. 68 1) 

 described the course taken by 

 the fibres of the posterior roots 

 on entering the cord. It is 

 obvious that through the main 

 fibres and their collaterals 

 an extensive connection 

 partly direct, partly by the link of intermediate neurons is estab- 

 lished with the motor cells on both sides of the cord. But the 

 facts of physiology demonstrate an even ampler connection than the 

 mere anatomical study of the distribution of the root fibres would 

 suggest. Indeed, the phenomena of strychnia-poisoning seem to 

 show that every afferent fibre is potentially connected with the 

 motor mechanisms of the whole cord. For in a frog under the 

 influence of this drug, stimulation of the smallest portion of the 

 skin will cause violent and general convulsions, which are unaffected 

 by destruction of the brain, but cease at once on destruction of the 

 cord (p. 768). 



Moderate stimulation of an afferent nerve causes contraction of 

 muscles connected with the same segment of the cord on its own 

 side, and it has been shown that the sensory nerves of a skeletal 

 muscle are derived from the spinal ganglion corresponding to the 

 segment of the cord containing its motor-cells. Stronger excitation, 

 particularly of the end-organs of a nerve, as in stimulation of the 

 skin, will be followed by more extensive movements involving higher 

 or lower segments of the cord, or crossing over to the opposite side. 

 Sometimes the reflex movements are co-ordinated to a high degree, 



FIG. 255. DIAGRAM OF A SIMPLE 

 REFLEX ARC. 



The arrows indicate the direction of the 

 afferent and efferent impulses. 



