198 CENTRAL NERVOUS SYSTEM 



Inhibition. Inhibition is one of the most fundamental 

 phenomena of spinal reflexes. If, in a frog, the cerebral hemi- 

 spheres are removed, stimulation of the exposed cut surface 

 with crystals of sodium chloride will greatly depress or entirely 

 inhibit the ordinary reflexes following stimulation of the skin. 

 Removal of the stimulating substance from the cut surface by 

 washing with physiological saline restores the reflex activities 

 of the cord. An example of an inhibition of a reflex is afforded 

 by the well-known device of preventing an act of sneezing by 

 pressing upon the upper lip. A change of conditions in the 

 spinal cord may permit excitation of a given group of muscles 

 by the stimulation of an afferent path which is usually inhibitory 

 for them. For instance, stimulation of the internal saphenous 

 nerve in a decerebrate dog always produces inhibition of the 

 quadriceps extensor involved in the knee-jerk. Poisoning with 

 strychnine so alters the cord that stimulation of the nerve now 

 produces reflex contraction instead of relaxation. The impor- 

 tance of inhibition in normal movements of the body is strik- 

 ingly shown by the phenomenon of reciprocal innervation. 

 By this is meant the simultaneous relaxation or loss of tone of 

 an extensor muscle when the antagonistic flexor is stimulated 

 reflexly. 



Spread of Impulses in Spinal Cord. As the strength of the 

 stimulus used in evoking a reflex movement is increased, the 

 effect becomes more and more extensive, spreading out or irra- 

 diating in various directions. For purposes of description of 

 reflex action the sensory surface of the skin is divided into a 

 number of areas related to definite portions of the cord, on the 

 one hand, and to definite muscles, on the other. Such areas 

 are the cervical, the brachial, the thoracic, the crural, and the 

 caudal. On this basis spinal reflexes are classed as long or short. 

 The short reflexes are those in which the muscular response 

 takes place in the same region as the application of the stimulus. 

 Long reflexes, on the other hand, involve the musculature of 

 one region when the stimulus is applied to the receptive area 

 of another region. For short reflexes a number of rules have 

 been determined which may be stated as follows: (1) It is 

 easier to excite reflex contractions involving a given efferent 

 nerve, upon stimulation of a given afferent nerve, the closer 

 their spinal segments lie together; (2) for each afferent nerve 



