148 PHYSIOLOGY OF CENTRAL NERVOUS SYSTEM. 



the time relations, intensity, or quality of the nerve impulses. 

 Regarding the fact, however, there can be no question, and it 

 constitutes a most important factor in the interaction of the dif- 

 ferent parts of the nervous system. It is probable that this factor 

 explains why a normal frog gives reflexes that are so much less 

 constant and less predictable than one with its brain removed. 

 A similar inhibition of spinal reflexes may be obtained by simul- 

 taneous stimulation of two different parts of the skin. The 

 usual reflex from pinching the toe of one leg may be inhibited in 

 part or completely by simultaneous stimulation of the other leg 

 or by direct electrical stimulation of an exposed nerve trunk. A 

 similar interference is illustrated, perhaps, in the well-known 

 device of inhibiting an act of sneezing by a strong sensory 

 stimulation from some part of the skin for instance, by pressing 

 upon the upper lip. The importance of the process of inhibition 

 in the normal movements of the body is illustrated strikingly 

 by the phenomenon known as reciprocal innervation, which has 

 been investigated chiefly by Sherrington.* This observer has 

 found that when a flexor muscle is stimulated reflexly there is 

 at the same time a relaxation or loss of tone in its antagonistic 

 extensor, which is explained as being due to an inhibition of the 

 motor cells of the extensor in the cord. Reflex stimulation of 

 the extensor is accompanied similarly by an inhibition of the 

 tone of the antagonistic flexor. This phenomenon has been 

 demonstrated not only for reflex stimulation of the cord but 

 also for voluntary movements (Athanasieu) and for electrical 

 stimulation of the cortical centers. The motor centers of the 

 muscles surrounding the joints are apparently so connected in 

 pairs that when one is excited the center of the corresponding 

 antagonist is inhibited. This reciprocating mechanism dis- 

 appears under conditions, such as strychnine-poisoning, in which,, 

 according to the usual belief, the irritability of the centers is 

 greatly increased. A relationship quite comparable to the 

 reciprocal innervation, although working in only one direction, 

 is exhibited by the peripheral nerve plexuses in the intestinal 

 canal in the so-called law of the intestines (see p. 732). A 

 brief statement of the more or less unsatisfactory theories of 

 inhibition is given in connection with the inhibitory action of the 

 vagus nerve on the heart beat (see p. 593). It should be added, 

 however, in this connection, that stimulation of the cord, and 

 probably of other parts of the nervous system, from two different 

 sources may result not only in an inhibition of the reflex normally 

 occurring from one of the stimuli, but under some circumstances 



* Sherrington, "The Integrative Action of the Nervous System," 1906, 

 p. 84. 



