140 PHYSIOLOGY OF CENTRAL NERVOUS SYSTEM. 



nificant experiment in this connection is that made by Setschenow.* 

 If in a frog the entire brain or the cerebral hemispheres are re- 

 moved, then stimulation of the exposed cut surface for instance, 

 by crystals of sodium chlorid will depress greatly or perhaps 

 inhibit entirely the usual spinal reflexes that may be obtained by 

 cutaneous stimulation. On removal of the stimulating substance 

 from the cut surface by washing with a stream of physiological 

 saline (solution of sodium chlorid, 0.7 per cent.) the reflex activities 

 of the cord are again exhibited in a normal way. This experiment 

 accords with many facts which indicate that the brain may inhibit 

 the activities of the spinal centers. In the reflex from tickling, 

 for instance, we know that by a voluntary act we can repress the 

 muscular movements up to a certain point; so also the limited 

 control of the action of the respiratory and micturition centers 

 is a phenomenon of the same character. To explain such acts we 

 may assume the existence of a definite set of inhibitory fibers, 

 arising in parts of the brain and distributed to the spinal cord, 

 whose function is that of controlling the activities of the spinal 

 centers. In view of the fact, however, that there is no independent 

 proof of the existence of a separate set of inhibitory fibers within 

 the central nervous system that is, a set of fibers whose specific 

 energy is that of inhibition it is preferable to speak simply of 

 the inhibitory influence of the brain upon the cord, leaving unde- 

 cided the question as to whether this influence is exerted through 

 a special set of fibers, or is brought about by some variation in 

 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 possible that this factor 

 may explain 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 

 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. Inhibition of spinal reflexes by such means is not so constant 

 nor so effective as by stimulation .of the central paths, but it forms 

 an interesting phenomenon which must be taken into account in 

 any hypothesis of the nature of inhibition that may be proposed. 



* Setschenow, " Physiologische Studien iiber d. Hemmungs-Mechanismen 

 f. d. Reflexthatigkeit im Gehirn d. Frosches," Berlin, 1863. 



