146 PHYSIOLOGY OF CENTRAL NERVOUS SYSTEM. 



equal to 0.01 to 0.02 sec. If the reflex was on the crossed side 

 about double this time was consumed in the cord. This delay 

 of the velocity of transmission of an impulse in the nerve centers 

 is a factor which must vary somewhat in different parts of the 

 nervous system. It has been shown that, in certain cases at 

 least, when strong stimuli are used the latent period of a reflex 

 is not greater than would be accounted for by transmission 

 through the nerve fibers and by the latency of the muscular 

 contraction. Thus Fran5ois Frank, in an experiment in which 

 the gastrocnemius muscle of one side was made to contract 

 reflexly by stimulation of the afferent root of a lumbar nerve 

 on the other side, records a latent period of only 0.017 sec. 

 Evidently in such a case there was no perceptible delay in 

 passing through the nerve centers of the lumbar cord. 



Inhibition of Reflexes. — One of the most fundamental facts 

 regarding spinal reflexes is the demonstration that they can be 

 depressed or suppressed entirely — that is, inhibited — by other im- 

 pulses reaching the same part of the spinal cord. The most sig- 

 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 centers of respiration and micturition 

 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 



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

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



