64O EXAMPLES AND NATURE OF INHIBITION. 



crystal of common salt or blood, the reflex movements are suppressed. The same 

 results obtain when only one side is operated on. Similar organs are supposed to 

 be present in the corpora quadrigemina and medulla oblongata of the higher verte- 

 brates. From 1 and 2 we may explain why reflex movements occur more regularly 

 and more readily after separation of the brain from the spinal cord. 



[Quinine greatlv diminishes the reflex excitability in the frog, but if the medulla oblongata 

 be divided, the reflex excitability of the cord is restored. The depression is ascribed by Chaperon 

 to the action of the quinine on Setschenow's centres.] 



3. Strong stimulation of a sensory nerve inhibits reflex movements. The 

 reflex does not take place if an afferent nerve be stimulated very powerfully (Goltz, 

 Lewisson). Examples : Suppressing a sneeze by friction of the nose, [compressing 

 the skin of the nose over the exit of the nasal nerve] j suppression of the move- 

 ments produced by tickling, by biting the tongue. Very violent stimulation may 

 even suppress the co-ordinated reflex movements usually controlled by voluntary 

 impulses. Violent pain of the abdominal organs (intestine, uterus, kidneys, bladder, 

 or liver) may prevent a person from walking or even from standing. To the same 

 category belongs the fact that persons fall down when internal organs richly 

 supplied with nerves are injured, there being neither injury of the motor nerves nor 

 loss of blood to account for the phenomenon. Excitement of the central organs 

 through other centripetal channels (nerves of special sense, and those of the 

 generative organs) diminishes the reflexes in other channels. 



4. It is important to note that in the suppression of reflexes, antagonistic muscles are often 

 thrown into action, whether voluntarily or by the stimulation of sensory nerves, i.e., reflexly. 

 In some cases, in order to cause suppression of the reflex, it appears to be sufficient to direct our 

 attention to the execution of such a complicated reflex act. Thus, some persons cannot sneeze 

 when they think intently upon this act itself {Darwin). The voluntary impulse rapidly reaches 

 the reflex" centre, and begins to influence it so that the normal course of the reflex stimulation, 

 due to an impulse from the periphery, is interfered with (Schlosscr). 



5. Poisons. Chloroform diminishes the reflex excitability by acting upon the 

 centre, and a similar effect is produced by picrotoxin, morphia, narcotin, thebain, 

 aconitin, quinine, hydrocyanic acid. [W. Stirling finds that chloral, potassic 

 bromide and chloride, ammonium chloride, but not sodium chloride, greatly diminish 

 the reflex excitability. Nicotin increases it in frogs (Freusberg),] 



A constant current of electricity passed longitudinally through the cord 

 diminishes the reflexes (Eanke), especially if the direction of the current is from 

 above downwards (Legros and Onimus, Uspenshy). 



[Some drugs affect the reflex excitability directly by acting on the spinal cord, e.g., 

 methylconine, but other drugs may produce the same result ingirectly by affecting the heart 

 and the blood-supply to the cord. If the abdominal aorta of a rabbit be compressed for a few 

 minutes to cut off the supply of blood to the cord and lower limbs, temporary paraplegia is 

 produced.] 



If frogs be asphyxiated in air deprived of all its 0, the brain and spinal cord become 

 completely unexcitable, and can no longer discharge reflex acts. The motor nerves and the 

 muscles, however, suffer very little, and may retain their excitability for many days (Aubert). 



[Nature of Inhibition. The foregoing view assumes the' existence of inhibitory centres, but 

 it is important to jxrint out that it has been attempted to explain this phenomenon without 

 postulating the existence of inhibitory centres. During inhibition the-funetion of an organ is 

 restrained during paralysis it is abolished, sq that there is. a sharp distiuction between the two 

 conditions. The analogy between inhibitory phenomena and tb,e. effects of interference of 

 waves of light or sound has been pointed out by Bernard and Ron^anes, while Lauder Bruntou 

 has tried to explain the question on a physical basis, indicating that inhibition is not dependent 

 on-the existence of special inhibitory centres, but that stimulation and inhibition are different 

 phases of excitement, the two terms being relative conditions depending on the length of the 

 path along which the impulse has to travel and the rate of its transmission. Brunton points 

 out that the known facts are more consistent with an hypothesis of the interference of waves, 

 one with another, than with the supposition that there are inhibitory centres for every so-called 

 inhibitory act in the body. In discussing this question great regard must be had to the action 

 of the vagus on the heart ( 369).] 



