INDIGESTION AND NERVOUS DEPRESSION. 231 



are much less sensitive to this process of choking than the delicate 

 structures of the nerve-centers. The gastrocnemius, or the heart of a 

 frog, may retain its irritability for very many hours after its sepa- 

 tion from the body, but the spinal cord of the same animal will rarely 

 retain its irritability for a single hour after the circulation through it 

 has been arrested. In warm-blooded animals the spinal cord is much 

 more sensitive than in the frog, and, if the circulation in the lower part 

 of the spinal cord be arrested in a rabbit by the pressure of a thumb 

 upon the aorta for three or four minutes, the hind-legs of the animal 

 will become completely paralyzed. Still more sensitive than the spinal 

 cord is the brain, and if the circulation in the latter organ be arrested, 

 consciousness is almost instantaneously abolished. In the animal body 

 as in the steam-engine, the governing and directing parts are much 

 more sensitive and easily acted upon than the working parts. A sin- 

 gle touch of the hand to the steam-valve will set the engine in action 

 or stop its movement, although the power of a thousand men applied 

 to the fly-wheel would avail little or nothing. And in animals the nerve- 

 centers are most sensitive and respond most readily to those circum- 

 stances which affect the organism. Not only are they exceedingly 

 sensitive to the accumulation within them of the products of their own 

 waste, but they are easily affected by alterations in the blood which 

 circulates through them, and which conveys to them not only the prod- 

 ucts of muscular and glandular waste formed in other parts of the 

 body, but also substances introduced from without, or absorbed from 

 the intestinal canal. A single whiff of nitrite of amyl is sufficient to 

 dilate the blood-vessels ; a fraction of a grain of pilocarpine will stim- 

 ulate the sweat-glands to the most profuse secretion ; and half a drop 

 of pure hydrocyanic acid is enough almost instantaneously to abolish 

 consciousness and destroy the functional activity of the entire nervous 

 system. In the case of the nitrite of amyl, the pilocarpine, or the 

 hydrocyanic acid, we are able to distinguish the relation of cause and 

 effect between the administration of the drug and the resulting changes 

 in the organism. We do this, however, because of our knowledge, 

 obtained by observation and experiment. Sometimes we can not do 

 this. I have seen, for example, a person become aware of a peculiar 

 sensation which, to the patient, was quite unaccountable, but of which 

 I understood the reason, as I knew it to be due to the fumes from a 

 bottle of nitrite of amyl, which the patient could not see. We may 

 notice a similar occurrence in poisoned animals. The poison of the 

 cobra causes paralysis of the spinal cord and nerves, and induces in- 

 tense weakness, so that the limbs of the animal fail under it, I have 

 seen an animal in this condition attempt to walk, and look round at its 

 legs with a puzzled air, as though it could not understand what was 

 the matter with it. It could not connect the weakness in its limbs 

 with the introduction of the poison some time previously, although the 

 connection between them was to me perfectly clear. 



