238 EXPErJMENTxVL INTESTTGATION OF THE ACTION OF MEDICINES. 



faster. But in the rabbit, its power is comparative!}^ small, and 

 the increased rapidity of the pulse after its division is but 

 slight. In most dogs, on tlie contrary, its power is great, and if 

 it be cut, the heart beats very much quicker, and sends more 

 blood into the arteries, so as to raise the pressure in them. If 

 we measure tlie pressure of the blood in tlie arteries of a rabbit 

 and of a dog, and then cause them to inhale nitrite of amyl, we 

 iind that the small vessels have become widened and allow the 

 blood to pass easily out of the arterial system into the veins, so 

 that the pressure sinks considerably in the rabbit, but it sinks 

 only slightly in the dog. The effect seems at first sight 

 different ; but w^hen we examine it more closely, we find that 

 the heart of the dog is no longer beating slowly, but very 

 quickly, so as to keep up the pressure, notwithstanding the 

 rapid flow of Ijlood tlirongh the widened vessels, while the 

 heart of the rabbit was going so fast before that it could not go 

 much more quickly. If we cut the vagi in the dog, so that the 

 "heart goes as quickly as in the rabbit before it begins to inhale, 

 the blood pressure sinks during the inhalation, just as it does in 

 the rabbit.* 



I have given these examples at length, because of their 

 important bearing on the question how far conclusions as to 

 the action of medicines on man may be drawn from those 

 which they exert on the lower animals. Now, the action of 

 ■curare in paralysing the ends of motor nerves is one of the 

 .-simplest and least complicated examples that we can take, as 

 the very nature of its action prevents disturbances in other 

 systems from showing themselves ; and we find that it is 

 •exactly the same in the Indian who accidentally wounds him- 

 self with his poisoned arrow, in the game which he shoots, or in 

 the frog on which we experiment. ^ 



Motor nerves, the structure on which curare acts, are alike 

 present, and in all are its results the same. 



As we have seen that in the lower animals, differences in 

 the action of drugs are produced by differences in the structure 

 of the animal, and that the former disappear when the latter 



* Lauder Brunton, " Action of Nitrite of Amyl on the Circulation," Journal 

 of Anatomy and Physiology ^ vol. v, p. 95 {vide antea, p. 176). 



