PHYSIOLOGY. 153 



medicines exciting the evacuations are actually conveyed and 

 applied to the secretories or excretories of the parts concerned ; 

 but this cannot possibly be supposed with regard to sweating, 

 not only from the small quantity of medicine employed, but per- 

 haps also from the nature of the excretion, which is certainly 

 not depending upon glands and their excretories. 



"Sixthly: Another circumstance inducing us to suppose me- 

 dicines to act only on the stomach, is, that of their being capa- 

 ble of being changed by the assimilating powers of the stomach 

 and intestines ; for such medicines, if they act at all, must act 

 immediately upon their entering into the stomach, or before 

 they are are changed by digestion. 



" It is true, with respect to vegetables, and also certain ani- 

 mal substances, it is often a certain portion of them only that 

 can be subjected to our digestive powers, while the medicinal 

 part of the same is hardly affected ; and therefore it may be 

 alleged, that their operation on the interior parts is not prevent- 

 ed by the powers of digestion. This indeed does certainly 

 sometimes happen ; but still, as digestion breaks down very 

 entirely the texture of vegetables, and evolves the several parts 

 of them more entirely than they were in the entire vegetable, it 

 thereby gives them an opportunity of acting immediately upon 

 the stomach, and even may thereby prevent their activity from 

 reaching beyond this organ. 



" Seventhly: Another circumstance which confines the oper- 

 ation of many medicines to the stomach, is their suffering a 

 change there, if not by digestion at least by mixture. 



" It appears to me very clear, that in all animals who take in 

 a quantity of vegetable aliments, and therefore in man, there is 

 an acid, and commonly in considerable quantity, very constant- 

 ly present in the stomach. It is therefore probable, that all 

 alkaline substances are more or less neutralized there ; and that 

 consequently, if they act at all as naked alkalines, they can act 

 only upon the stomach before they are neutralized. It appears, 

 however, that alkaline substances frequently prove powerful 

 medicines with respect to the remote parts of the system ; and 

 I think it must be concluded that their effects must be imputed 

 to their being changed into neutral salts in the stomach, and 

 operating in the other parts of the system as neutrals only ; or 



