Dr. Buchanan on the Effects of the Inhalation of Ether. 155 



if I were addressing a body of medical men, fully conversant with the 

 subject ; but I am persuaded that, in addressing a general audience on 

 such a subject, it will not be thought out of place for me to premise some 

 general remarks as to the mode in which medicines operate on the human 

 body, so that the place which ether occupies as a physiological agent, may 

 be the more readily understood. 



Medicines, then, may be divided into four classes, according to the 

 mode in which they affect the human body : 



Those of the first class act altogether locally. We have familiar 

 examples of them in mechanical agents applied to the surface of the 

 body, in tho diluted and strong acids, liquid ammonia, mustard, and 

 cantharides, all of which produce inflammation and other local effects on 

 the parts to which they are applied, but do not necessarily implicate 

 parts at a distance. Ether, and many other substances, have a local 

 irritant power of this kind, combined with a power of a more general 

 nature. 



The medicines of the second class operate by local sympathy. They 

 are, in so far, local agents, that they must always be applied to the same 

 spot; but the local impression influences distant organs sympathetically. 

 Tobacco and other irritants applied to the nostrils operate in this way; 

 the local impression they produce on the membrane of the nose is propa- 

 gated, through the nerves, to the diaphragm and abdominal muscles, 

 which are thus made to contract, and produce the act of sneezing : whence 

 we name them sternutatories. Many important medicines operate in this 

 way — many emetics, for instance, such as mustard, and the sulphates of 

 zinc and copper, which, exercising an irritant action on the stomach, call 

 into play, sympathetically, the muscles concerned in the act of vomiting. 

 Almost all purgatives also, such as castor and croton oils, jalap, senna, 

 and aloes, act in this way. They irritate the mucous membrane of the 

 bowels, and the impression is propagated, by sympathy, to the expulsive 

 muscles. It is a most erroneous idea, which some medical men entertain, 

 that such medicines will operate when applied to the skin, for they can 

 only operate when applied to the membrane of the bowels. Croton oil, for 

 instance, when used as a liniment to the skin, or even applied to an 

 abraded surface, never operates but as a local irritant. 



The medicines of the third class require to be absorbed by the blood 

 vessels, in order to produce their effects, which are thereafter exerted on 

 the organs of nutrition and secretion. Iodine is a good example of the 

 former; nitre, squill, and turpentine, of the latter. Such medicines 

 produce the same effects, to whatever part of the body they are applied, 

 provided it be an absorbing surface. Iodine and mercury act in the same 

 way, whether applied to the skin or to tho stoniaeh 



Tho modieiues in the fourth class act on the nervous system, either 

 after absorption, or directly. The former may be said to be their general 

 mode of action ; but there are some substances, such as the prussic acid, 

 <»{' which the effects arc manifested so instantanctni>l\ -, that we can scarcely 



