142 CHAPTER 17. 



or kidneys. They are respectively termed purgatives, diai)lioretics, and 

 diuretics. 



2nd. Depressants, including sedatives and nauseants, which lower the 

 action of the heart and nervous system. 



3rd. Stimulants, which rouse and excite the action of the heart and 

 nervous system. 



4th. Narcotics, which exert a primary stimulant, and secondary, but 

 more permanent, sedative effect. 



5th. Tonics, which impart tone and vigour to the system. 



The above medicines act on the system genei-ally. The following exert 

 chiefly a topical action. 



6th. Blisters, which cause irritation of the skin. 



7th. Caustics, which decompose solid tissues and fluids. 



8th. Astringents, which constringe muscular fibre, I'epress undue 

 graimlations, and cause contraction and condensation in the part to which 

 they are applied. In more familiar language, they dry up .sores and 

 induce the formation of a scab. 



9th. Demulcents, which act locally by protecting, so far as they reach 

 in an undiluted form, the lining of interior parts. 



10th. External dressings, which in cases of abrasions of the skin, 

 wounds, sores, &c., are useful in protecting exterior parts from the ins- 

 tating effect of the air and from flies, &c. 



11th. Emollients, which soften and relax the tissues and thereby lessen 

 pain, and allow the blood congested in the part to flow away more easily. 



12th. Cold applications, which constringe the structures, abstract heat, 

 and moderate inflammatory action. 



282. Mode in ivliich medicines produce their effects. 



Most medicines which are taken internally are absorbed into the blood 

 and circulation generally. But though absorbed, they do not seem to be 

 assimilated with it or with the structures of the body. On the contrary, 

 they are speedily ejected from the blood by their appropriate excretory 

 organs. This is especially the case with the large class of medicines in- 

 cluded under the comprehensive title of evacuants. They seem to pro- 

 duce their principal effect during the process of ejection, ^ — the organ 

 through which they are excreted being excited to increased action. 



Medicines, however, do not produce the same effects in all states of the 

 system. Their effects, even if not positively determined, are often much 

 modified by any morbid changes which may have taken place. Some 

 medicines, indeed, are completely changed when introduced into the 

 system. Alkalies, for instance, are neutralised when they meet with the 

 acid of the gastric juice. 



Again, most medicines which produce an effect on the skin, act also in 

 a certain degree on the stomach, intestines, or kidneys, because they are 

 more or less absorbed into the system, especially if applied in large 

 quantities. 



