8 PRELIMINARY CONSIDERATIONS 



possess to influence one organ or apparatus rather than the whole system. 

 Oftentimes this local action, in the case of secreting glands, is accom- 

 plished through stimulation of these parts during elimination of the drug. 

 Occasionally a medicine acts both on the part with which it comes in 

 contact and also through the circulation ; e. g., tartar emetic causes emesis 

 by local stimulation of the stomach and by stimulation of the vomiting 

 centre after absorption. Furthermore, remedies are said to exert a pri- 

 mary (or immediate) and secondary (or remote) action. 



The secondary effect is the result of the primary action; e. g., a 

 saline cathartic primarily removes serous fluid from the bowels and sec- 

 ondarily or remotely leads to absorption of serous exudations ; a counter- 

 irritant primarily produces irritation of the skin and sensory nerve-end- 

 ings, but secondarily relieves internal congestion by inducing reflex 

 contraction of the subjacent blood vessels. Most drugs are absorbed into 

 the blood after their ingestion and exert their action on various parts of 

 the body through the medium of the nervous system. Some drugs, how- 

 ever, may directly influence muscular tissue, as is seen in the action of 

 digitalis on the nerve-free heart's apex; while others may immediately 

 act on the cells of an organ, as pilocarpine on the sweat glands. As in 

 the latter instance, it is usually impossible to determine whether medi- 

 cines affect the cells of an organ or nerve-endings in the organ. The 

 action of most vegetable drugs is thought to arise from the chemical 

 affinity of their active principles for the part or parts acted upon. 



Thus the selective action of strychnine depends upon its forming a 

 chemical compound with the protoplasm of the cells of the spinal cord. 

 The affinity of certain cells of the tissues and micro-organisms for specific 

 substances is shown in the staining of the nervous system alone by intra- 

 venous injection of methylene blue. It is, in fact, the basis of all bac- 

 teriological stains, of Ehrlich's theory of immunity, and of his wonderful 

 discovery of specifics for nagana, syphilis and relapsing fever. 



All substances are divided into electrolytes and non-electrolytes. 



Electrolytes are capable of decomposition into ions. The action of 

 electrolytes, when used as medicines, is that of their ions. An ion is an 

 electrified molecule or a molecule of a substance having a charge of posi- 

 tive or negative electricity. 



The action of most salts, acids, and bases depends on their being in 

 great part dissociated in the weak solutions found in the tissues into 

 electrically positive (kation) and negative (anion) ions. Nor does the 

 action of an ion represent the chemical action of the atom, as when KCl 

 is dissociated into a positive K ion and a negative CI ion. The action of 

 the, ion is a physical or electrical action. Sometimes one ion is inert, as 

 the CI ion in KCl. Sometimes one is inert and the other very toxic, as 

 KCN, where the positive or K ion is practically without action (see action 

 of ions under special salts). In organic drugs the action of one ion is 

 usually so powerful that the other may be neglected, as morphine sul- 

 phate. In case the ions of inorganic salts are inactive their medicinal 

 effect may be due wholly to what is technically termed "salt action." 



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