SPECIAL AFFINITY FOR PARTICULAR ORGANS 31 



stream of blood each tissue takes up its appropriate nutrient 

 materials, and, in like manner, it appears to select its own 

 medicines. The characteristic effects are not developed 

 until medicines come into actual contact with the special 

 organs, or, it may be, the particular cells, on which alone 

 they operate. Curare does not exert its paralysing power 

 until it reaches the intramuscular endings of the motor 

 nerves. Magendie found that strychnine does not excite its 

 notable tetanic convulsions until it is in contact with the 

 spinal cord. Indeed, when a frog or other small animal, 

 immediately after receiving a full dose of strychnine, has 

 the spinal cord removed or destroyed from above downwards, 

 tetanic symptoms are prevented in the same order. 



On the particular part on which they act as, for example, 

 on the nerve-centres or nerve-endings that control blood- 

 vessels or glandular secretions some medicines exert 

 stimulant, others depressant or paralysing effects. These 

 effects, as already indicated in the case of drugs acting 

 locally, frequently produce reflexly indirect or remote 

 effects. The same medicine sometimes acts differently 

 when given in different doses. Thus, alcohol and ether 

 in small doses are stimulants, but in large doses are 

 depressants. 



Within the living body most medicines not only effect 

 changes, but themselves coincidently undergo changes, 

 notably of oxidation or deoxidation. Thus, many salts of 

 tartaric, acetic, and other organic acids are converted into 

 carbonates. Morphine has its chemical constitution altered, 

 and in an animal habituated to its use, it is all broken down 

 by the tissues so that none is excreted, and none can be 

 found by analysis on post-mortem. It has, in fact, been 

 destroyed. The activity of medicines depends materially 

 on their solubility, the rapidity of their absorption, and the 

 period during which they remain within the body in other 

 words, on the speed of their excretion. Some drugs, as lead, 

 mercury, silver, and digitalis, are apt to be retained for a 

 considerable period, and hence have more or less continuous 

 or cumulative effect. Unusual activity of such excreting 

 channels as the bowels or kidneys hurries most medicines 

 out of the body, and hence diminishes their action. 



