ACTION AND USES OF MEDICINES. 143 



Some medicines are believed to produce their effects by acting on the 

 nerves of the part which absorbs or excretes them. 



Though we can explain in some degree how medicines produce their 

 effects, yet we cannot explain why particular medicines affect particular 

 organs why, for instance, aloes acts on the intestines and resin on the 

 kidneys. It is only by continued observations and experiments that these 

 effects have been ascertained to be facts. In applying medicine to the 

 treatment of disease we endeavour to utilise our knowledge of facts, 

 though we cannot always explain the reason of them. 



283. Purgatives. 



Some purgatives seem to act generally on the intestinal canal. The 

 effects of others are confined to the large intestines, but the great majority 

 produce their effect on the small intestines. 



Though the immediate action of purgatives consists in causing evacua- 

 tion of the contents of the intestines, they also produce an effect on other 

 organs ; because the intestines when excited to increased action draw off 

 secretions from all parts. Thus a dose of purgative medicine will often 

 reduce swelled legs, because the increased action of the intestines drains 

 off the watery parts of the blood from other portions of the system. 

 Again, they often are useful in carrying off those noxious matters which, 

 from impaired secretion during disease, are apt to accumulate in the 

 blood, and tend, if not removed, to keep up fever and inflammation. 



Although the active operation of purgatives is only temporary, yet their 

 results are often permanent. By their action the intestines are relieved 

 from undigested materials or accumulated faeces, the blood is freed from 

 impurities, and the liver and other excretory organs are roused to healthy 

 action. 



Before purgative medicine can be safely given, it is absolutely necessary 

 that the horse should be well " prepared for physic," that is, deprived for 

 at least thirty-six, and if possible forty-eight, hours of all food except 

 cold bran mashes, which are in themselves laxative and tend to assist the 

 action of the medicine. Physic does not take any effect until it is brought 

 into contact with the mucous linings. If the stomach and intestines are 

 full, the purgative may never reach those linings. It may pass through 

 with a mass of food, and its properties may not be extracted, or it may 

 act violently in a mass on the part of the lining which it reaches. On 

 the other hand, when the stomach and intestines are somewhat empty, 

 the medicine is diffused over a large tract of mucous surface, and acts 

 more speedily and more safely. 



After the administration of the medicine the patient should be freely 

 supplied with tepid water. If, however, he refuses tepid water, he may 

 be allowed water from, which the chill has been removed by allowing it to 

 stand for a few hours in a warm room or kitchen. The diet must be 

 restricted to sloppy warm bran mashes. A little walking exercise or a 



