CLASSIFICATION OF CATHARTICS 109 



Drastic purgatives, such as croton oil, colocynth, ela- 

 terium, gamboge, and podophyllum, which greatly increase 

 peristalsis, violently stimulate intestinal contractions, caus- 

 ing more or less pain, promptly produce copious, fluid dis- 

 charges, and in large doses may cause serious intestinal 

 irritation and inflammation. Eserine and barium chloride 

 exert similar effects. 



The term hydragogue is often applied to those agents 

 which greatly increase the fluid contents of the faeces, 

 examples being the more active salines in large doses and 

 the drastics. 



Cholagogue purgatives, such as mercurial preparations, 

 podophyllum, and euonymin, remove bile, and will be 

 further noticed. 



Saline purgatives consist of neutral salts of the alkalies 

 and alkaline earths, such as magnesium sulphate and citrate, 

 sodium sulphate, potassium tartrate, and bitartrate. 



The salines produce purgation by greatly increasing the 

 amount of fluid in the bowel, and this they do by drawing 

 water from the blood and tissues through the bowel wall. 

 These salts in solution undergo dissociation to a greater 

 or less extent, and their purgative action largely depends 

 on their rate of absorption, which varies with each salt. 

 Potassium and sodium ions are freely absorbed, but mag- 

 nesium very slowly and with difficulty. Thus a magnesium 

 salt, other things being equal, will remain longer in the 

 bowel, and will exert its osmotic effects to draw out more 

 water than a sodium or potassium salt. Again, the Cl ions 

 are very rapidly and easily absorbed, the NO 3 ions less 

 rapidly, and the S0 4 ions hardly at all. Thus chlorides are 

 not nearly so effective as sulphates in producing purgation. 

 Since in magnesium sulphate neither of the ions is absorbed 

 to any extent, this salt is an excellent saline purgative, for 

 it remains in the bowel and exerts its osmotic power until 

 sufficient water has been drawn out to make it isotonic. 

 Salines act especially on the small intestines, but only very 

 slightly increase the secretion of bile or pancreatic fluid. 

 When the accumulated fluid mechanically distends and 

 stimulates the intestine, extra peristalsis is excited. These 

 salines excite very little intestinal secretion when injected 



