Purgatives are substances which cause intestinal evacuations. They are divided according to their nature into laxatives, simple, drastic, and saline purgatives, hydragogues, and cholagogues.

Laxatives are those which increase only slightly the action of the bowels and render the motions slightly more frequent and softer, without causing any irritation. Most articles of food which leave a large indigestible residue in the stomach act as laxatives : such are oatmeal, brown bread, and bran biscuits. Articles of food which contain salts of vegetable acids and sugar in considerable quantity also act as laxatives. The chief laxatives are: 1 Report to Brit. Assoc, 1874.

Honey.

Treacle.

Gingerbread.

Manna.

Cassia.

Tamarinds. Figs. Prunes. Stewed apples.

Sulphur. Magnesia. Castor-oil (in small doses).

Figs, raspberries, and strawberries, in addition to containing sugar and vegetable acids, have a number of small seeds which are absolutely indigestible, and these have probably a mechanical action in stimulating the bowel.

Simple purgatives also, when given in small doses, act as laxatives: such are carbonate of magnesium, magnesia, olive-oil, and sulphur.

Simple purgatives are more active than laxatives, and their administration is usually followed by one or more copious and somewhat liquid stools. Their action is sometimes accompanied by some irritation and griping. These are :Aloes.

Rhubarb.

Rhamnus (various species), e.g. Frangula and Cascara

Sagrada.

Senna. Castor-oil.

Drastic purgatives are those which cause violent action of the bowels, usually accompanied by evidences of greatly increased peristaltic action, such as borborygmi. They cause irritation of the intestine, and when taken in large doses produce inflammation and symptoms of poisoning. These are :Elaterium. Colocynth. Jalap. Scammony.

Gamboge.

Podophyllin.

Croton-oil.

Saline purgatives consist of neutral salts of metals of the alkalies or alkaline earths. The more commonly employed are :Sulphate of potassium.

,, sodium.

,, magnesium.

Phosphate of sodium. Tartrate of potassium.

Bi-tartrate of potassium. Tartrate of potassium and sodium. Citrate of magnesium. Sulpho-vinate of sodium.

Hydragogues are purgatives which excite a copious secretion from the intestinal mucous membrane and thus remove much water from the body; some of them belong also to the drastic group and others to' the saline.

Bi-tartrate of potassium.

Elaterium.

Gamboge.

Cholagogue purgatives are those which remove bile from the body. Some drugs aid the removal of bile by stimulating the secretion of the liver, but these, when they have no purgative action, are classed as hepatic stimulants. Cholagogue purgatives probably act by quickening peristaltic action of the duodenum and small intestine, thus preventing the absorption of the secreted bile.

Aloes. Rhubarb.

Mercurial preparations (blue pill, calomel, grey-powder).

Euonymin.

Iridin.

Podophyllin.

Action of Purgatives. - Purgatives may act in three ways: 1st, by quickening the peristaltic action of the bowels; 2nd, by increasing secretion of the intestinal membrane, and thus to some extent washing out its interior; 3rd, by hindering absorption of the fluids of the intestines.

Simple purgatives act chiefly by stimulating peristaltic movements, and have little effect on the secretion.

Hydragogue and cholagogue purgatives increase the secretion more than the peristaltic action, and drastics increase both. It has been held by several eminent German pharmacologists that the more watery stools produced by many purgatives are due only to more rapid peristaltic action, which hurries along the intestinal contents before there has been time for the absorption of their fluid constituents.

This opinion is chiefly based on the observations of Thiry and Radziejewski.

Thiry isolated a small piece of intestine, one end of which he attached to the abdomen and the other he sewed up. The part of the intestine from which this piece had been removed was again united by sutures, so that the intestine was perfect as before, though rather shorter. The small bag of intestine retained its vascular and nerve supply uninjured and secreted readily when tickled with a feather; but purgative medicines, such as croton-oil, senna, sulphate of magnesium, aloes, jalap, and sulphate of sodium, when applied to it, produced no increased secretion. These experiments led pharmacologists to believe that the ordinary idea that purgatives produced increased secretion from the intestine was erroneous; and the necessity for any such supposition seemed to be removed by an experiment of Radziejewski, who made an intestinal fistula in the ascending colon of a dog, and found that the intestinal contents as poured into the large from the small intestine exactly resembled the stools which ordinarily follow the administration of a purgative.

The ordinary phenomena produced by purgative medicines would therefore seem to be readily explained by increased peristalsis alone, but some other experiments by Colin and by Moreau have shown that the method employed by Thiry did not afford trustworthy results as to the action of purgatives on the intestines. Moreau isolated three loops of intestine by means of ligatures, carefully emptying the loops beforehand. He then injected a purgative medicine into the middle loop and returned the intestine to the abdomen. On examination some hours afterwards, it was found that, although all three loops had been under similar conditions, the one into which the purgative had been injected was distended with fluid while the others remained perfectly empty. These experiments were repeated by Vulpian, and afterwards by myself, with similar results. There can be no doubt whatever, then, that purgatives act both by increasing peristaltic action and intestinal secretion. Some purgatives act chiefly by the one, and some chiefly by the other.