Cathartics or Purgatives are substances which cause evacuation of the bowels either by direct local irritation of the intestinal mucous membrane or by setting up an osmotic current from the tissues toward the lumen of the intestines, causing an accumulation of fluid in the bowels - in both cases causing increased peristalsis and watery or semisolid evacuations.

Mechanism of Purgation. - It should be remembered that the epithelial surfaces through which the substances needful to the body enter it, and the waste-products leave it, are physiologically outside the body. The mucous membrane of the alimentary canal is in a sense as much external as the skin covering the surface of the body and is subject to the same irritating influences. The muscular mechanism of the intestines is somewhat peculiar in that it possesses the power to rhythmically contract and relax in a wave-like manner (peristalsis), the peristaltic wave traveling downward. These rhythmical movements of the intestines carry their contents along their lumen from the cardia to the rectum, and are to a large extent independent of the central nervous system, although controlled by nervous mechanisms. The normal contents of the small intestine are fluid, and are passed into the large intestine as such. In their passage along the large bowel the fluid part is largely absorbed and the semisolid part remaining is passed on into the rectum as feces.

In order that any substance may act as a purgative it must change the normal contents of the bowels in such a way as to cause fluid or semisolid evacuations. When a substance locally irritates the intestinal mucous membrane the intestines respond by increased peristalsis, which hurries the fluid contents of the small intestine through the large bowel so rapidly that absorption does not take place and the feces are evacuated in a fluid form.

Cathartics may be classified according to their various actions, the following table serving to show how and where the various drugs exert their several influences:

1. Classification according to their Mode of Action.

Laxatives.

Cassia. Castor oil. Cascara sagrada.

*Glycerin.

*Magnesia.

*Magnesium carbonate. Manna.

Sulphur. Taraxacum.

There are certain drugs which are not classed as cathartics, which are sometimes prescribed by physicians as laxatives, such as -

Belladonna.*

Ergot*

Hyoscyamus.*

Nux vomica.*

Physostigma.*

Stramonium.*

Certain articles of diet are laxative, such as bran biscuit, brown bread, gingerbread, oatmeal, figs, honey, molasses, prunes, raspberries, strawberries, tamarinds, olive oil, etc.

Simple purgatives.

Aloes. Calomel.* Cascara sagrada (full doses). Castor oil (full doses). Ox-gall. Rhubarb. Euonymus. Iris.

Juglans. Leptandra. Senna.

Hydragogue purgatives.

Croton oil (small doses). Elaterin. Gamboge.

Salines. Magnesium citrate. Magnesium sulphate. Potassium bitartrate.* Potassium sulphate. Potassium tartrate.* Potassium and sodium tartrate. Sodium phosphate. Sodium sulphate.

Drastic purgatives.

Cathartic acid (hypodermically). Colocynth. Croton oil. Elaterin. Gamboge. Jalap. Scammony. Podophyllin.

2. Classification according to their Manner of reaching the Intestinal Mechanism.

By first contact. Nearly all the drugs used as cathartics.

By circulation contact.

Morphine.*

Muscarine.*

Physostigma.*

Pilocarpine.*

Strychnine.*

By excretion contact.

Aloes. Castor oil. Croton oil. Colocynth. Elaterium. Podophyllin. Rhubarb. Senna.

3. Conditions of the Intestines affecting the Action of Drugs.

Drugs requiring the presence of an alkali or bile to act.

Aloes.

Elaterium.

Gamboge.

Jalap.

Scammony.

Sulphur.

Drugs requiring the presence of an acid to act.

Magnesium carbonate.* Magnesia.*

Drugs not requiring the presence of either alkali, bile, or acid.

Castor oil. Colocynth. Croton oil. Euonymin. Iris.

Leptandra. Magnesium citrate. Magnesium sulphate. Podophyllin.

Potassium and sodium tartrate. Rhubarb. Senna. Sodium phosphate.

(Drugs marked with an asterisk (*) are here given in detail; others are described elsewhere.)

4. Classification according to the Anatomical Portion of the Intestinal Canal on which they Act.

Small intestine.

Calomel.*

Castor oil.

Jalap.

Leptandra.

Podophyllin.

Rhubarb.

Scammony.

Senna.

Colon.

Colocynth. Elaterium. Gamboge. Magnesium citrate. Magnesium sulphate. Potassium bitartrate.* Potassium sulphate. Potassium tartrate.* Potassium and sodium tartrate. Sodium sulphate.

Descending colon and rectum. Aloes.

5. Classification of Cathartics according to Other Actions.

Stomachics.

Aloes.

Cascara sagrada.

Euonymin.

Leptandrin.

Iridin.

Rhubarb.

Hepatic stimulants and cholagogues.

Aloes.

Colocynth.

Colchicin.

Euonymin.

Iridin.

Leptandrin.

Podophyllin.

Sodium phosphate.

Sodium sulphate.

Galactogogues. Castor oil.

Rendering the milk purgative.

Aloes.

Castor oil.

Rhubarb.

Senna.

There are probably some other cathartics that affect the milk.

Increasing menstrual flow.

Aloes.

Cholagogues.

Aloes.

Calomel.*

Colocynth.

Euonymin.

Iridin.

Mercury with chalk.* Pil. hydrargyri.* Podophyllin. Rhubarb.

(Drugs marked with an asterisk (*) are here given in detail; others are described elsewhere.)

Intestinal peristalsis is increased probably by: Stimulation of:

1. The intestinal muscles (moderate stimulation);

2. The afferent nerves connecting the intestinal mucous membrane with Auerbach's ganglia;

3. Auerbach's ganglia;

4. The ends of the efferent nerves passing from Auerbach's ganglia to the intestinal muscles;