5. The ends of the afferent nerves passing from the intestinal mucous membrane to the brain;

6. The motor centers in the brain;

7. The ends of the motor nerves terminating in Auerbach's ganglia. Depression of:

8. The inhibitory motor center;

9. The ends of the inhibitory motor nerves terminating in Auerbach's ganglia;

10. The inhibitory motor center in the suprarenal plexus.

It will be seen that any substance which stimulates the motor apparatus or depresses the inhibitory motor mechanism will increase peristalsis.

Intestinal secretion may doubtless be promoted by any substance which serves to stimulate the secretory or the vasodilator apparatus, or to depress the inhibitory secretory or vasoconstrictor mechanism.

The methods by which absorption is diminished are not thoroughly understood, but it is known that:

1. By increasing peristalsis and hastening the removal of fluid from the bowels absorption takes place less rapidly;

2. By giving drugs - e.g., magnesium sulphate - having high osmotic equivalents, with a great affinity for water, the absorption of fluid is prevented;

3. Substances which in some manner affect the columnar epithelium of the intestinal glands retard absorption;

4. Drugs which diminish the circulation in the intestinal mucous membranes act as deterrents to the absorptive process.

It is apparent that certain drugs produce various effects, and that their mode of action varies according to the size of the dose and occasionally with the idiosyncrasy of the patient.

Nearly all cathartic drugs act by some local influence upon the intestinal mucous membranes previous to absorption; others, again, affect the bowels after they have entered the circulation - strychnine, for example, physostigmine, pilocarpine, etc., acting in this manner.

Certain other drugs, such as podophyllin, colocynth, etc., if injected into the circulation, are excreted by the mucous membrane of the intestines, and by their irritation produce catharsis.

The condition of the intestinal canal has much to do with the activity of certain drugs. Thus, certain medicines produce catharsis regardless of the reaction of intestinal fluids; others are inert without the presence of bile or other alkaline fluids or salts; and still a third class occasion catharsis only when after ingestion they come in contact with an acid. Of the last-mentioned, magnesium carbonate is an excellent example, the drug being inert unless it be acted upon by an acid in the stomach or bowels.

It is an interesting fact that, as shown by experience, different cathartics may act more energetically upon different portions of the intestines. The action of calomel, for instance, is almost entirely confined to the duodenum, while aloes acts largely upon the descending colon and the rectum.

In selecting a cathartic, therefore, a knowledge of the part of the intestinal canal to be acted upon and the locality in which the drug operates is necessary in order to secure the most satisfactory results.

Many cathartics contain principles which render them tonic to the stomach; some few are thought to directly stimulate the hepatic cells - mercury preparations; while the cholagogues merely hasten the expulsion of the bile from the intestinal canal, preventing its absorption.

Certain drugs, being excreted in the milk, which it renders purgative, are well adapted for administration to the nursing mother in order to produce catharsis in the infant. Castor oil, greatly augmenting the secretion of milk, is an excellent medium as a laxative in such cases.

Aloes increases the menstrual flow; other drugs promote the secretion of urine, etc.

Therapeutics. - Cathartics are employed:

To remove feces and produce a simple evacuation of the bowels. The laxatives are best adapted for this purpose.

For the relief of chronic constipation. For this purpose great judgment is requisite in the selection of a drug or combination of agents, it being important to determine whether there is diminished peristalsis or secretion; whether there exists an atonic condition of the intestinal muscles; or whether the disorder is located in the small intestine, the colon, or the rectum.

To remove from the bowels noxious substances or pathogenic matter. For this purpose the mercurial preparations, calomel or gray powder, are best, since they are not only active cathartics, but bactericides as well.

To stimulate the torpid liver. For this purpose the hepatic stimulants would naturally be employed.

To lessen the activity of the liver, as in bilious conditions. In such cases the cholagogue cathartics should be used.

To deplete the gastroduodenal mucous membrane, where the congested and swollen mucous membrane obstructs the outflow of bile, resulting in jaundice. In this condition the salines, especially the sodium salts, are the most efficient cathartics.

To promote absorption and remove dropsical effusions in certain diseases of the heart, liver, and kidneys. Here active catharsis is necessary, the hydragogue cathartics being indicated.

To remove urea, etc., from the blood. Occasionally in certain renal diseases the functional activity of the kidneys is so defective that waste-matter, urea, etc., rapidly accumulates in the body, occasioning uremic convulsions, coma, or other serious symptoms. In such cases it may be necessary to give a drastic purgative, such as croton oil, which acts rapidly, causing profuse watery stools.

To lower the blood-pressure where high arterial tension aggravates a malady, as at the onset of many acute diseases, and in cerebral hemorrhage, meningitis, etc. In these conditions it is necessary to employ such drugs which, by dilating the intestinal blood-vessels, drain the blood away from other organs and cause abundant watery discharges from the bowels. Hydragogue or drastic purgatives answer the required purpose.

For the relief of hemorrhoids, in which cases the mild laxatives, such as sulphur, senna, etc., are serviceable.

To aid the restoration of the catamenia. For this purpose aloes is usually employed, particularly if it be necessary to determine more blood to the pelvic organs. If depletion be required, the selection should be made from the hydragogue cathartics.

To purge the nursing infant through the mother's milk. For this purpose such drugs as rhubarb, senna, and castor oil may be administered to the mother.

To lower the temperature in fever, in which cases the saline cathartics may be advantageously employed.

Contraindications. - Active catharsis by the more powerful hydragogue or drastic purgatives would be contraindicated in appendicitis, peritonitis, typhlitis, intussusception, pregnancy, and typhoid fever, or where there is inflammation of the mucous membrane of the gastro-intestinal tract.

Administration. - Probably no group of medicines demands greater judgment in the administration than cathartics.

Ordinarily, the efficiency of these agents is increased and their operation rendered less irritant by associating drugs acting upon different portions of the alimentary canal. Their action, too, is more prompt and certain when the remedies are given upon an empty stomach and the efficiency of their operation is enhanced by exercise and diminished by sleep.

The action of cathartics is promoted by the addition of small doses of emetics, mydriatics, quinine, and bitters, quinine especially strengthening the action of magnesium sulphate. Mild diluent beverages also promote the activity of cathartics. Cold applied to the abdomen, enemata, massage of the abdominal walls, and electricity, all act as adjuvant measures in the employment of purgative medicines.

As has been previously suggested, a knowledge of the portion of the intestinal canal upon which the various cathartics act is of primary importance. Thus, if it be necessary to influence only the duodenum, calomel or podophyllum should be used; if the small intestine, senna or jalap; if the descending colon or rectum, aloes - these drugs acting chiefly upon these organs.

Moreover, due consideration should be given to the proper time for the administration of the different cathartics, the resinoid purgatives acting best when taken at night or before dinner, and the salines when taken in the morning before breakfast.

The mode of administration is also of great importance, in order to obtain from these agents the fullest benefit. The salines, for instance, act best when given in solution in either very cold or very hot water, their activity being enhanced by association with bitters, iron, or sulphuric acid. On the other hand, the resinoid drugs should be. administered in the form of pills, and if, for any reason, it is desirable that the drug should enter the intestine without coming in contact with the mucous membrane of the stomach, the drug may be given in the form of pills coated with keratin, which is unaffected by the gastric juice, but readily dissolved in the alkaline intestinal juices.

In the following detailed description cathartic drugs are grouped according to their modus operandi, the mildest drugs or laxatives being first considered.