tages, that they act with certainty, and produce a notable effect.

All medicines which by a mere external action augment the secretion of the bowels, or promote its evacuation, must be regarded as indirect Cathartics, for they do not operate on the eliminative principle. An outward irritation of the mucous membrane is sufficient to excite the peristaltic contraction of the bowel, and may even increase the secretion of that mucous surface by a reflex nervous action, in the same way that an irritation of the mucous lining of the mouth will cause the secretion of saliva. All substances which, after being taken into the stomach, are not absorbed, which are thus pushed onwards along the inner coat of the intestine, and by their mere accumulation excite its muscular contraction, must operate more or less as indirect purgatives. Of such a nature are the ligneous fibres of vegetables,-the bulky pulp of fruits, as in Prunes, Tamarinds, and Cassia,—and the husk of wheat in brown bread, all of which are known to act as laxatives. Very different from these in the intensity of their action are the resinous Cathartics, as Scammony and Gamboge, which, though capable of absorption, appear also to act in some cases by an external irritation of the intestinal surface. They are also true Eliminatives, for they are known to become absorbed; they may thus act in a double way, both directly and indirectly. (Vide p. 94.) In small doses it is probable that they are simply Eliminatives, but when given in large doses they may add to that an irritant external action, and produce a very powerful or even dangerous effect.

True Cathartics.-These are very many in number. At whatever part of the system they are introduced, their action is the same. If one of them be injected in solution into the veins, or absorbed from the surface of the skin, it passes at length to the intestinal canal, is excreted by the glands of the mucous surface, and causes purging by augmenting their na

tural secretion. When given as a medicine, the Cathartic is first received into the stomach. It is capable of absorption,— whether it be soluble in water, or in acid, or be of an oily or resinous nature, as was ascertained in the consideration of Proposition II. It is absorbed; and passes along in the circulation. But it is unnatural to the blood, and cannot remain in it; so that it is at length expelled by the eliminative force at the lower part of the intestinal canal, at a part which is more active in excretion than in absorption, though not very remote from the absorbent surface at which the remedy first entered. It is a common, but erroneous notion, that Cathartics do good simply by removing irritating matters from the intestines. In most cases they also purge the blood. This action of Cathartics by absorption and subsequent secretion is illustrated by the experiments which I have made on the operation of Sulphate of Magnesia-to be detailed in Chapter IV. We have many proofs of the fact that Cathartics will act from the blood as well as from the bowels. Solutions of Jalap, Gamboge, Rhubarb, and other such substances, will produce purging if introduced into the blood at any part. And Aubert finds that a solution of Sulphate of Magnesia, when injected into the veins of an animal, acts on the bowels.

Cathartics may be advantageously divided into three groups: -1. Mercurials, which tend to increase all secretions; 2. Some resins, oils, and acrid principles, which tend especially to the bowels; and 3. Salines, when given in such amount that they cannot pass off by the kidneys.

Mercurials, being also Cholagogues, are especially useful in bilious habits. When given to act on the bowels, a Mercurial is generally conjoined with another purgative, that it may not do damage in the system by remaining in the blood.

In the second group the great majority of Cathartics are included. They vary very much in the intensity of their action.

Some are mild, and may be administered in inflam

mations and fevers, or even in pregnancy, where a gentle action is required; others are powerful and drastic, and may cause serious congestion, or even inflammation of the bowel. All the resins and acrid principles are more or less heating, and should be administered very carefully in febrile states of the system. Of the resins, Jalap is comparatively mild, and may be given to children without risk; while Scammony, Colocynth, and Gamboge, are more drastic. Of the oils, Olive-oil is merely laxative; Castor-oil may be given in all cases without danger; but Croton-oil is a dangerous hydragogue Cathartic. Of the medicines which owe their efficacy to acrid principles, there are some which are mild in operation, and whose principles are soluble in water. Such are Rhubarb, Aloes, and Senna. Senna is somewhat irritant. Aloes appears to act on the lower part of the intestine, and is therefore objectionable in cases of pregnancy or of uterine disorder. Hellebore is a more powerful acrid, but is now seldom used. Elaterium is the most potent purgative known. In cases of dropsy, when all other medicines have failed, onetwelfth of a grain of this substance has been known to produce a copious evacuation.

Salines, i. e. salts of the alkaline and earthy metals, are all more or less purgative when given under certain conditions. Under other circumstances they may pass off from the body by the kidneys or the glands of the skin. The circumstances which determine the excretion of saline matters appear to be simple in nature. In the common condition of the body it is not possible for the secretion of the skin to be very largely increased, or rendered fluid. Suppose then a soluble saline, as the Sulphate of Soda or Magnesia, or Tartarized Soda, to have obtained entry into the blood, it has the choice of being excreted by the kidneys or the bowels. The alternative appears to depend mainly upon the amount of the dose. A small quantity may pass in the urine, and will not produce

purging. But a large quantity cannot so pass; it is excreted by the glands of the bowels, and acts as a Purgative. It has been supposed by some that a saline solution proves Purgative or Diuretic according to the degree of its dilution only. This matter we have already considered at some length; and the reasons which have induced me to arrive at a contrary conclusion have been detailed under the head of Proposition II. (See also Chap. IV., Art. Sulphate of Magnesia.)

Salines are hydragogue. Requiring water for their proper solution, and having further a great affinity for it, they convey a large quantity of the aqueous part of the blood with them through the glands of the bowels. When their action is very powerful, some of the albumen of the blood may be purged away along with this. This may also take place with the drastic resinous Cathartics. But the latter are much more violent in their action, producing a degree of griping and irritation which is dangerous in febrile cases. Salines, on the contrary, are cooling, and mild in their operation. They are appropriate in inflammations, not only for this reason, but because, while passing through the blood, they exert in it, as we have already seen, a mild action of an antiphlogistic nature. (Vide page 196.)

When the vegetable salts of the alkalies are given in diuretic doses, they are decomposed into carbonates while in the system. This does not appear to be the case when they are given in such quantity as to pass off quickly by the bowels.

These various Cathartic medicines are affirmed to act on the eliminative principle: it follows then that they must themselves pass out of the body along with the secretion which is augmented by their action. It is natural that the faces should have been less examined than other secretions. The resins of Jalap, Scammony, and other such substances, are affirmed to pass out along with them. From Rhubarb and Gamboge they derive a light yellow colour. Of the passage of saline Purga

tives we have long had an indirect proof; which is, that after their administration the amount of saline matter in the urine is not materially increased. But lately Buchheim has discovered the sulphates of soda and magnesia in the fæces, after purging has been produced by their administration. Castoroil is seen in the evacuations, sometimes little altered, at other times in the form of a solid fatty substance. M. Lehmann has detected Mercury in the fæces, whenever it was given as a Purgative, and whatever the colour of the evacuation. Herrmann and Merklein have done the same. (See Lehmann's Physiolog. Chemistry, vol. ii.) When combined with a sufficient amount of Opium, it neither acts as a Purgative, nor does it pass out by the bowels; and it has therefore in that case a better opportunity of exerting its operation in the blood. Buchheim, too, has prevented the action of saline purgatives by hindering their excretion by means of a dose of Morphia or Tannic acid. We always find that when a Cathartic passes off in some other way than by the glands of the bowels, it fails to produce purging. Thus a copious dilution with water may sometimes cause it to be excreted by the kidneys, which are the natural emunctories of water. In some

persons that are wont to perspire very freely it is difficult to produce purging. The case related by Dr. Ward, of a woman with whom a dose of Castor-oil was seen to pass off by the skin, and invariably failed to act on the bowels, is an extreme instance of this kind.

Of what use and intention, we may now venture to ask, is this intestinal function, the continual maintenance of which in a healthy condition is found to be so essential?

It was some time ago supposed that the fæces consisted simply of those parts of the food which remained unabsorbed, and that all Purgative medicines alike acted by exciting the peristaltic motion of the bowels, and causing thus the ejection of these undigested matters. Such an opinion is now rarely