These are medicines which increase the flow of bile. In this broadest acceptation of the term, it does not necessarily follow that they increase the secretion; for, as bile collects normally in the gall-bladder, and sometimes, from impediment to its passage, in the biliary ducts, whatever produces an extraordinary amount of pressure on the liver and its appendages will force it out into the duodenum, whence it may escape by the stomach or rectum, and thus become visible. Such a pressure is exerted in the act of vomiting, especially when severe and protracted; and emetic substances, consequently, not unfrequently act as cholagogues, in this sense of the word. The bile, under these circumstances, does not generally appear in the first discharge from the stomach; because some time must elapse after compression, before it can pass through the ducts, and ascend from the duodenum into the stomach. Hence, tartar emetic is more apt to occasion bilious discharges than ipecacuanha; as, independently of its greater violence, it is more frequently followed by repeated acts of vomiting.

It is not impossible that purgatives, which occasion much straining at stool, may have the same effect of emulging the liver by a mere mechanical action; in which case, also, the bile would appear, not in the first, but in some of the subsequent passages, in more than ordinary proportion.

The appearance, therefore, of bile in the discharged contents of the stomach, or in unusual quantity in the alvine evacuations, must not be considered as a certain evidence of an augmented secretion.

Nor is the absence of coloured bile from the feculent discharges a positive proof of suspended or diminished secretion of the liver. Leaving out of consideration those comparatively rare cases, in which mechanical obstruction is offered to the escape of the bile into the duodenum, as by the presence of biliary calculi in the tubes, and in which there is of course, without a necessary diminution of secretion, an absence of bile from the evacuations, there may be others, in which no obstruction exists, and in which, nevertheless, there may be the same want of colour, with an undiminished amount of secretion. When bile is chemically examined, it is found to be a complex fluid, of which the colouring matter is only one of the ingredients. The colour, therefore, like that of the blood, is not inherent in the whole fluid, but only in a particular principle contained in it. This colouring principle, however, is so generally found in bile, that it may be considered as a characteristic ingredient, as urea is of the urine; but, as urea may be morbidly wanting in the urine, so may this colouring matter also be in bile. Therefore, the absence of it in the alvine evacuations is not a positive proof of suspended action of the liver; but only that the organ does not secrete the yellow matter. it follows, too, that medicines may increase the biliary secretion, without any evidence of that increase being presented in the stools. They may increase the non-coloured ingredients, without the coloured; as diuretics may greatly augment the flow of urine, without any increase of urea. Unhappily, however, we have no means of deciding, in any particular case, whether there is or is not an increase of biliary matter, if unattended with the colouring principle, unless by an elaborate chemical examination, which few practitioners will be disposed to undertake. We cannot, therefore, determine, except conjecturally, in relation to any particular medicine, whether it is Cholagogue or not, when the evacuations are either of the natural colour, or deficient in colouring matter.

It is an opinion which I advanced forty years since, that the colouring matter of bile is not a product of secretion, but is merely separated by the liver from the blood, in which it exists normally, as a result of the disintegration of the tissues, or more probably of the blood-corpuscles. in this view of its origin, it may be increased or decreased in the bile, without a necessary alteration in the proportion of the other constituents. But, as its augmentation is the only certain evidence we have of an increase of the hepatic secretory function, and consequently the only proof of the excitant influence of any medicine on that function, we can consider as Cholagogue, so far as the act of secretion is concerned, only those substances which give this proof of their operation.

We may admit, therefore, two modes of obvious Cholagogue action; one by mechanical pressure, forcing out the accumulated bile; the other by an increased secretion, so far as it can be measured by an increased elimination of the bilious colouring matter. in reference to the first method, it may be left out of consideration here, as having only an incidental relation to the subject; the special object being at present the consideration of influences affecting the function of the liver. But there is one mode, in which this mechanical result may be produced through an increase of the secretory function. The bile is sometimes so thick and viscid that it will not flow readily through the ducts. if now the liver can be made to throw out thin liquid, with or without colouring matter, the thick bile may be diluted, and the tubes washed out as it were, thus giving to the evacuations a highly bilious appearance, although there may have been no increased production of the colouring matter. it is very probable that many of the general secretory stimulants may have this effect in emulging the liver, and thus seem occasionally to increase the ordinary quantity of bile evacuated. Many substances, supposed at various times to have a special influence on the liver, may have acquired their reputation in this way; but, not producing obvious bilious evacuations under ordinary circumstances, and as a general result, have not sustained this reputation.

In former times, during the prevalence of the absurd doctrine of signatures, a notion prevailed, that yellow substances were specially efficient in increasing the flow of bile; and hence rhubarb, turmeric, the yolk of eggs, etc. acquired some repute as cholagogues. This notion requires at present no refutation.