It is, in fact, not at all difficult to prove that the liver manufactures certain biliary constituents, while it merely excretes others. Thus, for example, the two substances glycocholic and taurocholic acids are never to be found either in the blood, tissues, or fluids of the healthy organism, with the single exception of those of the liver and gall-bladder; and after extirpation of the liver neither acid is to be found in the body at all. On the other hand, such substances as cholesterine and biliverdine, are not peculiar to the liver or its secretion, but are the products of several organs, and are always to be detected in the blood, independently of the presence or absence of the liver. These facts, therefore, clearly show that the liver {12} is both a formative and excretive organ to some, and an excretive only to others, of the biliary constituents. Lastly, the general opinion is that the secretion intermits, and, like the gastric, and pancreatic juices, bile is only formed during digestion. Were it so, however, where would be the necessity for a gall-bladder? Is it not to store up the secretion formed in the intervals of digestion, and to retain it until it is required? No doubt there are several animals, such as the horse, and the deer, that possess no gall-bladders; but there is undoubtedly in them some special arrangement of the digestive apparatus, rendering the presence of a gall-bladder unnecessary. In fact, it is easily shown that the biliary secretion in ordinary cases is continuous; for if in an animal possessing a gall-bladder a biliary fistula be established, and the secretion of bile carefully watched, it will be found that at no period of the day does it entirely intermit, although it is more active at one time than at another, the minimum of its activity being during sleep--the maximum during active digestion. The absolute quantity of bile secreted in the twenty-four hours is tolerably uniform, although the daily amount is slightly influenced by the kind of food.[3] [Footnote 3: Arnold found that dogs secreted more bile on a bread, than on an animal diet. "Zur Physiologie der Galle," Mannheim, 1854.] {13} IS BILE ESSENTIAL TO LIFE?