The intestinal secretion contains no trace of a ferment acting on native protein, but a ferment—erepsin—is present in fair amount in the intestinal mucous membrane and in small amount in the secretion, which acts in an alkaline medium on proteoses, peptones, and on casein, converting them into amino acids. A lipase has also been detected which is very similar to pancreatic lipase; it, however, attacks only emulsified fats.
Several carbohydrate hydrolysing enzymes have been described in the small intestine. Invertase, the ferment which splits cane sugar, is present in small amount in the secretion, more abun dantly in the extract of mucous membrane. In all probability it deals with the saccharose after or in process of absorption. Mal tase is also present in large amount, and here again in greater amount in the extract than in the secretion. The question of the presence of lactase has been much discussed, and it seems prob able that suckling animals do possess this enzyme.
Bile.—This fluid, in all probability, has little direct action in ordinary digestion, although it contains substances which act in directly. The bile salts act as solvents for fats and fatty acids, and as activators of pancreatic lipase. The salts also serve to keep cholesterol in solution. Bile is to be looked upon rather as an excretion, the result of the hepatic metabolism, than as a digestive juice. It has an orange-brown colour, but the colour varies according to the pigment present. It is more or less viscous (not so viscous as bile taken from the gall bladder) and has a specific gravity of about mu). It has a slightly alkaline reaction, a bitter taste and a characteristic smell. On analysis it is found to have over 2% of solids, of which more than half are organic. It contains in addition to a nucleo-albumin, derived mainly from the bile passages and gall bladder, bile acids, bile pigments, choles terol, lecithine, etc. The most abundant solids are the salts of the bile acids, of which in man the most important is sodium glyco cholate, sodium taurocholate being present in small amount. The bile acids are formed in the liver cells, and when the duct is liga tured they tend to accumulate in the blood.
The pigments amount to only about 0.2%. In human bile the chief pigment is bilirubin, whilst in herbivores biliverdin is more abundant. They are derived from the haemoglobin of the blood, but the pigments are iron-free. They may be regarded as purely excretory products arising from the breakdown of the haemo globin of effete blood corpuscles.
Cholesterol is a monatomic alcohol, and is probably a waste product. Fats and lecithine are both derived from the liver cells. Of the inorganic constituents calcium phosphate is the most abundant.
The secretion of bile is practically continuous, but it seems to enter the duodenum intermittently. The taking of food increases the flow of bile. The entry of the acid chyme into the duodenum is the stimulus which brings about the ejection of the bile.
Faeces.—The bulk of the waste matter arising from the foods along with the secretions from the alimentary canal form the faeces. On a diet giving little residue the faeces are almost purely intestinal in origin. As a channel of excretion of nitrogenous metabolic waste products they are not very important, although they do play a certain part. The nature of the excreted nitrog enous substances has not been fully examined. It is, however, very difficult to come to any definite conclusion as to what is un absorbed material and what excreted.