232 DIGESTION AND ASSIMILATION OF THE FOODSTUFFS 



celluloses is most especially important in those animals, such as the 

 herbivora, which have very long intestines. In man, however, too 

 large a proportion of indigestible carbohydrate, as for example in the 

 dietary of vegetarians, may lead to incomplete absorption of the 

 digestible foodstuffs and promote, in this way, bacterial activities to 

 an undesirable extent. 



The Digestion of the Fats. Very slight lipolytic action is exerted by 

 the Gastric Juice. Not only is the lipase-content of gastric juice low, 

 or the lipase weak in action, but the fats, while they remain in the 

 stomach, being insoluble in water and in the form of relatively large 

 masses, present only a limited surface of contact with the gastric 

 juice, and the Lipase which it contains can hydrolyze the fat-masses 

 only at their surface. However, the slight action which is exerted by 

 the gastric lipase is probably of no little importance, for it ensures 

 that upon the entry of the fats into the upper part of the small intestine 

 they contain a small admixture of fatty acid, which greatly promotes 

 their rapid emulsification by the alkaline fluids with which they here 

 come into contact. 



The path of Absorption of the fats is quite different from that which 

 is followed by the carbohydrate. Instead of passing into the blood- 

 stream, after having traversed the epithelial lining of the intestine, 

 they are deflected into the lymphatics and carried thence into the 

 thoracic duct. After a meal rich in fats, the numerous small lymph- 

 atic vessels coming away from the small intestine are full of milky 

 fluid and stand out distinctly from the surrounding tissues, by reason 

 of their whiteness and opacity, whereas under resting conditions, when 

 digestion is not proceeding, they are transparent and difficult to 

 distinguish. 



It is, therefore, possible to follow the absorption of fat from the 

 small intestines by naked-eye examination, and it was in this way that 

 Claude Bernard in 1846, discovered that the Pancreatic Juice is essen- 

 tial for the digestion and absorption of fats, for no absorption is 

 evidenced by the appearance of fat in the lacteals until the foodstuffs 

 have reached the point at which the pancreatic duct opens into the 

 duodenum. In man the ducts from the liver and the pancreas join to 

 form a common channel of discharge, in the dog the two ducts enter 

 the intestine very close together, but in the rabbit a considerable 

 interval separates the openings of the two ducts, the Bile from the 

 liver being discharged into the intestine at a point considerably above 

 that at which the pancreatic duct opens. In the space between these 

 two ducts no absorption of fat whatever is to be observed after a meal, 

 but immediately below the pancreatic duct the lacteals are seen to be 

 filled with emulsified fat. 



Not only the pancreatic juice, but also the bile is essential for the 

 absorption of fat, however, for if by surgical procedures the bile-duct 

 be made to open into the intestine below, instead of above the pan- 

 creatic duct, the lacteals in the space between the ducts, notwithstand- 



