THE ABSORPTION OF THE FOOD-STUFFS 743 



particles has been actually dissolved out by the digestive juices and has 

 been absorbed in a state of solution. 



We may sum up the processes involved in digestion and absorption of 

 fat as follows. Neutral fat is hydrolysed into fatty acid and glycerin under 

 the action of the gastric juice, the pancreatic juice, and the succus entericus, 

 the effect of the gastric juice being, however, extremely limited unless the 

 fat be presented to it in a finely divided condition. The lipolytic action of 

 the pancreatic juice and succus entericus is largely aided and increased by 

 the simultaneous presence of bile, which, in virtue of the bile salts lecithin 

 and cholesterin it contains, enables the pancreatic juice to enter into close 

 relation with the fat, and dissolves the products of the activity of the ferment, 

 and so enables it to attack renewed portions of the neutral fat. As the result 

 of this lipolysis there are formed glycerin, which is soluble in water, and 

 fatty acids or soaps, according as the reaction of the medium is acid or 

 alkaline. The alkaline soaps are soluble in water, the soaps of magnesium 

 and calcium are soluble in bile, free fatty acids are soluble in bile acids. The 

 fat is thus reduced to a condition in which it is soluble in the intestinal 

 contents whatever their reaction. In this state of solution its constituents 

 are taken up by the cells of the intestinal mucosa. Within the cells a 

 process of synthesis takes place, the soaps being split and the fatty acids thus 

 set free or absorbed, being combined with glycerin with the elimination of 

 water to form neutral fat, which appears as fine granules in the cell proto- 

 plasm. By an active process of excretion these granules are extruded in a 

 somewhat more finely divided form into the intercellular clefts and into the 

 spaces of the villus, whence by the contractions of the musculature of the 

 villus they are forced with the lymph transuding from the capillary blood- 

 vessels into the central lacteal, and thence along the mesenteric lymphatics 

 to the thoracic duct. This description would apply to about 60 per cent, of 

 the fat which is absorbed. It is probable that all the fat which is absorbed 

 is taken up in a dissolved condition, but whether the remaining 40 per cent, 

 enters the blood stream, or is utilised and broken down in the tissues of the 

 intestinal wall itself, we have no means of judging. Under normal circum- 

 stances the utilisation of fat is almost complete. By the time the intestinal 

 contents have arrived at the lower end of the ileum 95 per cent, of the fat 

 has been absorbed. Removal of the whole large intestine was found by 

 Vaughan Harley not to affect fat absorption. 



THE ABSORPTION OF CARBOHYDRATES 



As a result of the action of the various digestive juices all the carbo- 

 hydrate constituents of the normal diet of man are reduced to the state of 

 monosaccharides. The absorption of these digestive products may take 

 place at any part of the alimentary canal, the greatest part in the act of 

 absorption being taken by the small intestine. By the time that the food 

 has arrived at the ileocaecal valve practically the whole of the carbohydrate 

 constituents of the food have been absorbed. All experimenters are agreed 

 that the carbohydrates pass into the body by way of the vessels of the portal 



