v DIGESTION 75 



The exact mode of absorption of the fats is not 

 thoroughly understood. It is usually supposed that only 

 a small proportion of them are decomposed into fatty 

 acids and glycerine, and that the greater part is merely 

 broken up into particles so small that they can be taken 

 up by the epithelial cells of the intestine. This emulsifica- 

 tion of fat is effected by the combined action of the 

 pancreatic juice and bile, the fats being reduced to the 

 condition in which they exist in milk and in the emulsions 

 of cod-liver oil so much used in place of the natural form 

 of that medicine. 



Thus during the passage of the food through the 

 intestine the remainder of the prcteids, the whole of the 

 fats, and any starch which may be present, are rendered 

 capable of being absorbed : they pass through the 

 mucous membrane into the blood, and by the time the 

 rectum is reached all the nutriment is extracted from the 

 food, and there remains only a small quantity of indigest- 

 ible matter which is passed out in the form of faeces. 



Peristaltic Movements. The passage of the food 

 through the enteric canal is effected by the contraction 

 of its muscular layer, which is really double, and which 

 is composed of muscular fibres (see p. in, and Figs. 39 

 and 40). In the inner layer these fibres have a trans- 

 verse direction, encircling the tube, and by their contrac- 

 tion narrowing it ; the outer layer consists of longitudinal 

 fibres, which by their contraction shorten it. By the 

 alternate contraction and relaxation of the two layers 

 are produced a series of peristaltic movements, not unlike 

 those by which an earthworm makes its way over the 

 ground : they can be seen in a freshly-killed frog, and 

 still better in a rabbit or rat. 



Summary of Chapter. The body is constantly under- 

 going waste, and in consequence needs continual repair. 

 The waste matters chiefly take the form of carbon di- 



