V DIGESTION 75 



ferment, which converts starch into sugar, and a fat- 

 decomposing ferment. 



The exact mode of absorption of the fats is not thoroughly 

 understood. It is usually supposed that only a small pro- 

 portion of them are decomposed into fatty acids and gly- 

 cerine, and that the greater part is merely broken up into 

 particles so small that they can be taken up by the epi- 

 thelial cells of the intestine. This emulsification 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 proteids, 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 re- 

 mains only a small quantity of indigestible 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 Chapter vii.. Figs. 39 and 40). In the 

 inner layer these fibres have a transverse direction, encircling 

 the tube, and by their contraction 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 oi peristaltic movements, 

 riot unlike those by which an earthwprm makes its way over 

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

 still better in a rabbit or rat. 



Stunmary of Chapter.— The body is constantly under- 



