138 HUMAN PHYSIOLOGY 



The amount of faeces daily discharged is about 120-150 

 grams, containing 30-37 grams solids. 



2. Utilization of foodstuffs. As the faeces contain un- 

 digested food, not all the food taken into the alimentary 

 canal is utilized by the body. The amount utilized by the 

 body can be found by subtracting the undigested food present 

 in the faeces from all the food taken by the mouth. As the 

 amount of undigested food in the faeces has never been 

 definitely determined, no accurate figure of the amount of 

 food absorbed can be given. 



The amount absorbed must be taken into consideration in 

 formulating a diet. The animal foods are better absorbed 

 than the vegetable foods. There are absorbed : 



Proteid Fat. Carbohydrate. 



of meat and eggs 97$ 95$ 



" milk 89-99$ 95-97$ J oo$ 



" white bread 78$ 99$ 



" black bread 68-78$ 89$ 



" potatoes 68$ 92$ 



1 * turnips 61$ 



The reasons why vegetable foods are so poorly absorbed 

 are: 



(1) The food in vegetables is often inclosed in cellulose 

 membranes which prevent the action of digestive juices 

 upon the food. Hence vegetable foods are the better 

 absorbed in proportion as they are freed from the cellulose 

 membranes. 



(2) The cellulose stimulates, perhaps mechanically, the 

 peristalsis. Vegetable foods are therefore more quickly 

 carried through the intestine than the animal foods ; in fact, 

 so rapidly that they are discharged as faeces before they have 

 been fully acted upon and absorbed. 



(3) Defaecation. The anus is kept closed by the tonic 

 contraction of the sphincters ani, internus and externus. 

 The action of the external sphincter is increased by the 

 voluntary contraction of the levator ani, which is placed like 



