FATE OF THE FOOD ABSOEBED 381 



Phosphorus is undoubtedly absorbed in organic combination, 

 but the mode of absorption and the channels by which it passes 

 from the intestine have not been investigated. 



(2) Carbohydrates. Although the chief monosaccharid 

 formed in digestion is dextrose, others are also produced 

 laevulose from cane sugar and galactose from milk sugar. 

 All these are absorbed in solution, and are carried away in 

 the blood of the portal vein. 



(3) Fats. After being split up into the component acids and 

 glycerin, fats pass, as soluble soaps or as fatty acids soluble 

 in the bile, through the borders of the intestinal epithelium. 

 Here they appear to be again converted into fats by a synthesis 

 of the acid with glycerin. Fine fatty particles are found to 

 make their appearance in the cells at some distance from the 

 free margin and to increase in size. A similar synthesis occurs 

 even when free fatty acids are given, so the cells must be 

 capable of producing the necessary glycerin to combine with the 

 acids. The fats are sent on from the cells, through the lymph 

 tissue of the villi, into the central lymph vessels, and thus on, 

 through the thoracic duct, to the blood stream. Unlike the pro- 

 teins and carbohydrates, they are not carried directly to the liver. 



IV. FATE OF THE FOOD ABSORBED 



The food absorbed may be 



(A) used immediately as a source of energy, for (1) the Con- 

 struction or Reconstruction of Tissues; (2) the Production of 

 Mechanical Work; (3) the Production of Heat ; 



Or (B) it may be stored for future use in the body. 



The processes of construction and repair of the tissues and 

 the production of mechanical work have already been considered 

 (p. 13 et seq.), and the production of heat and the regulation 

 of temperature may now be dealt with. 



I. PEODUCTION OF HEAT AND REGULATION OF 

 TEMPERATURE 



1. Production of Heat 



A. Muscle. The production of heat in muscle has been 

 already studied (p. 61). It has been shown that muscle, from 



