ABSORPTION OF THE VARIOUS FOOD SUBSTANCES 445 



follow as regards the absorption of fat: (i) Each cell may be capable 

 of dealing with the original neutral fats of the food, and of adapting 

 them to its needs by decomposing and resynthesizing them so far 

 as is necessary in its own substance. In this case it would not be 

 necessary for assimilation by the cells that the fats should be com- 

 pletely split or even split at all in the alimentary canal, however 

 important this might be for their absorption from its lumen. 

 (2) If it were necessary for absorption that decomposition of the 

 fats should take place in the lumen of the digestive tube, the whole 

 of the fat, or at any rate such portion of it as was not at once needed, 

 might without disadvantage for the tissues be resynthesized after 

 absorption. It is not difficult to see that it might even be advan- 

 tageous that not only the relatively fixed reserve in the fat cells, 

 but also what might be termed the floating or circulating reserve 

 constituted by the emulsified fat in the blood should be in the 

 insoluble form of neutral fat. 



Absorption of Carbo- Hydrates. Carbo-hydrates are normally 

 absorbed from the alimentary canal only in the form of mono- 

 saccharides, such as dextrose, levulose or fructose, and galactose, 

 but especially dextrose. These monosaccharides are readily 

 formed from polysaccharides like starch and dextrin, and the disac- 

 charide maltose, which they yield on digestion with amylase, as 

 well as from disaccharides like cane-sugar and lactose, by the fer- 

 ments already studied. That, as a matter of fact, the hydrolysis 

 in the intestine must convert practically all the carbo-hydrate into 

 monosaccharides before absorption, can be shown in various ways. 

 The ferment lactase, while present in the small intestine of all 

 young mammals, is regularly absent in some mammalian groups 

 in the adult. In other species, including man, it is found in some 

 adults, but not in all. In birds and other animals below the mam- 

 mals, it has not hitherto been found at any age. It has been sur- 

 mised that these differences depend upon the presence or absence 

 of lactose (milk) as a regular constituent of the food. (But see 

 p. 410.) If, now, lactose is introduced into a loop of intestine in an 

 animal which does not possess lactase e.g., an adult rabbit it is 

 not absorbed, but remains in the lumen till it is at last decomposed 

 by bacterial action. In animals in which lactase is present the 

 lactose is rapidly absorbed. Maltose is easily taken up from the 

 intestine because of the action of the ferment maltase, which is the 

 most widely spread of all the inverting ferments. The dextrose formed 

 by the maltase is so rapidly absorbed that none, or only traces, of it 

 can be detected in the contents of the intestinal loop. But if absorp- 

 tion be interfered with by injuring the intestine, maltose disappears, 

 and the dextrose produced from it accumulates in the lumen. The 

 reason for the discrimination exercised by the intestinal mucosa in 

 favour of the monosaccharides becomes apparent when an attempt is 



