4i 6 A MANUAL OF PHYSIOLOGY 



Since, however, the amount of neutral fat recovered from the 

 thoracic duct is not equivalent to more than one-third of the 

 fatty acids given, it has been suggested that this synthesis of 

 fat is only apparent, and that the whole of the fat which appears 

 in the chyle after a meal of fatty acids comes from the fat 

 excreted into the intestine (Frank) , which is increased when fatty 

 acids are given by the mouth. But the suggestion is more in- 

 genious than the evidence advanced in its support is convincing. 

 And, as we have seen (p. 415), a part of the deficit may be 

 accounted for by absorption directly into the bloodvessels. 



Absorption of Carbo-hydrates. Carbo-hydrates are normally 

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

 saccharides, such as dextrose, levulose, and galactose, but 

 especially dextrose. These monosaccharides are readily formed 

 from polysaccharides like starch and dextrin, and the disaccharide 

 maltose, which they yield, as well as from disaccharides like 

 cane-sugar and lactose, by the ferments 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 mammals, it has not 

 hitherto been found at any age. It has been surmised that these 

 differences depend upon the presence or absence of lactose (milk) 

 as a regular constituent of the food. (But see p. 382.) 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 absorption be interfered with by injuring the 

 intestine, maltose disappears, and dextrose accumulates in the 

 lumen. The reason for the discrimination exercised by the in- 

 testinal mucosa in favour of the monosaccharides becomes 

 apparent when an attempt is made to circumvent it by injecting 

 the sugars subcutaneously. Cane-sugar and lactose so intro- 

 duced are excreted unchanged in the urine. Dextrose, levulose, 

 and galactose are used up in the body, and maltose likewise, 

 thanks to the presence of lactase in the blood and tissues. The 

 cells of the body in general will burn only monosaccharides, and 



