106 NUTRITION OF FARM ANIMALS 



In the higher animals the extent of resorbing surface is greatly 

 increased by certain projections of the interior surface of the 

 small intestine known as the mill. Those are round or club- 

 shaped protuberances of the inner surface of the intestine. 

 They are covered, like all parts of the intestinal surface, with the 

 epithelial cells just described, which are underlaid by a deli- 

 cate membrane, beneath which are found numerous minute 

 capillary blood-vessels, a layer of smooth (involuntary) mus- 

 cular fibers and a network of nerves. In the center of each 

 villus ends a vessel called a lacteal, belonging to the lymphatic 

 system. Figure 14 shows a longitudinal section of three villi. 



The villi are absent in the stomach and in the large intestine. 

 Although some resorption takes place in the stomach, and while 

 a considerable amount of water and more or less of the fermen- 

 tation products are resorbed in the large intestine, the small 

 intestine is the special resorptive organ. 



150. Mechanism of resorption. Since the processes of 

 digestion are apparently directed toward the conversion of feed 

 substances into soluble and diffusible forms, it was quite natural 

 that resorption should be regarded as an osmotic process. In 

 this conception of it, the epithelial cells and other tissues be- 

 tween the cavity of the digestive organs and the blood and lymph 

 vessels constituted a membrane through which osmosis took 

 place. On the one side of this membrane were the contents 

 of the digestive tract, containing the soluble products of, diges- 

 tion, while on the other side were the blood and lymph, contain- 

 ing little or none of these products. Under these conditions, 

 osmosis was assumed to set in and transfer the digested nutrients 

 to the blood and lymph. 



Undoubtedly osmosis plays an important part in resorption, 

 but its effects are profoundly modified by the properties of the 

 resorbing cells of the intestinal epithelium in ways which as yet 

 are but very partially understood, and resorption can by no 

 means be explained by a simple analogy with the parchment 

 dialyzing -tube of the laboratory. 



Differences in the permeability of the epithelial cells and of the 

 intercellular substance for the various dissolved substances in the 

 digestive tract doubtless play their part in bringing about the phe- 

 nomena of selective resorption, while variations in the affinity of the 

 cell colloids for water may be assumed to influence the resorp- 



