214 TEXT-BOOK OF PHYSIOLOGY. 



The Absorption of Intercellular Lymph. From the fact that lymph 

 is being discharged from the thoracic duct into the blood, more or less con- 

 tinually, it is evident that lymph is being absorbed from the intercellular 

 spaces; from which fact it may be inferred that the production of lymph is a 

 continuous process and that it is passing through the capillaries in amounts 

 greater than is necessary for the immediate needs of the tissues. Should 

 this excess accumulate there would soon arise the condition of edema and 

 an interference with the functional activities of the tissues. Therefore 

 so soon as the accumulation attains a certain volume it is absorbed in large 

 measure by the lymph-capillaries and transmitted to the lymph-vessels and 

 thoracic duct. Because of the general belief that the lymph-capillaries 

 were in open communication with the tissue spaces it was assumed that 

 the absorption of lymph and its flow through the lymph-vessels was the 

 result of a difference of pressure between the lymph in the tissue spaces and 

 the blood in the innominate veins. But if the lymph-capillaries are closed 

 vessels, as recent investigations indicate, then additional factors, in explana- 

 tion of lymph absorption, must be sought for. 



It is quite possible under even normal conditions of pressure in the tissue 

 spaces that some of the more diffusible constituents of the lymph are 

 absorbed by the capillary blood-vessels. As to whether the relatively feebly 

 diffusible colloids are so resorbed is as yet a matter of investigation. 



ABSORPTION OF FOODS. 



The most important of the absorbing surfaces, especially in its relation 

 to the absorption of new material, is the mucous membrane of the alimentary 

 canal, and more particularly that portion lining the small intestine, provided 

 as it is with specialized absorbing structures the villi. Though certain 

 substances can be absorbed from the mouth, it is not probable that any food 

 is so absorbed. From the changes which the food principles undergo in the 

 stomach it might naturally be inferred that their absorption would promptly 

 follow. Experimental researches have demonstrated, however, that this 

 takes place, if at all, but to a slight extent. If, however, solutions of inor- 

 ganic salts, sugars, and peptones possessing a concentration of at least 5 per 

 cent. a degree of concentration seldom realized under normal conditions 

 are introduced into the stomach, their absorption will be effected, the rate of 

 absorption following in a general way the increase, within limits, in concen- 

 tration. Water is practically not absorbed from the stomach. The absorp- 

 tion of the products of digestion i.e. y dextrose, levulose, peptones, amino- 

 acids, soaps, glycerin, fat acids, salts, along with water, in which for the 

 most part they are held in solution is therefore limited very largely to the 

 small intestine, and is accomplished by the villous processes projecting 

 from the surface of the mucous membrane. 



Structure of the Villi. The villi are small filiform or conical processes, 

 from 0.5 to i mm. in length, and from 0.2 to 0.5 mm. in breadth, covering 

 the surface of the mucous membrane from the pyloric orifice to the upper 

 surface of the ileo-cecal valve. Each villus consists of a basement mem- 

 brane (see Fig. 91) supporting tall columnar epithelial cells. Each cell is 

 composed of granular bioplasm containing a distinct nucleus. At its free 



