170 ANATOMY AND PHYSIOLOGY 



of the intestine (so-called lacteals), which open into the lymph 

 vessels in the submucous coat. By these vessels the chyle finally 

 reaches the thoracic duct and is carried to the blood, to be distrib- 

 uted to the tissues of the body by way of the general circulation. 



Osmosis 



The forces which regulate absorption include the process of 

 osmosis, which has been described as the passage of " diffusion 

 streams" whereby solutions of different strengths or densities 

 pass through animal membranes. This does not explain all that 

 happens; it is recognized that certain very important chemical 

 processes must be involved in the cell walls of the intestines, the 

 nature of which is beyond our present understanding. We 

 bid farewell to peptones and amino-acids in the intestinal canal 

 and greet albumins and fats in the blood-vessels which leave it; 

 we find solutions of soaps and fatty acids on the outside of the 

 villus emulsified fat in the lymph tube within. 



The same forces by which the nutritive fluids were absorbed 

 into the vessels, are again at work to effect their transference 

 from the vessels to the tissues of the body. 



In the tissues. The solution of nutritive substances, having 

 been carried by the blood-vessels to the minutest channels in the 

 body, passes into the tissue spaces as lymph, which bathes the cells 

 themselves, so that they may receive the material necessary for 

 their action and upbuilding. 



Different tissues appropriate their different foods, and each 

 gives back the products of its own activities as tissue wastes, 

 which in turn enter the blood to be carried to tissues which can 

 make another use of them, or to organs which can dispose of them 

 as excretions. 



The next chapter will introduce the study of the blood, heart 

 and blood-vessels, or the system of circulatory organs for dis- 

 tributing the blood throughout the body. 



