The nutritive products of digestion reach the blood and 

 have to be distributed to the muscles and other tissues. 

 The oxygen of the air has also to be conveyed to the tissues, 

 while the products of combustion must be removed. 



The way in which the circulation of the blood is carried 

 out must next be considered. 



SECTION IV. 



A. The Manner in which the Nourishing Fluids are 

 Brought to the Tissues. 



THE CIRCULATION. 



I GENERAL CONSIDERATIONS. 



The arrangement by which the blood and lymph are dis- 

 tributed to the tissues may be compared to a great irrigation 

 system. 



It consists of a central force pump — the systemic heart 

 (fig. 162, S.H.) — from which passes a series of conducting 

 tubes — the arteries — leading otf to every part of the body, 

 and ending in innumerable fine irrigation channels — the 

 capillaries {Gap.) — in the substance of the tissues. From 

 these some of the blood constituents are passed into the 

 spaces between the cells as lymph. From these spaces the 

 fluid either passes back into the capillaries, or flows away in 

 a series of lymph vessels, which carry it through lymp)h 

 glands (Ly.), from which it gains certain necessary con- 

 stituents, and finally bring it back to the central pump. 



The fluid, which has not passed out of the capillaries into 

 the tissues, has been deprived of many of its constituents, 

 and this withdrawal of nutrient material by the tissues is 

 made crood bv some of the blood beino- sent throufrh the 

 walls of the stomach and intestine (Al.C), in which the 

 nutrient material of the food is taken up and added to the 



382 



