THE CIRCULATING LIQUIDS OF THE BODY 51 



much as the whole of the blood. The flow^has been calculated 

 in various animals at one-eighteenth to one-seventh of the body- 

 weight in the twenty-four hours. The quantity of lymph in 

 the body is unknown, but it must be very great perhaps two 

 or three times that of the blood. 



Allied to tissue-lymph, but not identical with it, are the fluids 

 present in health in very small amount in such serous cavities 

 as the pericardium. The synovial fluid of the joints differs from 

 lymph especially in containing a small amount of a mucin-like 

 substance. 



The gases of the blood and lymph will be treated of in 

 Chapter III., the formation of lymph in Chapter V. 



The Functions of Blood and Lymph. 



We have already said that these liquids provide the tissues 

 with the materials they require, and carry away from them 

 materials which f have served their turn and are done with. 

 These materials ! are gaseous, liquid, and solid. Oxygen is 

 brought to the tissues in the red corpuscles; carbon dioxide 

 is carried away from them partly in the erythrocytes, but chiefly 

 in the plasma of the blood and lymph. The water and solids 

 which the cells of the body take in and give out are also, at 

 one time or another, constituents of the plasma. The heat 

 produced in the tissues, too, is, to a large extent, conducted 

 into the blood and distributed by it throughout the body. 

 It is not known whether the leucocytes play any part in the 

 normal nutrition of other cells, although it is probable that 

 they exercise an influence on the plasma in which they live ; 

 but they have important functions of another kind, to which 

 it is necessary to refer briefly here. 



Phagocytosis. Certain of the amoeboid cells of blood and 

 lymph, and the cells of the splenic pulp, are able to include 

 or ' eat up ' foreign bodies with which they come in contact, in 

 the same way as the amoeba takes in its food. Such cells are 

 called phagocytes ; and it is to be remarked that this term 

 neither comprises all leucocytes nor excludes all other cells, 

 for some fixed cells, such as those of the endothelial lining of 

 bloodvessels, are phagocytes in virtue of their power of sending 

 out protoplasmic processes, while the small, immobile, uninuclear 

 leucocyte, or lymphocyte, is not a phagocyte. 



Although it is not at present possible to assign a physiological 

 value to all the phenomena of phagocytosis, either as regards 

 the phagocytes themselves or as regards the organism of which 

 they form a part, there seems little doubt that under certain 

 circumstances the process is connected with the removal of 



42 



