FUNCTIONS OF BLOOD AND LYMPH 59 



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

 Chapter IV., the formation of lymph in Chapter VIII., its circulation 

 in Chapter III. 



SECTION VI. 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 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. 

 The leucocytes, as will be seen farther on, aid in some measure in the 

 absorption of certain of the food substances from the intestine. It is 

 not known whether, apart from this, they play any role 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 impor- 

 tant 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, 

 relatively immobile 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 structures which in the 

 course of development have become obsolete, or with the neutral- 

 ization or elimination of harmful substances introduced from with- 

 out, or formed by the activity of bacteria within the tissues. During 

 the metamorphosis of some larvae, groups of cilia and muscle-fibres 

 may be absorbed and eaten up by the leucocytes. In the metamor- 

 phosis of maggots, for example, the muscular fibres of the abdominal 

 wall, which are used in creeping, and are therefore not required in 

 the adult, degenerate, and are devoured by swarms of leucocytes 

 which migrate into them. In the human subject an example of 



