BLOOD AS THE COMMON CARRIER 103 



explicable as disturbances of function in a delicate^ balanced 

 endocrine system. It is probably true that these effects are in the 

 last analysis due to disturbances in the metabolic reactions of the 

 protoplasm, which are dependent in some way upon the presence in 

 certain amounts of the several endocrine secretions. 



Blood as the Common Carrier. — In this chapter the main 

 topic of consideration has been metabolism, or the chemical 

 phenomena characteristic of the protoplasm found in the cells 

 making up the bodies of animals. In complex multicellular ani- 

 mals, such as the vertebrates, it is impossible for each cell to obtain 

 its food materials directly from the animal's surroundings, or to 

 discharge its excretions into the external environment. Conse- 

 quently, we find cells organized into organs and systems which 

 function in such a way that the individual cells may be able to 

 carry on the metabolic reactions that are responsible for the 

 animal's life. The digestive system serves to prepare nutrients, 

 other than oxygen, so that they can be assimilated by the proto- 

 plasm; respiratory organs obtain oxygen for the organism; 

 excretory organs are the places of discharge for the waste products 

 of metabolic reactions; and endocrine glands produce secretions 

 essential for metabolic balance. Blood and lymph, the circu- 

 lating fluids of the body, place these systems in intimate relation- 

 ship with the individual cells in which assimilation and dissimila- 

 tion occur. 



The blood, circulating in a system of closed tubes, is composed 

 of a liquid, the plasma, in which float two kinds of cells : red blood 

 cells, or erythrocytes; and white blood cells, or leucocytes. Lymph 

 consists of blood plasma that filters out of the thin-walled capil- 

 laries into spaces between the cells. White blood cells migrate out 

 through the capillary walls and are found in the lymph. Reference 

 to the diagram (Fig. 60) will show that lymph is collected by the 

 dehcate vessels known as the lymphatics (Fig. 20, p. 41). These 

 finally discharge their contents into the veins. The individual 

 cells of the body are surrounded by lymph which is derived from 

 the blood, and, by means of these two fluids, materials are trans- 

 ported to and from all cells. When the simple sugars, amino 

 acids, vitamins, water, and mineral salts are absorbed from 

 the small intestine they pass into the plasma of the blood. Fats 

 and lipins, absorbed into the Ijonph, are soon discharged by way of 

 the great veins into the blood plasma for distribution. Oxygen, 



