138 THE HUMAN MECHANISM 



4. The red blood corpuscles. The red corpuscles are pig- 

 mented, biconcave disks with no nucleus ; they are normally 

 confined to the blood vessels and are carried around passively 

 in the blood current without active movements of their own. 

 The main function of these corpuscles is to carry oxygen 

 from the lungs to the tissues, a function which will be fur- 

 ther studied in connection with respiration. They contain a 

 pigment, hemoglobin, which gives to the blood its red color 

 and carries the oxygen. 



5. The blood plasma is an exceedingly complex fluid 

 whose general composition is represented as follows: water, 

 90 parts; solids, 10 parts (proteins, 8 parts; inorganic salts, 

 1 part; extractives, 1 part). 



Under the extractives are included a very large number 

 of substances which, though present in small quantities, are 

 interesting to the physiologist because they are largely prod- 

 ucts of the chemical activities of the body and as such 

 give information about the nature of the chemical changes 

 occurring in the organs. 



Finally, it should be remembered that the cells of the 

 body generally are bathed with lymph, not with blood ; 

 in other words, that the lymph and not the blood is the 

 immediate environment of the cells. Lymph is sometimes 

 described as blood minus its red corpuscles; but this state- 

 ment, though convenient, is not strictly correct, since the 

 amount of waste products in lymph must be greater than 

 in blood, while the amount of food material must be less 

 (see Chap. IV). Much as the blood is a product of the 

 united chemical activity of all the organs of the body, so 

 the lymph of each organ is derived from the cells of that 

 organ and from the blood flowing through it. Lymph thus 

 has a double origin and of course shows very considerable 

 differences of composition in different organs. 



