HISTOLOGY 



107 



The fluid part of blood, the plasma, is water containing all the other 

 substances which enter into the constitution of protoplasm together with 

 various hormones and the waste products of metabolism. In its inorganic 

 chemical constitution, the plasma resembles sea water. 



In the coagulation of blood, on exposure to air or under some other 

 circumstances, a nitrogenous substance, fibrinogen, carried by the plasma 

 in solution, becomes transformed into fine solid filaments of fibrin (Fig. 

 104). The uncoagulated portion of the plasma is called serum. The 

 "clot" is a mass of fibrin with blood cells caught in its meshes. 



Fig. 104. — Coagulated blodd. Biconcave red corpuscles arranged in "rouleaux"; 

 filaments of fibrin radiating from minute blood plates. (From Bremer, "Textbook of 

 Histology"; after Da Costa.) 



Blood cells are of two main kinds, red corpuscles or er5rthroc)rtes and 

 white corpuscles or leucocytes. The red cells are much more numerous. 

 In human blood the red cells outnumber the white in the ratio of five 

 or six hundred to one. 



Erythrocytes (Figs. 104 and 105) are relatively small and usually 

 have the form of flat discs with elliptical outlines. These blood cells 

 are the oxygen-carriers, being heavily loaded with hemoglobin, a complex 

 protein substance containing iron and having a strong afl^nity for oxygen 

 which the cells pick up at the respiratory surfaces of the animal. Their 

 color is due to the hemoglobin. The mature erythrocytes of all verte- 

 brates except mammals are nucleated. In adult mammals, the red cells 

 in course of their differentiation lose their nuclei, thereby acquiring the 

 form of biconcave discs. (Fig. 104) 



