HISTOLOGY 



159 



and must be relieved of their waste products. Its circulatory function 

 requires that it be fluid but various special services are rendered by cells 

 suspended in the fluid, some of them passively carried by it, others capable 

 of independent motion somewhat Hke that exhibited by an ameba. 



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

 substances which enter into the constitution of protoplasm. In its 

 inorganic chemical constitution, the plasma resembles sea water. Derived 

 from the food are various proteins and carbohydrate substance (sugar), 



c.t.- 





-c.t. 



bl.v. i bl.v. 



Fig. 121. — Fat cells in subcutaneous tissue of a human embryo of four months. 

 U.V., blood vessel; c.t., white connective-tissue fibers; ^6., young fibrocyte; mes., mesen- 

 chymal cell; X, young fat cell, nucleus not in section; i, 2, 3, developing fat cells. (From 

 Bremer, Text-book of Histology.) 



while fat may be carried as minute droplets suspended in the plasma. 

 Other important constituents of the plasma are hormones produced by 

 such endocrine glands as the pituitary, thyroid and suprarenal, and the 

 various wastes of metabolism. 



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. 

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

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



Blood cells are of two main kinds, red corpuscles or erythrocytes and 

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



