THE VERTEBRATE ANIMAL — SUBPHYLUM VERTEBRATA 395 



skeletal muscles require another fourth ; and the remaining fourth is 

 distributed through all of the other organs. Human blood contains 

 normally about 5,000,000 red corpuscles (erythrocytes) per cubic 

 millimeter of volume in the male and about 4,500,000 in the female. 

 The average person, weighing 150 pounds, then, would possess ap- 

 proximately 20,000,000,000,000 (20 trillion) of them. Each erythro- 

 cyte is essentially a little capsule filled with hemoglobin which is a 

 compound peculiarly fitted to unite with atmospheric oxygen. When 

 united with oxygen it is known as oxyhemoglobin, which is readily 

 reduced to give up the oxygen to the cells when the blood reaches 

 the tissues. The carbon dioxide given off by the cells is collected 

 principally in the plasma and returned to the lungs. 



The leucocytes or white corpuscles are quite variable in form and 

 number from 6,000 to 10,000 per cubic millimeter. They are amoe- 

 boid and therefore not confined to the blood vessels. One of their 

 chief functions is the destruction of bacteria and other foreign mate- 

 rial in the tissues. This process is known as phagocytosis. The ac- 

 companying table summarizes essential information concerning blood 

 cells. 



The plasma of the blood contains a group of substances called 

 antibodies. These have been produced by various tissues of the body 

 upon contact with certain foreign proteins. Since bacteria and patho- 

 genic Protozoa react as foreign protein, they stimulate the body tis- 

 sues to the production of specific protective antibodies and physicians 

 have come to make use of these antibodies in sterile serum for pre- 

 vention and treatment of several diseases. Some of these antigen 

 substances bring about the clumping or agglutination of foreign bac- 

 teria, others dissolve the bacteria, and still others cause them to be 

 precipitated. The chemical nature of these bodies is not yet known. 

 There are individuals known as hemophiliacs or bleeders whose 

 blood will not clot, and any wound is likely to be fatal. The plasma 

 normally contains a soluble protein, called fibrinogen and calcium in 

 solution. Howell 's theory of coagulation of blood holds that there is 

 also an inert substance, antithrombin, which prevents the activation 

 of the prothrombin of the plasma to become thrombin. When blood is 

 shed and exposed to air, the blood cells and platelets produce a sub- 

 stance, cephalin, which, in the presence of calcium, neutralizes the 

 antithrombin, allowing the formation of thrombin. Thrombin reacts 

 with fibrinogen to produce fibrin, the solid fibers of the clot. The rate 



