180 ESSENTIALS OF ZOOLOGY 



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 plasma of the blood contains a group of substances called 

 antibodies (immune bodies). These have been produced by various 

 tissues of the body upon contact with certain foreign proteins. Since 

 bacteria and pathogenic Protozoa react as foreign protein, they stimu- 

 late the body tissues to the production of specific protective antibodies 

 and physicians have come to make use of these antibodies in sterile 

 serum lor prevention and treatment of several diseases. Some of 

 these antigen substances bring about the clumping or agglutination 

 of foreign bacteria, 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 liemopJiiliacs 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 protliroyiibin 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 

 of the heartbeat for an average adult man at rest is about 72 times 

 per minute. The contraction phase of the heartbeat is called the 

 systole and the relaxation phase is the diastole. It has been estimated 

 that an average circuit of the circulation of blood in man can be com- 

 pleted in twenty- three seconds, with about two seconds of this time 

 being spent in capillaries. 



Nervous Function — Reception and Conduction 



Irritability and conductivity are fundamental functions of all 

 protoplasm, whether it be in the body of Amoeba or man. The 

 responsiveness of organisms to change of conditions both externally 

 and internally determines their behavior. Living protoplasm is not 

 only excitable, but it possesses the power to record or store up the 



