130 ADVANCED PHYSIOLOGY 



events, it is an undisputed fact that while blood is flowing 

 through the vessels, it contains no fibrin ferment, and con- 

 sequently the fibrinogen remains unchanged; but as soon as 

 the blood comes into contact with any material other than 

 the regular lining of the blood vessels, fibrinogen at once 

 changes to fibrin and the blood clots. Even contact with 

 other tissues, e.g. muscle or skin of the same animal, pro- 

 duces a clot almost immediately; so, too, will an injury to a 

 blood vessel or simple exposure of blood to the air. 



At times when it is desired to prevent blood from clotting, 

 this can be done by adding to it certain chemicals, e.g. sodium 

 sulfate or magnesium sulfate, pepsin, trypsin or peptones; the 

 extracts of the salivary glands of leeches or snake's venom will 

 very effectually prevent clotting. Rapid cooling will retard 

 and sometimes entirely check it. Blood can be made to clot 

 more rapidly by bringing it into contact with foreign sub- 

 stances, for example, by covering with cloth a wound from 

 which it is flowing. 



Purpose of Blood Clotting. Blood clotting is nature's 

 method of checking the flow of blood from wounds, thus pre- 

 venting possible fatal consequences. All our lives we are 

 thus guarded, not only from annoyance from small injuries, 

 but from the very serious results which would follow from bad 

 accidents or from surgical operations. 



DISEASES OF THE BLOOD 



It is often affirmed that a person's health is poor because 

 his blood is "out of order" or needs "toning up." In most 

 cases, this is a mistake. Health is not determined by the con- 

 dition of the blood, but on the contrary the state of the blood 

 is determined by the rest of the body. The blood may not be 

 in good condition, it is true, but the reason is usually be- 

 cause the living cells of the rest of the body are out of order. 

 For example, a person is pale and white and suffers from lassi- 

 tude and other uncomfortable symptoms. The physician, 



