26o TEXT-BOOK OF PHYSIOLOGY 



pig and bat), they are now regarded as normal constituents of the blood and 

 designated sometimes as the third corpuscle. When blood is freshly drawn 

 from the body, the plaques rapidly undergo disintegration and disappear; 

 but by treating the blood with osmic acid, the form and structure of the 

 plaque may be retained. They may also be preserved by preparing and 

 staining the tissues with Wright's blood stain. 



The blood-platelet may be denned as a colorless, grayish- white, homo- 

 geneous or finely granular protoplasmic disc, varying in diameter from 1.5 

 to 3'. 5 micro-millimeters. The edges are rounded and well defined, but it 

 is not certain whether they are only flattened or are slightly biconcave. 

 There is, however, no nucleus, though the central portion is granular and 

 the peripheral portion clear. The ratio of the plaques to the red corpuscles 

 is i to 1 8 or 20, and the total number per cubic millimeter has been estimated 

 to be 250,000 to 300,000 or more. 



When blood is shed they tend to adhere to each other and form irregular 

 masses known as Schultze's granular masses. If threads are suspended in 

 blood, the plaques accumulate in enormous numbers upon them and appear 

 to form a center from which fibrin filaments radiate as coagulation proceeds. 

 The white thrombi which form in blood-vessels in consequence of diseased 

 states e.g., endocarditis, atheromatous ulceration, etc. are composed very 

 largely of blood-plaques and fibrin threads. 



The blood-plaques can be seen with high powers of the microscope in the 

 blood-vessels of the omentum of the guinea-pig and rat, especially when the 

 blood-stream begins to slow. They are also readily seen in the blood-vessels 

 of subcutaneous connective tissue of various animals, and especially in that 

 of the new-born rat. A small quantity of this tissue moistened with normal 

 saline and examined microscopically with suitable powers will show large 

 numbers of plaques within the blood-vessels. 



As to the origin of the blood-platelets there has been much difference of 

 opinion. Many theories have been proposed, none of which have been 

 accepted. As a result of long-continued observations Wright has recently 

 published results which make it probable that they are fragments or detached 

 portions of the cytoplasm of giant cells, megakaryocytes, found in the 

 marrow of the bones. The cytoplasm is prolonged into pseudopod-like 

 processes which become detached, and as they are in close relation to 

 the blood channels they are soon taken up and carried into the blood 

 of the general circulation when they are known as blood-platelets or 

 plaques. 



The function of the blood-plaques is unknown, but it has been surmised 

 that in some way they are, like the leukocytes, concerned in the coagula- 

 tion of the blood. Whenever they are diminished in number, as in purpura 

 and hemophilia, coagulation takes place very slowly. 



THE TOTAL QUANTITY OF THE BLOOD; ITS GENERAL 

 COMPOSITION 



The determination of the total quantity of the blood in an animal is best 

 made by the chromometric method, somewhat modified at present, of 

 Welcker. This consists, first, in bleeding an animal, collecting all the blood 

 it yields, and weighing it; second, in washing out the vessels with a normal 



