258 TEXT-BOOK OF PHYSIOLOGY. 



applied to them the term hematoblasts, on the supposition that they 

 were the early stages in the development of the red corpuscles. This 

 is now known to be erroneous. On account of their specific, distinct 

 characters, and their constant presence in the blood of living animals 

 (guinea-pig and bat), they are now regarded as normal constituents 

 of the blood and designated as the third corpuscle. When blood is 

 freshly drawn from the body, the plaques rapidly undergo disintegra- 

 tion and disappear; but by treating the blood with osmic acid, the 

 form and structure of the plaque may be retained. 



The blood-plaque may be defined as a colorless, grayish- white, 

 homogeneous or finely granular protoplasmic disk, varying in diam- 

 eter from 1.5 to 3.5 micro-millimeters. The edges are rounded and 

 well defined, but it is not certain whether they are on^y flattened or 

 are slightly biconcave. There is, however, no nucleus>\ The ratio of 

 the plaques to the red corpuscles is i to 1 8 or 2oyand the total number 

 per cubic millimeter has been estimated to be 250,000 to 3oo,ooo\V 



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 function of the blood-plaques is 

 , unknown, but it has been surmised that in some way they are, like 

 the leukocytes, concerned in the coagulation of the blood.^When- 

 ever they are diminished in number, asnt purgura and hemophilia, 

 coagulation tends to take place very slowly/ 



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

 scope 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 exam- 

 ined microscopically with suitable powers will show large numbers 

 of plaques within the blood-vessels. 



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 saline solution until the fluid comes 

 from the veins clear and free from blood; third, in mincing the tissues 



