24 THE BLOOD. 



Why does not blood clot in the living vessels? 



The reason is not clearly understood, but it is supposed that the 

 living blood-vessels exert a restraining influence upon the formation 

 of the fibrin ferment. The formation of a clot may occur in a vessel 

 after injury of the lining by ligation, by the introduction of a for- 

 eign substance, or by disease of the vessel. This fact is made 

 apparent in the ligation of vessels, in the treatment of aneurism, 

 and in the formation of emboli. 



What are the principal forms of blood-corpuscles ? 



The red or colored, and white or colorless. They make up 

 nearly half (40-45 per cent.) of the total weight of fluid blood. 

 The proportion of the red to the white is about 500 to 1. 



Describe the red blood-corpuscles. 



Human colored blood-corpuscles are circular, biconcave disks 

 with rounded edges ; in diameter they are about 3 J^ in., in thick- 

 ness about YYoTnr "* ^ n water tne J swell and become flat or 

 convex. When seen singly they appear yellow, but their color is 

 red when seen in groups. Microscopic examination shows that 

 they have no nucleus and no limiting membrane (which defects 

 preclude the name "cell"); but they have an elastic framework 

 or stroma, which retains an individuality for each corpuscle, and 

 allows changes of shape to adapt them for capillary circulation, 

 and brings them back to the original form after such distortion. 



Why are they red? 



Because the stroma of each corpuscle is infiltrated with a red 

 coloring matter, haemoglobin. 



Mention some other peculiarities of red cells. 



(1) Blood-corpuscles (sp. gr. 1088) are somewhat heavier than 

 plasma (sp. gr. 1030), and therefore tend to sink when drawn in 

 a vessel. In blood that coagulates slowly the corpuscles have an 

 opportunity to do this, and the result is a formation of fibrin at 

 the surface of the clot. This surface is of a light yellowish color, 

 and is known as the " buffy coat." 



(2) The red corpuscles in the process of clot-formation form in 

 rolls or columns like piles of coins, the corpuscles adhering to one 

 another at the edges, and the columns so formed adhere to one 

 another by their ends, so as to make clusters. 



