CHAPTEK XIV. 



THE BLOOD. 



General Considerations. The blood of man and of almost all 

 vertebrate animals is a slightly viscid, somewhat opaque-looking 

 fluid, which, according to its origin from an artery or a vein, pre- 

 sents a color that varies from a bright scarlet to a dark bluish-red. 

 On microscopical examination it is seen to contain a large number 

 of cellular elements, which are partly colored and partly colorless. 

 The former, which greatly predominate, are the red corpuscles, or 

 erythrocytes of the blood. In man they are homogeneous, normally 

 non-nucleated, circular, biconcave disks, measuring on an average 

 7.5 p, in diameter. When viewed through the microscope they are 

 of a faint greenish-yellow color, while en masse they present the 

 ordinary color of the blood. The colorless bodies, or leucocytes, on 

 the other hand, are all nucleated and in part capable of executing 

 amoeboid movements. Some of them are of about the same size 

 as the red corpuscles, while others are larger. The nucleus may 

 be single or multiple, and it will be noted that in the mononuclear 

 forms the surrounding protoplasm is more or less homogeneous, 

 while in the polynuclear varieties it is distinctly granular. The 

 total number of the leucocytes per cubic millimeter varies under 

 normal conditions between 3000 and 10,000, and is thus much 

 smaller than the number of the red corpuscles, which is generally 

 placed at between 4,000,000 and 5,000,000 in the same volume of 

 blood. 



In addition to the red corpuscles and the leucocytes, we further 

 find a large number of minute colorless disks, measuring less than 

 one-half the diameter of a red corpuscle, and usually occurring in 

 bunches of from six to a dozen or more. They are termed the 

 plaques or blood-plates of Bizzozero. On an average, about 635,000 

 are found in the cbmm. Other morphological elements are not 

 found in the blood under normal conditions, while in disease nucleated 

 red corpuscles, both of the adult and the embryonic type, as well as 

 other forms of leucocytes, may be encountered. 



When blood is drawn from the living body and is allowed to 

 stand, it will be noted that after a variable length of time the entire 

 mass is transformed into a semisolid, jelly-like material, which is 

 termed the placenta sanguinis or blood-clot. On microscopical ex- 

 amination this will be seen to consist of a dense network of fibres, 

 in the meshes of which the corpuscles of the blood are found. If 

 the clot is now carefully separated from the walls of the vessel, it 



306 



