CHAPTER III. 



THE BLOOD. 



Ix man and most vertebrates the blood consists of a clear 

 fluid, the liquor sanguinis or plasma, in which a large num- 

 ber of corpuscles are very evenly distributed. Of these there 

 are two prominent varieties, differing much in character the 

 red and the colorless cr white. The former are greatly in ex- 

 cess, and give to the liquid its characteristic red appearance. 



In relative proportion the two vary greatly within certain 

 limits. Usually there is but one of the white to 600 or 1,200 of 

 the red ; but these numerical relations are disturbed by vari- 

 ous diseases, and the white may equal the red, or even, in rare 

 cases, exceed them. 



In fresh liquid blood the corpuscles are the only solid mat- 

 ters visible tinder the microscope ; nor is there any difference 

 in this respect with coagulated blood, when the quantity is 

 large. If, however, a little should be allowed to dry, fibrin 

 may be deposited under the form of delicate filaments, which 

 are superimposed on one another without definite order. 



In one hundred volumes of blood there are said to be thirty- 

 six volumes of corpuscles and sixty- four of plasma. This ratio, 

 however, is altered somewhat by different conditions, such as 

 the age and health of the individual. 



The red corpuscles in man and other mammals, with very 

 few exceptions, are bi-concave bodies, circular in outline. In 

 birds, amphibia, and almost all fishes they are also bi-concave 

 or hollowed out at the centre, but have an elliptical contour. 

 In the human species nuclei or central bodies appear at a very 

 early period of life, but subsequently are invisible, unless arti- 

 ficial means are used to display them. In birds, amphibia, and 

 fishes a rounded prominence is also seen at the centre, which is 

 particularly well marked when the corpuscle happens to be 



