THE BLOOD. 171 



present it also forms an unstable compound. Its color is dark. 

 When oxyhaemoglobin is in excess, as it is in arterial blood, the color 

 is a bright red. When carbon dioxide is in the ascendancy, the 

 blood is bluish red in color and the oxygen-gas is present in dimin- 

 ished amounts. 



Arterial blood contains more of the assimilable products of the 

 digestive processes, so that it is better fitted to supply the cells with 

 their proper nourishment. It also contains greater quantities of 

 salts, fats, and sugars. 



Venous blood contains less nutriment, but more waste-products, 

 particularly urea and carbonic acid resulting from catabolic 

 processes. 



Composition of the Blood. Apparently the blood-stream, as 

 viewed by the naked eye, is composed of one homogeneous, red sub- 

 stance; but when examined histologically with the microscope this 

 impression becomes entirely dispelled. It is then found to be com- 

 posed in reality of a transparent liquid portion, known as the plasma, 

 or liquor sanguinis, in which, as a medium, float an immense num- 

 ber of blood-corpuscles. The great majority of these latter are col- 

 ored, and it is to them that the blood owes its color. There are at 

 least three different kinds of blood-corpuscles, commonly known as 

 the red corpuscles ; the white, corpuscles, or leucocytes; and the blood- 

 plates. 



The red corpuscles of mammalia the camel and others of the 

 group of Camelidce alone being excepted are circular plates, bicon- 

 cave, and without nuclei. Those of the birds and reptiles are elliptical, 

 biconvex and nucleated. 



Human red blood-corpuscles are biconcave, disc-shaped bodies 

 with rounded edges and slight central depressions. They have been 

 tersely described by one author as "circular, biconcave, nonnucleated 

 discs." 



Weidenreich holds that the red corpuscles are bell-shaped and 

 not biconcave, the latter condition being due to evaporation and con- 

 centration of blood outside the body. 



The corpuscles are formed of a semisolid, homogeneous, iron- 

 holding mass which appears to have no membrane or nucleus, for 

 a nucleus is normally met with in them only during embryonic life 

 of mammals and in the blood of the lower vertebrates, as the 

 amphibia. In size, they are about %ooo ]ncn ^ n diameter and 

 MOOOO inch in thickness. Various causes and conditions may, how- 

 ever, slightly increase or decrease their size. 



