160 THE BLOOD 



layer formed by them. The indirect method, suggested by Hoppe- 

 Seyler, is analytical in its nature and necessitates the following pro- 

 cedures. To begin with, the total amount of proteins in a definite 

 quantity of defibrinated blood is determined and secondly, also the 

 total protein content of the washed corpuscles contained in an equal 

 amount of the same blood. It must be evident that the value ob- 

 tained as a difference, corresponds to the amount of proteins contained 

 in the serum of this sample of blood, and if the quantity of proteins 

 in a special sample of the same blood is now ascertained, the propor- 

 tion of serum in the blood as a whole can readily be calculated. 



The proportion of plasma and corpuscles differs widely not only 

 in animals of different species but also in animals of the same group. 

 As a rule, the volume of the plasma is found to be much greater than 

 that of the corpuscles, a relationship of 2:1 having been observed at 

 times. The figures for human blood vary between 48 and 54 per 

 cent., the average value for the corpuscles being about 50 per cent, 

 by weight,^ or 35 to 40 per cent, by volume. In the dog, the corpus- 

 cles constitute 36 per cent, and in the horse 34.5 per cent, by weight. 



Color of the Blood. — When present in larger amounts, the blood 

 exhibits a very characteristic color, varying between scarlet red and 

 purple. Blood free from oxygen is dichroitic, dark red in reflected 

 light and green in transmitted light. The color impression, actually 

 derived from it, is in accord with the amounts of oxygen and carbon 

 dioxid, or, more correctly speaking, with the amounts of oxyhemo- 

 globin and reduced hemoglobin present therein. 



The blood, or rather, the body-fluid, of the lower forms embraces 

 pigments of different color. Red, violet, brown, green, and blue 

 blood has been found. Furthermore, the pigment is not always held 

 in the corpuscular elements, but may also occur free in suspension 

 in the plasma. The earthworm, for example, possesses colorless 

 corpuscles, while the blood-pigment, called hemerythrin, floats in the 

 plasma. The red coloring material in the echinoderms is known as 

 echinochrome, while the blue pigment of the molluscs and crusta- 

 ceans is called hemocyamine, and the green pigment of worms, 

 chlorocruorine. 



The plasma itself is a clear, amber-colored liquid and does not impart 

 a distinct color to the blood as a whole. Moreover, inasmuch as the 

 leukocytes and platelets are colorless, the only constituents to which an 

 influence of this kind may be attributed, are the red corpuscles. It 

 should be emphasized, however, that single red cells give solely a sen- 

 sation of very faint yellow, and that a distinct reddish color is obtained 

 only when many of them are grouped together. The coloring matter 

 of the red cells is the pigment of the hemoglobin. 



On entering the lungs, the blood exhibits a dark red color, while 

 that leaving them is much lighter. Obviously, therefore, its passage 



^Arronet, Maly's Jahresber.; xvii; Schneider, Zentralblatt fur Phvsiol., v, 

 1891, 362; and Stewart, Jour, of Physiol, xxiv, 1899, 356. 



