STRUCTURE AND FUNCTIONS OF BLOOD AND LYMPH 261 



slowly, but is constantly renovated by the blood, which is kept in 

 rapid movement, and which, besides containing a store of new 

 food-matters for the lymph, carries off the wastes which the 

 various cells have poured into the latter, and thus is also a sort 

 of sewage stream into which the wastes of the whole Body are 

 primarily collected. 



Composition of the Blood. The average specific gravity of 

 human blood is 1,055. It has an alkaline reaction to litmus. 

 About one-third its mass consists of moist corpuscles and the re- 

 mainder of the liquid part or plasma. Exposed in a vacuum, 

 100 volumes of blood yield about 60 of gas consisting of a mixture 

 of oxygen, carbon dioxid, and nitrogen. 



Microscopic Characters of Blood. If a finger be pricked, and 

 the drop of blood flowing out be spread on a glass slide, covered, 

 protected from evaporation, and examined with a microscope 

 magnifying about 400 diameters, it will be seen to consist of in- 

 numerable solid bodies floating in a liquid. The solid bodies are 

 the blood-corpuscles, and the liquid is the blood-plasma. 



The corpuscles are not all .alike.. While currents still exist in 

 the freshly-spreacj drop of blood, the great majority of them are 

 readily carried to and fro ; but a certain number more commonly 

 stick to the glass and remain in one place. The former are the 

 red, the latter the pale or colorless blood-corpuscles. With proper 

 precautions a third sort of corpuscles, the blood-plates, may also 

 be seen. 



Red Corpuscles. Form and Size. The red corpuscles as they 

 float about frequently seem to vary in form, but by a little at- 

 tention it can be made out that this appearance is due to their 

 turning round as they float, and so presenting different aspects to 

 view; just as a silver dollar presents a different outline according 

 as it is looked at from the front or edgewise or in three-quarter 

 profile. 



Sometimes the corpuscle (Fig. 98, B) appears circular; then it is 

 seen in full face; sometimes linear (C), and slightly narrowed in 

 the middle; sometimes oval, as the dollar when half-way between 

 a full and a side view. These appearances show that each red 

 corpuscle is a circular disk, slightly hollowed in the middle (or 

 biconcave) and about four times as wide as it is thick. The av- 

 erage transverse diameter is 0.008 millimeter (^ inch). Shortly 



