124 ELEMENTARY PHYSIOLOGY less. 



changes effected in the blood is that which results, in 

 most parts of the body, from its simply passing through 

 capillaries, or, in other words, through vessels the walls 

 of which are thin enough to permit a free exchange 

 between the blood and the fluids which permeate the 

 adjacent tissues (Lesson II.). 



Thus, if l)lood 1)0 taken from the artery which supplies 

 a limb, it will be found to have a bright scarlet colour ; 

 while blood drawn, at the same time, from the vein of the 

 limb, will be of a dark j)urplish hue. And as this con- 

 trast is met witli in the contents of the arteries and veins in 

 general (except the pulmonary artery and veins), the 

 .scarlet blood is commonly known as arterial and the 

 dark blood as venous. 



This conversion of arterial into venous blood takes 

 place in mo.st parts of the body, while life persists. Thus, 

 if a limb be cut off" and scarlet blood be forced into its 

 arteries by a syringe, it will issue from the veins as dark 

 blood. 



When specimens of venous and of arterial blood are 

 subjected to chemical examination, the differences pre- 

 sented by their solid and fluid constituents are found to 

 be very small and inconstant. But the gaseous contents 

 of the two kinds of blood differ widely in the i)roportion 

 which the carbonic acid gas bears to the oxygen ; there 

 being a smaller quantity of oxygen and a greater quantity 

 of carbonic acid, in venous than in arterial blood. 



Every 100 volumes of blood contain about 60 volumes 

 of gases. These may be extracted by boiling the blood 

 in a vessel connected with the vacuum of a mercurial 

 pump. The reduction of pressure on the surface of the 

 blood leads to a ra[)id exit of the gases into the vacuum ; 

 they can now be collected and measured and their respec- 

 tive volumes determined. The composition of the blood- 

 is thus found to be the following : — 



