404 THE HUMAN BODY. 



and the left auricle is bright red. Let, however, the artificial 

 respiration be stopped for a few seconds and, consequently, 

 the renewal of the air in the lungs (since an animal cannot 

 breathe for itself when its chest is opened), and very soon the 

 blood returns to the left auricle as dark as it left the right. 

 In a very short time symptoms of suffocation show them- 

 selves and the animal dies, unless the bellows be again set at 

 work. 



The Blood Gases. If fresh blood be rapidly exposed to 

 as complete a vacuum as can be obtained, it gives off certain 

 gases, known as the gases of the Mood. These are the same 

 in kind, but differ in proportion, in venous and arterial 

 blood; there being more carbon dioxide and less oxygen ob- 

 tainable from the venous blood going to the lungs by the 

 pulmonary artery, than from the arterial blood coming back 

 to the heart by the pulmonary veins. The gases given off by 

 venous and arterial blood, measured under the normal pres- 

 sure and at the normal temperature, amount to from 58 to 62 

 volumes for every 100 volumes of blood, and in the two cases 

 are about as follows 



Venous Blood. Arterial Blood. 



Oxygen 10 20 



Carbon dioxide 46 40 



Nitrogen 2 2 



It is important to bear in mind that while arterial blood 

 contains some carbon dioxide that can be removed by the 

 air-pump, venous blood also contains some oxygen removable 

 in the same way; so that the difference between the two is 

 only one of degree. When an animal is killed by suffocation, 

 however, the last trace of oxygen which can be yielded up in 

 a vacuum disappears from the blood before the heart ceases 

 to beat. All the blood of such an animal is what might be 

 called suffocation blood, and has a far darker color than 

 ordinary venous blood. 



The Cause of the Bright Color of Arterial Blood. The 

 color of the blood depends on its red corpuscles, since pure 

 blood plasma or blood serum is colorless, or at most a very 

 faint straw yellow. Hence the color change which the blood 

 experiences in circulating through the lungs must be due to 

 some change in its red corpuscles. Now, minute solid bodies 

 suspended in a liquid reflect more light when they are more 

 dense, other things being equal; and the first thing that sug- 



