376 THE HUMAN BODY 



readily demonstrated. If a rabbit be rendered unconscious by 

 chloroform, and its chest be opened, after a pair of bellows has 

 been connected with its windpipe, it is seen that, so long as the 

 bellows are worked to keep up artificial respiration, the blood in the 

 right side of the heart (as seen through the thin auricle) and that in 

 the pulmonary artery, is dark colored, while that in the pulmonary 

 veins 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 themselves and the animal dies, un- 

 less the bellows be again set at work. 



The Blood Gases. If fresh blood be rapidly exposed to as com- 

 plete a vacuum as can be obtained, it gives off certain gases, known 

 as the gases of the blood. These are the same in kind, but differ in 

 proportion, in venous and arterial blood ; there being more carbon 

 dioxid and less oxygen obtainable 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 60 

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

 about as follows: 



Venous Blood. Arterial Blood. 



Oxygen 12 20 



Carbon dioxid 45 38 



Nitrogen 1.7 1.7 



It is important to bear in mind that while arterial blood contains 

 some carbon dioxid 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 



