378 THE HUMAN BOD I. 



fortunately, fit quite tight, fresh air gets even into closed 

 rooms, in tolerable abundance for one or two inhabitants, 

 if there be outlets for the air already in them. 



Changes undergone by the Blood in the Lungs. These 

 .are the exact reverse of those undergone by the breathed air 

 what the air gains the blood loses, and vice versa. Con- 

 sequently, the blood loses heat, and water, and carbon 

 dioxide in the pulmonary capillaries; and gains oxygen. 

 These gains and losses are accompanied by a change of color 

 from the dark purple which the blood exhibits in the pul- 

 monary artery, to the bright scarlet it possesses in the pul- 

 monary veins. 



The dependence of this color change upon the access of 

 fresh air to the lungs while the blood is flowing through them, 

 can be 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 pulmo- 

 nary 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, 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 blood. These are the same 

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

 blood; there being more carbon dioxide 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 

 pressure and at the normal temperature (see Physics), 



