226 TECE PRINCIPLES OF AGRICULTURE 



thin and permeable that gases pass through it 

 with gi'eat rapidity,— the oxygen from the air 

 to the blood, and the carbon dioxid from the 

 blood to the air. 



ob. Blood -changes in respiration 



414. The heart of warm-blooded animals 

 is composed of two double cavities, right and 

 left, which are quite distinct from each other. 

 The left side pumps the blood into the arte- 

 ries of the system at large, whence it returns 

 through the veins to the right side. The right 

 side, in its turn, pumps the blood into the arte- 

 ries of the lungs, whence it returns by the lung- 

 veins to the left side. In this way the blood 

 is circulated first through the lungs, and then 

 through the tissues of the rest of the body. 



415. The blood is of a dark red or purple 

 color as found in the veins, in the right side 

 of the heart, and in the arteries of the lungs. 

 It is of a bright crimson hue as it returns 

 from the lungs and passes through the left 

 side of the heart and the arteries to all parts 

 of the body. The varying color is determined 

 by the presence of a larger amount of oxj^gen 

 in the arterial (bright crimson) blood, and by 

 its comparative absence, and by the presence 

 of an excess of carbon dioxid, in the venous 

 (dark red) blood. 



