86 THE FROG chap'. 



blood is aerated in the lungs and skin ; hence the blood 

 returned from those organs by the pulmonary and musculo- 

 cutaneous veins is aerated. On the other hand, the blood 

 in the pulmo-cutaneous artery is non-aerated. 



Flow of Blood. — We must now try to understand the 

 function of this complicated blood-system and the reason 

 why every part of the body has two vessels, an artery and a 

 vein. That there is some kind of movement of the blood 

 has been hinted in the foregoing description, in which 

 arteries have been described as branching out to various 

 parts, veins as formed by the confluence of smaller veins 

 from various'parts. 



Were an artery to be cut in a living frog, the blood would 

 be found to flow out in a series of jerks corresponding with 

 the beats of the heart. Moreover the blood would flow 

 from the side of the cut nearest to the heart, and the flow 

 might be stopped by tying or compressing the artery on that 

 side, i.e., between the heart and the cut. Evidently, then, 

 the blood in the living animal flows from the heart along 

 the arteries to the various parts of the body, and is propelled 

 by the pulsation of the heart. 



If a vein were cut the result would be very different. 

 The blood would flow in a comparatively slow stream and 

 without jerks ; it would flow, moreover, from the side of the 

 cut furthest from the heart, so that, in order to stop the 

 bleeding, the vein must be tied or compressed on the far 

 side of the cut. The blood in the veins flows, therefore, 

 towards the heart in an even stream, unaffected by the 

 heart's pulsations. 



Thus the blood is driven by the heart to the various 

 parts of the body through the arteries, and is returned from 

 the various parts of the body to the heart by the veins. Two 

 questions thus naturally arise : how is it that the blood 



