86 THE FROG CHAP. 



veins in respect of the blood they contain. As a rule, 

 the veins contain purple or non-aerated blood, the 

 arteries more or less thoroughly aerated or scarlet blood. 

 But there are certain exceptions. As we shall see in a 

 later chapter, the 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. 



If an artery were 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, 

 a pulse being observable in the arteries. 



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 com- 

 pressed 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. 



