COMPARATIVE ZOOLOGY 



from venous to arterial, returning by the pulmonary 

 veins to the left auricle. This sends it past the mitral 

 valves into the left ventricle, which drives it past the 

 semilunar valves into the aorta, and thence, by its rami- 

 fying arteries and capillaries, into all parts of the body 

 except the lungs. From the systemic capillaries, the 

 blood, now changed from arterial to venous, is gathered 

 by the veins, and conveyed back to the heart. 



The Rate of the Blood Current generally increases with 

 the activity of the animal, being most rapid in birds. 117 



In insects, however, 

 it is comparatively 

 slow ; but this is 

 because the air is 

 taken to the blood 

 the whole body 

 being bathed in air, 

 so that the blood has 

 no need to hasten 

 to a special organ. 

 However, activity 

 nearly doubles the 

 rate of pulsation in 

 a bee. The motion 

 in the arteries is 

 several times faster 

 than in the veins, 

 but diminishes as 

 the distance from the heart increases. In the carotid 

 of the horse, the blood moves 12^ inches per second ; in 

 that of man, 16 ; in the capillaries of man, I to 2 inches 

 per minute ; in those of a frog, i. 



The Cause of the Blood Current may be cilia, or the 

 contractions of the body, or pulsating tubes or hearts. 

 In the higher animals, the impulse of the heart is not the 



FIG 273. A, Plan of Circulation in Amphibia and 

 Reptiles; B, Plan of Circulation in Birds and 

 Mammals: a, right auricle receiving venous blood 

 from the system; b, left auricle receiving arterial 

 blood from the lungs; c, c', ventricles; d, e,f t 

 systemic artery, vein, and capillaries; g, pulmon- 

 ary artery; h, k, vein and capillaries. 



