172 FOUNDATIONS OF BIOLOGY 



which consists of right and left auricles and ventricles. The 

 RIGHT AURICLE receives blood from the venous system of 

 the body and passes it through the TRICUSPID VALVE into the 

 right ventricle to be pumped through the PULMONARY ARTERY 

 to the lungs. After traversing the capillaries of the lungs the 

 blood is returned by the PULMONARY VEIN to the LEFT AURI- 

 CLE, thence through the MITRAL VALVE into the LEFT VENTRI- 

 CLE, which forces it into the AORTA and so on its way about 

 the body as a whole. To all intents and purposes, the higher 

 Vertebrates have two hearts which act in unison a right, or 

 pulmonary, heart receiving non-aerated blood from the entire 

 body and pumping it to the lungs, and a left, or systemic, heart 

 receiving aerated blood from the lungs and delivering it to 

 the body as a whole. (Fig. 92, C.) 



In this way the blood vessels of the primitive aquatic res- 

 piratory apparatus are transformed by gradual additions and 

 subtractions into the pulmonary system of the higher Verte- 

 brates, including Man the most convincing evidence that 

 nature, whenever possible, turns to structures at hand to 

 construct what is to be, and thereby weaves in the woof and 

 warp of higher forms a record of their lowly origin. 



The blood vascular system of the higher Vertebrates, in 

 spite, shall we say, of its makeshift origin, is a highly efficient 

 apparatus. Day in and day out throughout life the human 

 heart, beating rhythmically at an average rate of 70 times 

 per minute, does about 175,000 foot-pounds of work. This 

 power is expended in moving the weight of the blood, in 

 imparting to it the velocity of its motion, and in raising the 

 pressure in the aorta and pulmonary artery. 



The RATE of flow is greatest when the blood leaves the heart 

 and gradually diminishes until, in the capillaries of both the 

 pulmonary and systemic systems, it is reduced to a minimum. 

 On the return trip from the capillaries through the veins the 



