THE HEART. 119 



auricles are seen to contract first, and to distend the ventri- 

 cles with blood; the ventricles contract immediately after- 

 wards, and then there is a pause before the auricles become 

 quite distended and contract again. The contraction of the. 

 auricles is completed in about a third of the time taken by 

 the ventricles to contract; but it is not thorough, for it 

 proceeds in a wave forwards from the venous trunks to the 

 tips of the appendages, so that the appendages are at first 

 distended, and when in turn they contract, the rest of the 

 auricular walls are already relaxed. On account of this 

 mode of contraction of the auricles, there is in health little 

 tendency of the blood to regurgitate into the venous 

 trunks ; and the mouths of these vessels are unguarded 

 in mammals, although protected by competent valves in 

 other 'animals. There is in man a fold of membrane, the 

 Eustachian valve, in front of the vena cava inferior; but it 

 can have little action as a valve after birth, for it is frequently 

 nearly absent in the adult. The ventricles contract in a 

 different way from the auricles. The muscular fibres in the 

 middle depth of their walls embrace them circularly, while 

 the successively deeper and more superficial layers have suc- 

 cessively steeper degrees of obliquity, and are continuous 

 one with another both at base and apex ; and in consequence 

 of this arrangement, these cavities are contracted throughout 

 their whole extent by both shortening and narrowing at the 

 same time, till they are completely emptied. But no matter 

 how completely or forcibly the ventricles might contract, 

 they would make but an inefficient engine of propulsion were 

 there not some means of preventing the blood being pushed, 

 during their contraction, back into the auricles, and re- 

 coiling, after their contraction, back into them from the 

 arteries. Such waste of power is prevented by the presence 

 of valves, which guard the arterial and auriculo- ventricular 

 orifices. 



87. The arterial valves guarding the entrances into the pul- 

 monary artery and aorta are named semilunar, because they 

 consist each of three delicate pouches, with semilunar attach- 

 ments to the wall of the artery. The pouches are placed in a 

 circle, with their mouths turned away from the heart, and 

 are pushed flat against the arterial walls when the blood is 



