PHYSIOLOGY. 



37 



rounds of the circulation ; which, like an inverted tree, become 

 larger and less numerous as they approach the heart, where 

 they terminate in the right auricle. The auricle on the left 

 side of the heart receives the blood that has been distributed 

 through the lungs for purification. Where the veins terminate 

 in auricles, there are valves placed. The coronary vein, 

 which enters the right auricle, has its mouth protected by a 

 valve called semilunar, or half-moon shape, which opens only 

 towards the heart, and prevents the blood taking a retrograde 

 course. The different tubes coming from and entering into 

 the heart, are also provided with valves to prevent the blood 

 from returning. For example, the blood proceeds out of the 

 heart, along the aorta, (see plate ;) the valve opens upwards, 

 the blood also moves upwards, and pushes the valve asunder, 

 and passes through ; the pressure from above effectually closes 

 the passage. The valves of the heart are composed of elastic 

 cartilage, which enables them to work with ease. In some 

 diseases, however, they become ossified. This, of course, is 

 fatal. The heart and its appendages are also subject to other 

 diseases, called dilatation, softening, hardening, &c. Now, the 

 blood, having been brought from air* parts of the system by 

 the veins, enters into the vena cava, (see plate 3, fig. o, q, 

 ascending and descending portion,) which empty themselves 

 into the right auricle ; and this, when distended with blood, 

 contracts, and forces its contents into the right ventricle, 

 which, contracting in its turn, propels the blood into the pul- 

 monary arteries, (see plate,) whose numerous ramifications 

 bring it in contact with the air-cells of the lungs. It 

 then assumes a crimson color, and is then adapted to 

 build up and supply the waste. Having passed through 

 the vessels of the lungs, it continues on, and passes into the 

 left auricle : this also contracts, and forces the blood through 

 a valve into the left ventricle. This ventricle then contracts 

 in its turn, and the blood passes through another valve into 

 the great aorta, from which it is distributed into the whole 

 arterial structure : after going the rounds of the circulation, 

 it is again returned to the heart by the veins. 



