THE ACTION OF THE HEART. i6 



round the stomacli to the antennse (aa). Behind these, 

 yet two other arteries are given off from the under side of 

 the heart, and supply the Uver (lia). All these arteries 

 hranch out and eventually terminate in fine, so-called 

 cajnllary, ramifications. 



In the dorsal wall of the heart two small oval aper- 

 tures are visihle, provided with valvular lips (fig. 16, 

 sa), which open inwards, or towards the internal cavity 

 of the heart. There is a similar aperture in each of the 

 lateral faces of the heart {la), and two others in its 

 inferior face {ia), making six in all. These apertures 

 readily admit fluid into the heart, but oppose its exit. 

 On the other hand, at the origins of the arteries, there 

 are small valvular folds, directed in such a manner as to 

 permit the exit of fluid from the heart, while they prevent 

 its entrance. 



The walls of the heart are muscular, and, during life, 

 they contract at intervals with a regular rhythm, in such 

 a manner as to diminish the capacity of the internal cavity 

 of the organ'. The result is, that the blood which it 

 contains is driven into the arteries, and necessarily forces 

 into their smaller ramifications an equivalent amount of 

 the blood which they already contained ; whence, in the 

 long run, the same amount of blood passes out of the 

 ultimate capillaries into the blood sinuses. From the 

 dis])osition of the blood sinuses, the impulse thus given 

 to the blood which they contain is finally conveyed to the 

 blood in the brancliise, and a proportional quantity of that 



