THE ACTION OF THE HEART. 73 
round the stomach to the antenne (aa). Behind these, 
yet two other arteries are given off from the under side of 
the heart, and supply the liver (ha). All these arteries 
branch out and eventually terminate in fine, so-called 
capillary, ramifications. 
In the dorsal wall of the heart two small oval aper- 
tures are visible, 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 (Ja), 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 
disposition of the blood sinuses, the impulse thus given 
to the blood which they contain is finally conveyed to the 
blood in the branchiz, and a proportional quantity of that 
