THE RABBIT. 29 



of course, a succession of contractions and dilatations of its 

 muscular wall. The contraction, or systole, commences at 

 the bases of the venae cavae and passes to the auricles, 

 driving the blood before it into the ventricles, which then 

 contract sharply and drive it on into the aorta or pulmonary 

 artery ; a pause and then a dilatation, the diastole, follows. 

 The flow of the blood is kept in one constant direction by 

 the various valves of the heart. No valves occur in the 

 opening of the precavals, but an imperfect one, the Eus- 

 tachian valve, protects the postcaval ; the direction of the 

 heart's contraction prevents any excessive back-flow into the 

 veins, and the onward tendency is encouraged by the suck 

 of the diastole of the ventricles. Between the left ventricle 

 and auricle is a valve made up of two flaps of membrane, 

 the mitral valve, the edges of the flaps being connected 

 with the walls of the ventricle through the intermediation of 

 small muscular threads, the chordce tendinece, which stretch 

 across its cavity to little muscular pillars, the papillary 

 muscles ; these attachments prevent the mitral valve from 

 flapping back into the auricle, and as the blood flows into 

 and accumulates in the ventricle it gets behind the flaps of 

 the valve and presses its edges together. When the systole 

 of the ventricle occurs, the increased pressure of the blood 

 only closes the aperture the tighter, and the current passes 

 en into the aorta, where we find three watch-pocket valves, 

 with the pocket turned away from the heart, which are 

 also closed and tightened by any attempt at regurgitation 

 (back-flow). A similar process occurs on the right side of 

 the heart, but here, instead of a mitral valve of two flaps 

 between auricle and ventricle, we have a tricuspid valve 

 with three. The thickness of the muscular walls, in view 

 of the lesser distance through which it has to force the 

 blood, is less for the right ventricle than the left. 



13. The Systemic Arteries. From the aortic or sys- 

 temic arch come off two main pairs of arteries anteriorly 

 the common carotids to the neck and head (each dividing 

 into internal and external carotids as they near the head), 

 and the subclavians to the fore- limbs. On the right side 

 these two arteries are united for a short distance as tho 



