170 THE HUMAN HEART. 



we have to regard the heart in its own peculiar relations to physics, 

 physiology and psychology. 



The anatomical heart is a conical, hollow, muscular organ, lying 

 obliquely in the chest between the two lungs, the base of the cone 

 pointing upwards in the direction of the right shoulder, the apex 

 pointing to the space between the fifth and sixth ribs. It rests upon 

 the tendinous portion of the diaphragm, which is the partition be- 

 tween the chest and abdomen ; and it is encased in a peculiar bag or 

 capsule, the pericardium, which consists of two layers, the outer 

 fibrous, by which the pericardium is attached to the great vessels at 

 the root of the heart ; the inner, a serous layer, continuous with the 

 serous membrane that covers the outer surface of the heart. The 

 cavity between the heart and pericardium, thus lined by a serous 

 covering, generally contains more or less fluid, whereby the heart 

 is lubricated on the outside, and its local motions are rendered easy. 



The heart comprises four cavities, two auricles and two ventricles, 

 one auricle and one ventricle being on each side, and the right pair 

 of cavities being devoted to the circulation of the venous blood, the 

 left pair, to that of- the arterial blood. The auricles are at the top, 

 constituting the base of the heart; the ventricles form the apex; the 

 latter are much stronger than the auricles, consisting of very thick 

 muscular walls, the reason of which we shall see presently. 



The heart is a peculiar muscle, and when any of its four cavities 

 contract, they have the power of expelling their contents, the force 

 of the expulsion being the prime mover of the circulation of the 

 blood. TVe may begin the circle where we please, and we shall find 

 that it returns into itself. Starting for instance from the left ven- 

 tricle, we see that the blood is driven by the contraction of that ca- 

 vity into the aorta, the highway which leads into all the arteries of 

 the body; through these the blood is discharged into the veins, which 

 unite to form at last only two great trunks, the vense cavse, which are 

 again the thresholds of the heart, and debouch into the right auricle. 

 The blood which has now passed from the left ventricle to the right 

 auricle, has still a journey to make before it completes its course. 

 Accordingly from the right auricle it is forced into the right ventri- 

 cle, and by the right ventricle, into the pulmonary artery, which 

 conveys it to the lungs, where it ramifies through the multiple 



