THE HEART. 117 



clironously, and are structurally one heart. Anatomically 

 considered, the natural division of the heart is into an 

 auricular and ventricular part, separated by a deep sulcus, 

 the auriculo-ventricular groove. The ventricular part is a 

 strong musculaj 1 structure invested completely with the serous 

 covering of the pericardium, and unconnected with other 

 viscera. It is directed downwards, forwards, and to the left 

 side, resting on the diaphragm^ in man, and narrowing to the 

 apex, which is felt beating opposite the interval between the 

 sixth and seventh costal cartilages of the left side. The apex 

 is formed entirely by the walls of the left ventricle, which 

 are three times as thick as those of the right ventricle, the 

 blood requiring much greater force to propel it through the 

 system than to send it through the lungs; and if the ven- 

 tricles be cut across, the section of the left ventricle will be 

 seeii to be circular, while the right is curved crescentically 

 round it. 



Above the auriculo-ventricular groove, ascending from the 

 base of the ventricles, the two arterial trunks issuing from 

 those two cavities lie close together behind the breast-bone, 

 each twisted somewhat round the other. That which rises 

 from the right ventricle is the pulmonary artery, and divides 

 into a right and left branch, one going to each lung; while 

 the systemic artery, arising from the left ventricle, and con- 

 cealed at its origin by the pulmonary artery, is called the 

 aorta. 



The auricles have exceedingly thin muscular walls, their 

 whole function being to receive the blood, which continues 

 pouring in during the contraction of the ventricles, and to 

 pass it into them through the large auriculo-ventricular aper- 

 tures as soon as they relax. They lie behind and to the sides 

 of the arterial trunks, and each is prolonged into a pointed 

 cul-de-sac in front, which, from a fancied resemblance to a 

 dog's ear, is called the auricular appendage; and these ap- 

 pendages have given their name of auricles to the cavities 

 to which they belong. The cavities are separated one from 

 the other by a thin septum, which, as seen from the right 

 auricle, presents a depression and, in front of it, a crescentic 

 border, the fossa and annulus ovalis, marking the position of 

 an opening which exists, and is made use of, in foetal life, 



