INTIMATE STRUCTURE. 



367 



right cartilage, the base of the ensiform process, and the seventh and sixth left car- 

 tilages, to meet the other margin at the apex. 



The apex of the heart (fig. 318, 6') is situated about 3J inches to the left of the 

 middle line, in the fifth intercostal space, just internal to the end of the fifth rib. 



The auriculo -ventricular openings lie slightly to the left of the line of the auriculo- 

 ventricular sulcus. The right orifice lies behind the sternum, on a level with the 

 fourth intercostal space and fifth cartilage. The left is behind the inner end of the 

 fourth left costal cartilage and adjacent part of the sternum. The orifice of the pul- 

 monary artery is placed immediately to the left of the sternum, at the-upper edge of 

 the third cartilage, and the pulmonary trunk extends up to the second left cartilage. 

 The aortic orifice is mainly behind the left half of the sternum, on a level with the 

 lower border of the third costal cartilage : it is a little below and to the right of the 

 pulmonary opening, by which it is covered to the extent of one-fourth of its dia- 

 meter ; and it lies exactly behind the upper part of the infundibulum of the right 

 ventricle. The ascending aorta passes upwards behind the sternum to the inner end 

 of the second right costal cartilage. 



The position of the heart is subject to some degree of individual variation, and the extent 

 of the several divisions will necessarily be dependent upon the stage of their action : the 

 condition represented in fig. 318 is that met with after death, when all the cavities are in a 

 state of relaxation. It is also influenced to a slight extent by the respiratory movements 

 and the posture of the body (p. 355). In children the heart is relatively broader and projects 

 more to the left side of the chest than in the adult, so that' the apex-beat is often perceived 

 in, or even external to, the nipple-line. In old persons the heart frequently occupies a much 

 lower position than that given above. 



INTIMATE STRUCTURE OF THE HEART. 



The heart is closely invested by a serous membrane, the cardiac pericardium 

 (epic&rdntm), and its cavities are lined by a smooth membrane, termed the endocar- 

 dium. The main substance of the organ is composed of muscular tissue (myo- 



Fig. 319. ANTERIOR VIEW OF HEART OP A 



YODNG SUBJECT DISSECTED AFTER LONG BOIL- 

 ING, TO SHOW THE SUPERFICIAL MUSCULAR 



FIBRES. (Allen Thomson. ) f 



This figure is planned after one of Luschka's, 

 but its details were chiefly taken from an ori- 

 ginal preparation. The aorta, &', and pulmonary 

 artery, a', have been cut short close to the 

 semilunar valves, so as to show the anterior 

 fibres of the auricles, a, superficial layer of 

 the fibres of the right ventricle ; Z>, that of the 

 left ; c, c, anterior interventricular groove ; 

 d, right auricle ; d', its appendix, both showing 

 chiefly perpendicular fibres ; e, upper part of 

 the left auricle ; between e, and &', the trans- 

 verse fibres which behind the aorta pass across 

 both auricles ; e', appendix of left auricle ; /, 

 superior vena cava, around which, near the 

 auricle, circular fibres are seen ; g, g', right 

 and left pulmonary veins with circular bands of 

 fibres surrounding them. 



cardium), with interstitial areolar tissue 



and numerous blood-vessels, lymphatics, 



and, in some parts, nerves and ganglia. There is also a considerable amount of fat 



chiefly collected at the base of the heart and beneath the pericardium, while in 



connection with the large orifices at the base of the ventricles a mass of fibrous 



tissue and fibro-cartilage occurs, which in some animals, as the ox, is bony, and is 



