THE HEART 



565 



the segments the wall of the aorta presents slight dilatations, the sinuses of 

 Valsalva, which are larger than those at the origin of the pulmonary artery. 



The columnae carneae are of three kinds, T ike those upon the right side; but they 

 are more numerous, and present a dense interlacement, especially at the apex, 

 and upon the posterior wall. The musculi papillares are two in number, one being 

 connected to the anterior, the other to the posterior wall ; they are of large size, and 

 terminate by free rounded extremities, from which the chordae tendineae arise. 

 The chordae tendineae from each papillary muscle are connected to both cusps 

 of the mitral valve. 



The interventricular septum (septum ventriculorum) is directed obliquely back- 

 ward and to the right, and is curved with the convexity'toward the right ventricle ; 

 its margins correspond with the interventricular grooves. The greater portion 

 of it is thick and fleshy (septum musculare ventriculorum), but its upper and 

 posterior part, which separates the aortic vestibule from the lower part of the right 

 auricle and upper part of the right ventricle is thin and fibrous, and is termed 

 the undefended or membranous part of the interventricular septum (septum mem- 

 branaceum ventriculorum). It is derived from the lower part of the aortic septum 

 cf the fetus, and an abnormal communication may exist at this part, owing to 

 defective development of this septum. 



\RIGHT AURICULO- 

 rENTRICULAR ORIFICE 



PULMONARY 



LEFT AURICULO- 

 VENTRICULAR ORIFICE 



FIG. 420. Fibrous rings at the base of the ventricles. (Poirier and Charpy.) 



Capacity of the Cavities of the Heart. Each of the cavities of the heart 

 is capable of holding about 100 c.c., but this is subject to considerable variation 

 among different individuals. 



Structure of the Heart. The heart is a hollow muscular organ, and its walls are divisible 

 into three coats the endocardium, myocardium, and epicardium, or visceral layer of the peri- 

 cardium (page 552). 



The endocardium is a thin, smooth, serous membrane which lines and gives the glistening 

 appearance to the internal surface of the heart; it assists in forming the valves by reduplications, 

 and is continuous with the endothelial coat of the bloodvessels which pass to and emerge from 

 the heart. It is composed of endothelial cells resting upon a fibroelastic membrane which 

 contains unstriated muscle cells. The endocardium is more opaque on the left than on the right 

 side of the heart, thicker in the auricles than in the ventricles, and thickest in the left auricle. 

 It is thin on the musculi pectinati and on the columnae carneae, but thicker on the smooth 

 parts of the auricular and ventricular walls and on the tips of the musculi papillares. 



The fibrous rings (annuli fibrosi] surround the auriculo ventricular and arterial orifices; 

 they are stronger upon the left than on the right side of the heart, and are composed of dense 

 white fibrous connective tissue. The auriculoventricular rings serve for the attachment of the 

 muscle fibres of the auricles and ventricles, and also for the mitral and tricuspid valves; the 



