HEART 195 



HEART 



The wall of the heart consists of interlacing bundles of cardiac mus- 

 cle fibers, the myocardium, which are covered externally by the epicar- 

 dium, a serous membrane which forms the visceral layer of the peri- 

 cardium. Internally the muscular wall of the heart is lined by the 

 endocardium, which resembles the serous membranes in that it consists 

 of pavement epithelium supported upon a layer of fibre-elastic connective 

 tissue. The endocardium lines all the cavities of the heart, and its endo- 

 thelium is directly continuous with that of those arteries and veins 

 which are connected with the cavity of the heart. Thus the entire 

 vftscular system heart, arteries, capillaries, lymphatics, and veins may 

 be said to be lined by an uninterrupted sheet of pavement epithelial cells, 

 the endothelium. 



Myocardium. The muscle cells of the myocardium are so disposed 

 as to form long fibrous bundles which by their figure-of-8 arrangement 

 are interwoven with one another to form a dense interlacing mass of mus- 

 cle bundles. (For detailed description see Mall, Amer. Jour. Anat., 11, 3, 

 1911.) The structure of these cardiac muscle fibers has already been 

 described. Because of the irregularity of their disposition, transections 

 of the cardiac wall present sections of muscle fibers which have been 

 cut in every conceivable direction. 



Between the muscle fibers is a very delicate framework of fibrous 

 connective tissue, the endomysium, which surrounds the muscle fibers 

 and supports the abundant capillaries, arterioles, and venules, with 

 which they are supplied. The proportion of connective tissue in the 

 normal myocardium as compared with the muscle is, nevertheless, very 

 small. 



In certain portions of the myocardium connective tissue is more abun- 

 dant. Thus it is slightly increased in the vicinity of the endocardium, in 

 the papillary muscles, and near the bases of the cardiac valves. At the 

 surface of the heart, beneath the epicardium, especially in the various 

 grooves on the surface of the heart, the connective tissue is still more 

 abundant, and may contain groups of fat cells. It is through these 

 accumulations of connective tissue that the larger blood-vessels are dis- 

 tributed to the myocardium. 



Epicardium. The epicardium, like the other serous membranes, 

 consists of a layer of pavement cells, so joined edge to edge as to form 

 a complete mesothelial coat. Here and there the mesothelium present, i 



