HEART 97 



cations of the tunica intima between which is a slightly increased 

 amount of connective tissue. The valves therefore are suspended 

 free in the lumen of the vessel and are covered on either side with 

 a layer of endothelium which is continuous with that lining the 

 vein. 



The fact should be borne in mind that it is because of their 

 relative infrequency that valves are not often met with in those 

 transections of the smaller veins which are seen in nearly all 

 microscopical preparations. 



HEART. The wall of the heart consists of interlacing bundles 

 of cardiac muscle fibres, the myocardium, which are covered ex- 

 ternally by the epicardium, a serous membrane which forms the 

 visceral layer of the pericardium. Internally the muscular wall of 

 the heart is lined by the endocardium, which resembles the serous 

 membranes in that it consists of pavement endothelium supported 

 upon a layer of connective tissue. The endocardium lines all the 

 cavities of the heart, and its endothelium is directly continuous 

 with that of those arteries and veins which are connected with the 

 cavity of the heart. Thus the entire vascular system heart, ar- 

 teries, capillaries, lymphatics, and veins may be said to be lined 

 by an uninterrupted sheet of pavement epithelial cells, the endo- 

 thelium. 



Myocardium. The muscle cells of the myocardium are so dis- 

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

 arrangement are interwoven with one another to form a dense in- 

 terlacing mass of muscle bundles. The structure of these cardiac 

 muscle cells has already been described. Because of the irregu- 

 larity of their disposition, transections of the cardiac wall present 

 sections of muscle fibres which have been cut in every conceivable 

 direction. 



Between the muscle fibres is a very delicate framework of 

 fibrous connective tissue, the endomysium, which surrounds the 

 muscle fibres 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 abundant. 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 epi- 

 cardiurn, and especially in the various grooves on the surface of 

 8 



