136 HISTOLOGY. 



of the epicardial blood vessels. The capillaries of the heart are derived 

 from venous outgrowths of the coronary sinus which unite in the epicar- 

 dium with arterial outgrowths from the root of the aorta. The branches 

 of these vessels invade the myocardium where they form abundant capil- 

 lary networks and finally reach the endocardium. Some of them, especially 

 in the right atrium, empty into the cavities of the heart as small veins, 

 the venae minimae [of Thebesius]. Since under certain conditions the 

 blood may flow from the heart cavity to the myocardium through these 

 vessels, they are of considerable importance. Their embryological history 

 is unknown, so that nothing can be said concerning their possible relation 

 to the sinusoids. 



The myocardium consists of cardiac muscle, the structure of which 

 has been described on page 81-85, together with intervening connective 

 tissue, poor in elastic elements but containing many capillaries, motor 

 nerve fibers, and tissue spaces. Some lymphatic vessels pass through it. 

 The musculature of the atria is not completely separated from that of the 

 ventricles; there is an uninterrupted portion in the median septum. An 

 outer oblique layer of muscle covers both atria extending from one to the 

 other. Each has a separate inner layer of longitudinal bundles, which, 

 as found in the prominent ridges seen in the interior of the right atrium, 

 are called pectinate muscles. There are similar but less prominent struc- 

 tures in parts of the left atrium. Besides these two layers, more or less 

 definite, there are irregularly placed cardiac muscle fibers, and some which 

 extend over the terminal parts of the large veins. The annuli fibrosi serve 

 for the attachment of the ventricular muscles. The right annulus is larger 

 than the left. Similar bands of fibrous tissue surround the openings of the 

 arteries. The complex muscle layers of the ventricle may be separated 

 by maceration into bands which arise in the annuli, wind spirally around 

 the heart, and terminate in the opposite ventricle. The deeper layers 

 pass through the septum and are arranged in 8 or S shaped figures. Mus- 

 cular elevations projecting into the ventricles are called trabeculae carneae 

 if columnar, or papillary muscles, if conical. The latter may be connected 

 with the margins of the cuspid valves by fibrous prolongations, mostly 

 non-muscular, named the chordae tendineae. These structures represent 

 the trabecular framework of the embryonic heart. 



The epicardium consists of the single layered, very flat mesothelium 

 and the underlying layer of connective tissue, which contains groups of 

 fat cells. Its elastic fibers are continuous with those in the externa of 

 the large veins, but they cannot be traced beyond the roots of the aorta 

 and pulmonary artery. The epicardium contains lympathic vessels, the 

 main branches of the coronary blood vessels, and important nerves. 



