STRUCTURE OF THE HEART. 



141 



The myocardium is covered externally by a layer of serous membrane 

 the epicardium (cardiac pericardium, fig. 163, A) composed, like 

 other serous membranes, of connective tissue and elastic fibres, the 

 latter being most numerous in its deeper parts. Underneath the 

 epicardium run the blood-vessels, nerves, and lymphatic vessels of the 

 heart, embedded in areolar and adipose tissue, this tissue being con- 

 tinuous with that which lies between the muscular bundles ; and the 

 free surface of the membrane is covered by serous endothelium. 



FIG. 165. SECTION THROUGH ONE OF THE FLAPS OF THE AORTIC VALVE, AND PART OF 

 THE CORRESPONDING SINUS OF VALSALVA, WITH THE ADJOINING PART OF THE 

 VENTRICULAR WALL. (From a drawing by V. Horsley.) 



a, endocardium, prolonged over the valve ; 6, sub-endocardial tissue ; c, fibrous tissue of the 

 valve, thickened at c' near the free edge ; d, section of the lunula ; e, section of the fibrous 

 ring ; /, muscular fibres of the ventricle attached to it ; g, loose areolar tissue at the base 

 of the ventricle ; s. V. sinus Valsalvse ; 1, 2, 3, inner, middle, and outer coats of the aorta. 



The endocardium (fig. 163, B) has a structure not very unlike the 

 pericardium. It is lined by a pavement-epithelium or endothelium, 



