STRUCTURE OF THE HEART 



117 



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. 



The endocardium (fig. 142) has a structure not very unlike the 

 pericardium. It is lined by a pavement-epithelium, like the epithelium 

 of a serous membrane, and consists of connective tissue with elastic 

 fibres in its deeper part, between which there may, in some parts, be 



c- 



FIG. 142. SECTION OF THE ENDOCAR- 

 DIUM OF THE EIGHT AURICLE. 



a, lining epithelium ; 6, connective tissue with 

 fine elastic fibres ; c, layer with coarser 

 elastic fibres ; d, sub-endocardial connective 

 tissue continuous with the intermuscular 

 tissue of the myocardium ; ft, muscular 

 fibres of the myocardium ; m, plain muscu- 

 lar tissue in the endocardium. 



FIG. 143. SECTION THROUGH ONE OF 

 THE FLAPS OF THE AORTIC VALVE, 

 AND PART OF THE CORRESPONDING 

 SINUS OF VALSALVA, WITH THE AD- 

 JOINING PART OF THE VENTRICULAR 

 WALL. 



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 ven- 

 tricle attached to it ; g, loose areolar tissue 

 at the base of the ventricle ; s. V. sinus 

 Valsalvae ; 1, 2, 3, inner, middle, and outer 

 coats of the aorta. 



found a few plain muscular fibres. Fat is sometimes met with under 

 the endocardium. 



In some animals, e.g. the sheep, and sometimes also in man, large 

 beaded fibres are found under the endocardium. These are formed of 

 large clear cells joined end to end, and generally containing in their 

 centre two nuclei, whilst the peripheral part of the cell is formed of cross- 

 striated muscular tissue ; they are known as the fibres of Purkinje. 



The valves of the heart are formed of folds of the endocardium 

 strengthened by fibrous tissue (fig. 143). This tissue forms a thicken- 



