THE HEAKT. 



499 



Anastomosing muscu- 

 lar fibres from the hu- 



works on anatomy. The muscular structures of FJ g- 330. 

 the auricles and ventricles are completely distinct; 

 both, however, originating chiefly from the ostea 

 venosa of the ventricles, where the so-called fibro- 

 cartilaginous rings are situated. 



The endocardium is a whitish membrane cover- 

 ing all the internal surface of the heart, as well as 

 the columnae carneae, the chords tendineae, and the 

 valves. It is thickest (fa of an inch) in the left 

 auricle, and thinnest in the ventricles. It consists 

 of two layers: 1st, a scaly epithelium of one or 

 two layers of clear, flattened, nucleated cells, rest- 

 ing, without any apparent basement-membrane, ma n heart. (K;:mker.) 

 upon the surface of, %dly, the elastic layer. The 

 latter, determining the varying thickness of the endocardium, has 

 its superficial layer made up of very fine, longitudinal, elastic 

 fibres, and the remainder of areolar tissue, with scattered nuclei. 

 In the auricles, this membrane becomes almost entirely an elastic 

 membrane, and is therefore quite yellow. It is very delicate over 

 the chorda3 tendineaB. Under the endocardium lies a very deli- 

 cate stratum of areolar tissue, attaching it to the muscular fibres. 

 The chordce tendinece are composed of collagenous tissue, like the 

 tendons. 



The auricula-ventricular valves present three layers a middle 

 one of areolar tissue with numerous elastic networks, and two 

 lamellae of the endocardium. Towards their free borders, these 

 three are condensed, as it were, 



into a single layer of areolar tis- Fig- 331. 



sue and elastic networks, over 

 which the epithelium is contin- 

 ued. The semilunar valves pre- 

 sent the same condition as the 

 free borders of the preceding. 

 (Fig. 331.) 



The vessels of the heart present only the following peculiarities. 

 The capillaries often encompass several of the fibres in common, on 

 account of the small size of the latter. The endocardium has very 

 few vessels, while they are plentifully distributed to the subjacent 

 layer of areolar tissue. A few vessels are seen in the auriculo- 

 ventricular valves, but never exist in the semiluuar. Lymphatics 



Elastic layer of a semilunar valve, beneath the 

 endocardium. 



