4-SO THE ENDOCARDIUM. [sect. 212. 



Hum, an elastic layer upon which the various thicknesses of the 

 endocardium at different places depends, and a thin layer of con- 

 nective tissue. The epithelial layer is usually simple, but accord- 

 ing- to Lusclika, is sometimes a double stratum : its cells are 

 flattened and polygonal, generally somewhat elongated, clear, and 

 nucleated, measuring 0007'" to 0012'" in length. They are seated 

 immediately upon the most superficial lamina of the elastic coat, 

 which may be said to be composed entirely of very fine longitudinal 

 fibres. The remainder of this middle coat is formed by an or- 

 dinary matrix of connective tissue, with interspersed nuclei, and 

 is traversed by numerous networks of finer and coarser elastic 

 fibres; indeed, in the auricles, these fibres are present in such 

 numbers, even intermingled with true fenestrated membranes (see 

 § 25), that the endocardium is here quite converted into a yellow 

 elastic membrane of several layers. Lastly, there follows an outer- 

 most layer of connective tissue, which, although thin, may yet 

 readily be pulled off as a whole in the ventricles, as well as in the 

 auricles. It still contains fine elastic elements in the parts adjoin- 

 ing the elastic layer, and forms a loose layer, analogous to a 

 subserous connective tissue, uniting the muscular substance to 

 the proper endocardium. Upon the chordae tendineae, the endo- 

 cardium only consists of the epithelium, and of the innermost 

 elastic layer, whilst the loose layer of connective tissue is entirely 

 absent : it is very thin also upon the trabecular of the right ven- 

 tricle, and upon the musculi pectinati. 



The auviculo -ventricular valves are laminar proceeding from the 

 fibrous rings of the ostiavenosa ; and we may distinguish in them, 

 when they are tolerably thick, a middle layer (thicker on the left 

 side) of connective tissue, with numerous networks of elastic fibres, 

 in whose formation the radiations of the chordae tendineae are very 

 essentially concerned ; and en either side of this layer is a lamella 

 of the endocardium in connexion with it. Towards the free bor- 

 ders, these three layers nearly coalesce into a single one composed 

 of connective tissue, with fine networks of elastic fibres, over 

 which the epithelium is still continued. — The semilunar valves 

 present the same arrangement as those just described, except that 

 they are thinner. In both sorts of valves, the endocardium is 

 thicker on the side which is most stretched during life. A few 

 muscular fibres of the auricles arise here and there from the outer- 

 most edge of the middle of the auriculo-ventricular valves ; beyond 

 these, the valves are free from muscular elements. 



The blood-vessels of the muscular substance of the heart are 



