670 SPECIAL HISTOLOGY. 



elastic layer, upon which the varying thickness of the endocardium in 

 different situations depends, and a thin layer of connective tissue. The 

 first is a single, or, according to Luschka, perhaps a double layer of 

 polygonal, usually elongated, clear, flattened, nucleated cells, 0-007 to 

 0.012 of a line long, resting immediately upon the most superficial layer 

 of the elastic membrane, which may be said to consist of nothing but 

 very fine, longitudinal, elastic fibres. The remainder of this middle 

 layer is constituted of a matrix of common connective tissue with scat- 

 tered nuclei, through which the very abundant finer and coarser elastic 

 networks penetrate. This elastic element is so abundant, in fact, in the 

 auricles, and even mixed with true fenestrated membranes {vid. § 23) that 

 the endocardium of those cavities is rendered almost entirely a yellow, 

 elastic membrane, consisting of several lamina. Most externally, there 

 succeeds to this elastic layer, a stratum of connective tissue, of great 

 tenuity indeed, but which, nevertheless, both in the ventricles and in 

 the auricles, may easily be raised as an entire membrane. In the por- 

 tion bordering upon the elastic tissue this layer contains fine elastic 

 elements. It represents, in fact, a somewhat loose layer, like a sub- 

 serous connective tissue, uniting the muscles and the true endocardium. 



The auriculo-ventricular valves are lamince springing from the fibrous 

 rings of the ostia venosa, in the thicker parts of which, a middle layer 

 of connective tissue with numerous elastic networks, and two lamellce of 

 the eiidocardium united with it, may be clearly distinguished. Towards 

 the free border, these three layers are conjoined so as to form a single 

 one, composed of connective tissue and fine elastic networks, over which 

 again the epithelium is continued. The semilunar valves present the 

 same conditions as the free border of the others, and with respect to the 

 cliordoe tendinea^ they are constituted of common tendinous tissue, 

 covered by a very thin layer of endocardium — consisting, indeed, merely 

 of epithelium and a fine elastic lamella. 



The bloodvessels of the muscular substance of the heart are very 

 numerous, but differ in no respect from those of transversely striped 

 muscle (§ 77), except that the capillaries, owing to the slenderness of 

 the muscular fibre, often encompass several of them in common. The 

 endocardium is tolerably well supplied with vessels in its layer of con- 

 nective tissue, whilst they are more scanty in the proper endocardium.. 

 In the auriculo-ventricular valves a few vessels are readily seen, not 

 only in animals but also in Man {vid. Luschka, 1. c, p. 182, Fig. 1), 

 some of which enter them from the papillary muscles, but chiefly from 

 the basis of the valves, and are also distributed in part, though sparingly, 

 in their proper endocardial investment. The semilunar valves possess 

 no vessels. Only a few lymphatics are found in the external lamella of 

 the 2:)e7'icardium, whilst they occur in greater abundance on the inner 

 lamella on the muscular substance, and may there be demonstrated 



