284 SPECIAL HISTOLOGY. 



there presents its natural colour. As regards its structure, it 

 consists of three layers; an epithelium, an elastic layer, upon 

 which the varying thickness of the endocardium in different situa- 

 tions 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, 

 , 007 /// to 0*01 2'" 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 scattered 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 laminae. Most 

 externally, there succeeds to this elastic layer, a stratum of con- 

 nective 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 portion bordering upon the 

 elastic tissue this layer contains fine elastic elements. It repre- 

 sents, in fact, a somewhat loose layer, like a subserous con- 

 nective 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 lamella of the endocardium 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 chorda 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 blood-vessels of the muscular substance of the heart are 

 very numerous, but differ in no respect from those of trans- 

 versely striped muscle, (§ 77), except that the capillaries, 

 owing to the slenderness of the muscular fibres, often encompass 



