130 MICROSCOPIC ANATOMY OF TllK ORGANS. 



1. Endocardium; 2. Myocardium; ami :>. Epicardium (vis- 

 ceral layer of the pericardium). 



(1) The endocardium is a connective-tissue membrane 

 which contains smooth muscle and elastic tissue fibres. It is 

 situated immediately outside the endothelial sac which lines 



x 



the cavity of the heart. The endocardium is spoken of usually 

 as including both the endothelial layer and the smooth muscle 

 and elastic fibres outside it. The cells making up the endo- 

 thelial layer are polygonal, and are continuous with the 

 endothelial lining of the vessels. 



(2) The myocardium forms the main part of the heart wall. 

 The layer is much thicker in the left ventricle than elsewhere. 

 The finer structure of the muscle cells has already been de- 

 scribed. By joining together laterally the branched cells form 

 a network, the strands of which are bound together by con- 

 nective tissue. The course of these strands of cells is not the 

 same in different parts of the heart wall. In the auricles we 

 find a superficial layer common to both, and a deeper layer 

 belonging to each chamber. In the ventricles the most super- 

 ficial layers are seen to run at right angles to the deepest. 

 Between these there are fibres in all stages of transition. At 

 the apex they form a whorl or vortex, disappearing from the 

 surface in the depths. This very complicated structure is much 

 simplified by a study of embryonic hearts by macerating 

 methods. If hearts be taken from pigs' embryos, about 

 150 mm. in length, and macerated in nitric acid (commer- 

 cial), 1 part; glycerin, 2 parts; water, 2 parts, the con- 

 nective tissue binding the muscle strands together is dis- 

 solved or destroyed. The course of the fibres may then be 

 traced by dissection, and has been described in some detail 

 (J. B. MacCallum). The superficial fibres are found to have 

 their origin in either auriculoventricular ring, to wind about 

 the heart spirally, and to end in tendons of the papillary 

 muscles of the opposite ventricle. The deep layers also begin 

 in the tendon of one auriculoventricular ring, pass around to 

 the interventricular septum, cross over forward or backward 

 in this septum, and end in the papillary muscles of the other 



