CHAPTER III. 



CIRCULATORY SYSTEM, BLOOD, MARROW, AND 

 LYMPHATIC ORGANS. 



HEART. 



The heart is a muscular organ. Its wall consists 

 of three layers, endocardium, myocardium, and 

 epicardium. 



1. The endocardium is a serous membrane that 

 covers the inner surface. Histologically it consists 

 of two layers, an inner lining of simple squamous 

 epithelial cells (endothelium or mesothelium), and 

 an outer layer composed of connective-tissue fibers, 

 connective-tissue cells, and smooth muscle cells. 

 The endocardium is reflected over the heart valves 

 where the smooth muscle is particularly abundant. 



2. The myocardium is the middle layer and forms 

 the mass of the heart wall. It consists of muscle 

 tissue, the cardiac muscle already described (page 87) . 

 This muscle consists of many layers that course in 

 different directions with connective-tissue elements 

 intervening, in which branches of the coronary 

 blood-vessels ramify. 



3. The epicardium is the outer covering, a serous 

 membrane, and histologically similar to the endo- 

 cardium, with a greater deposit of fat. The epi- 

 cardium is reflected to form the pericardium, the 



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