76 



THE CIRCULATORY SYSTEM. 



Cardiac muscle-tissue is not compact, but the sides of the 

 muscle-cells are separated by slight intervals or interstices 

 through which delicate connective tissue (the interstitial tissue), 

 capillaries, lymphatics, and nerves permeate the entire myo- 

 cardium. The blood-supply of the heart, including the endo- 

 cardium and visceral pericardium, is derived from the coronary 

 vessels; the muscle-cells are richly supplied with capillaries. 



The pericardium is a typical serous sac, consisting of a fibro- 

 elastic membrane (fibrous pericardium) lined on the free interior 

 surface by a single layer of polygonal endotheli urn-cells (serous 



FIG. 37. 



Cross-section of artery (Dunham), a, endothelial lining ; 6, internal elastic lamina ; 

 c, tunica media ; d, tunica adventitia ; e, adipose tissue ; /, small nerve. 



pericardium) ; beneath which is the subserous or subpericardial 

 areolar tissue, which in this situation often includes masses of 

 fatty tissue. The visceral portion of the serous pericardium, 

 sometimes called the epicardium, envelops the outside of 

 the heart, while its parietal portion is reflected on and lines 

 the inner surface of the fibrous pericardium. Under the portion 

 of the pericardium which covers the heart the subpericardial 

 tissue, areolar and fatty, is continuous with and merges into 

 the interstitial connective tissue of the myocardium. 



