THE CIRCULATORY SYSTEM. 93 



Capillaries 4 Cement 



[ Sti 



f Endothelial cells. 

 Cement. 

 Stigmata. 



THE HEART. 



The heart is composed of three distinct layers pericardium; 

 muscular layer, or myocardium; and endocardium. 



The pericardium is lined on its outer and inner surfaces with a 

 single layer of endothelium. Between these layers are fibrous and 

 areolar tissues, which send out septa into the myocardium. 



The muscular layer, or myocardium, is composed of striated 

 fibres which branch, and the branches anastamose with each other. 

 The fibres run in all directions, interweaving with each other, thus 

 making possible the peculiar contraction of the heart. The fibre is 

 without a sarcolemma and has centrally-located nuclei. 



The endocardium, or inner layer, is composed of elastic fibres, 

 the substance proper, and endothelium. The substance proper con- 

 tains smooth muscle fibres, which are surrounded by a delicate peri- 

 mysium. 



The annuli fibrosi consist of ligaments which lie between the 

 auricles and ventricles, and form an attachment for numerous 

 muscle fibres. 



The valves of the heart are modified endocardium. They consist 

 of endocardium and fibrous connective tissue which is continuous 

 with that of the annuli fibrosi. 



THE ARTERIES. 



Arteries comprise three coats Tunica adventitia, tunica media, 

 and tunica intima. The tunica adventitia is the external layer and 

 consists of bundles of connective tissue and elastic fibres, longitudi- 

 nally disposed. The tunica media is composed of involuntary mus- 

 cle ancl elastic tissue, circularly disposed. The tunica intima con- 

 sists of the internal elastic membrane, which is structureless in 

 character ; the subendothelial tissue, which consists of flattened cor- 

 puscles and elastic fibres ; and the endothelium, consisting of a sin- 

 gle layer of cells lining the internal cavity of the artery. 



