CHAPTER YL 

 THE CIRCULATORY SYSTEM. 



THE apparatus by which the circulation of the blood 

 throughout the body for the nourishment of the tissues is 

 accomplished consists of the heart, covered by the pericar- 

 dium ; and the bloodvessels : arteries, veins, and capillaries. 



The heart is a hollow muscular organ, whose walls are made 

 up of a thick mass of muscle, the myocardium, lined on the 

 inside by a delicate endothelial membrane, the endocardium ; 

 and on the outside by a serous membrane, the visceral portion 

 of the pericardium. 



The endocardium, which lines the interior of the heart, is 

 similar to the lining of the bloodvessels, of which it is a 

 continuation. It is thinner than the pericardium. Its free 

 surface is lined by polygonal endothelium-cells, which lie on 

 a firm connective-tissue basis of interlacing white and elastic 

 fibres. The lowermost tissue of the endocardium merges 

 gradually into the delicate connective tissue which occupies 

 the interstices between the muscle-cells of the myocardium. 



The valves of the heart and arterial orifices consist of folds 

 or reduplications of the endocardium, fortified and strength- 

 ened by additional white and elastic fibrous tissue. The tis- 

 sues at the bases of the valves are strengthened into well- 

 marked fibrous rings, which serve as a basis for the attach- 

 ment or insertion of the valves and muscular bundles of the 

 myocardium. 



The myocardium is a muscular mass making up the main 

 substance of the heart. The cardiac muscle-cells of which it 

 is composed are of a kind peculiar to the heart, and have been 

 already described (Fig. 29). The muscular fibres are arranged 

 in bundles or layers which in different places run in different 

 directions, transverse, longitudinal, oblique, spiral, in an intri- 

 cate manner. 



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