THE HEART. 305 



do, while at the time no lymph is flowing in the lymphatic vessels 

 of the part. When lymph accumulates, whether in a serous 

 cavity or in the cellular tissue beneath the skin, it constitutes 

 dropsy or edema. 



The lymph is collected by the lymphatic vessels and ultimately 

 reaches the blood-circulation again (p. 330). 



CHYLE. 



The term chyle is applied to that portion of the lymph which 

 comes from the small intestine during the period of digestion. 

 The tissues of this portion of the body are, like all others, bathed 

 in lymph ; but during digestion such products as enter the lacteals 

 change its composition to a considerable extent, and the fat gives 

 to it a milky appearance. The following is an analysis of chyle 

 taken from a fistula of the thoracic duct in man (Paton) : 



Water 95.34 



Proteids 1.37 



Fats 2.40 



Cholesterin 0.06 



Lecithin 0.03 



Inorganic constituents 0.56 



CIRCULATORY SYSTEM. 



The blood in carrying nutrition to, and in carrying waste 

 products from, the tissues passes through the entire circulatory 

 system, and this constitutes the circulation of the blood. Before 

 studying this process in detail it is essential to have a knowledge 

 of the organs concerned in carrying it on i. e., the circulatory 

 organs. These are (1) the heart, (2) the arteries, (3) the capillaries, 

 and (4) the veins. 



THE HEART. 



The heart, together with the great blood-vessels at its base, is 

 enclosed in the pericardium, a fibroserous membrane having an 

 external fibrous and an internal serous layer. The serous layer 

 not only lines the inner surface of the sac, forming the parietal 

 portion, but it also covers the heart itself; this portion is the 

 visceral portion or epicardium ; its structure is similar to that 

 of other serous membranes, being composed of connective tissue 

 and elastic fibers, beneath which are the blood-vessels, nerves, and 

 lymphatics of the heart. 



The myocardium, or muscular structure of the heart, is 

 composed of transversely striated muscular fiber-cells, each con- 

 taining a single nucleus. They diifer from voluntary muscle in 

 possessing no sarcolemma, in branching and uniting with adjoining 

 cells, and in having their striae less pronounced (p. 61). 



The endocardium, which lines the heart and takes part in 

 the formation of the valves, resembles the epicardium in struct- 

 ure, and is covered by endothelium. 



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