CHAPTER XIX 

 THE ANATOMY OF THE HEART AND BLOOD-VESSELS 



General Statement. During life the blood is kept flowing with 

 great rapidity through all parts of the Body (except the few non- 

 vascular tissues already mentioned) in definite paths prescribed 

 for it by the heart and blood-vessels. These paths, which under 

 normal circumstances it never leaves, constitute a continuous set of 

 closed tubes (Fig. 99) beginning at and ending again in the heart, 

 and simple only close to that organ. Elsewhere it is greatly 

 branched, the most numerous and finest branches being the 

 capillaries. The heart is essentially a bag with muscular walls, 

 internally divided into four chambers (see figure). Those at one 

 end receive blood from vessels opening into them and known as the 

 veins. From there the blood passes on to the remaining chambers 

 which have very powerful walls and, forcibly contracting, drive the 

 blood out into vessels which communicate with them and are 

 known as the arteries. The big arteries divide into smaller; these 

 into smaller again (Fig. 100) until the branches become too small 

 to be traced by the unaided eye, and these smallest branches 

 end in the capillaries, through which the blood flows and enters 

 the commencements of the veins; and these convey it again to 

 the heart. At certain points in the course of the blood-paths 

 valves are placed, which prevent a back-flow. This alternating 

 reception of blood at one end by the heart and its ejection from 

 the other go on during life steadily about seventy times in a minute, 

 and so keep the liquid constantly in motion. 



The vascular system is completely closed except at two points 

 in the neck where lymph-vessels open into the veins; there some 

 lymph is poured in and mixed directly with the blood. Accord- 

 ingly everything which leaves the blood must do so by passing 

 through the walls of the blood-vessels, and everything which enters 

 it must do the same, except matters conveyed in by the lymph 

 at the points above mentioned. This interchange through the 



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