CHAPTER XIV. 



THE ANATOMY OF THE HEART AND BLOOD-VESSELS. 



General Statement. During life the blood is kept flow- 

 ing 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 circum- 

 stances it never leaves, constitute a con- 

 tinuous set of close^tubes (Fig. 87) 

 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 (I and a) 

 being the capillaries. The heart is essen- 

 tially a bag with muscular walls, internally 

 divided into four chambers (d, g, e, /). 

 Those at one end (d and e) receive blood 

 from vessels opening into them and known 

 as the veins. From there the blood passes 

 on to the remaining chambers (g and/) 

 which have very powerful walls and, for- 

 cibly contracting, drive the blood out into 

 vessels (m and b) which communicate with 

 them and are known as the arteries. The 

 big arteries divide into smaller; these into smaller again 

 (Fig. 88) 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 alternat- 

 ing reception of blood at one end by the heart and its ejec- 

 tion from the other go on during life steadily about seventy 

 times in a minute, and so keep the liquid constantly in 

 motion. 



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