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 pe.ths, which under 

 normal circumstances it never leaves, consti- 

 tute a continous set of closed tubes (Fig. 100) 

 beginning at and ending again in the heart, 

 and simple only close to that organ. Else- 

 where it is greatly branched, the most numer- 

 ous and finest branches (I and a) being the 

 capillaries. The heart is essentially 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 power- 

 ful walls and, forcibly contracting, drive the 

 FIG 100 The heart blood out into vessels (m and b) which corn- 

 and blood-vessels dia- municate with them and are known as the 



grammatically repre- . . . 



sented. arteries. The big arteries divide into smaller; 



these into smaller again (Fig. 101) 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 



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