Tim POSITION OF Tim HEART. 195 



vessels (i and Z>) which communicate with them and 

 are known as the arteries. The big arteries divide into 

 smaller; these into smaller again (Plate IV), 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 ; the veins convey it again to the heart. At certain 

 points in the course of the blood-path valves are placed, 

 which prevent a back-flow. This alternating reception of 

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

 other occurs about seventy times a minute during health. 



The Position of the Heart. The heart (h, Fig. 4) lies 

 in the chest, immediately above the diaphragm and opposite 

 the lower two thirds of the breast-bone. It is conical in 

 form, with its base or broader end turned upwards and pro- 

 jecting a little on the right of the sternum, while its nar- 

 row end or apex, turned downwards, projects to the left of 

 that bone, where it may be felt beating between the carti- 

 lages of the fifth and sixth ribs. The position of the organ 

 in the body is, therefore, oblique. It does not, however, lie 

 on the left side, as is so commonly believed, but very nearly 

 in the middle line, with the upper part inclined to the 

 right, and the lower (which may be more easily felt beat- 

 ing hence the common belief) to the left. 



The Pericardium. The heart does not lie bare in the 



What then happens to it? How are the small arteries formed? 

 In what vessels do they end? What becomes of blood which has 

 flowed through the capillaries? Where do the veins carry it? How 

 is back- flow prevented? How frequently does the heart receive and 

 pump out blood? 



Where is the Heart situated? What is its form? Where does its 

 base lie? Where its apex? Where may we feel the apex beat? 

 What is the origin of the common belief that the heart is on the left 

 side? 



