THE ANATOMY OF THE HEART AND BLOOD-VESSELS 323 



walls of the vessels takes place only in the capillaries, which form 

 a sort of irrigation system all through the Body. The heart, 

 arteries, and veins are all merely arrangements for keeping the 

 capillaries full and renewing the blood within them. It is in the 

 capillaries alone that the blood 

 does its physiological work. 



The Position of the Heart. 

 The heart (h, Fig. 1) 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 projecting a little 

 on the right of the sternum, 

 while its narrow end or apex, 

 turned downwards, projects to 

 the left of that bone, where 

 it may be felt beating between 

 the cartilages of the fifth and 

 sixth ribs. The position of the 

 organ in the Body is therefore 

 oblique with reference to its 

 long axis. It does not, how- 

 ever, lie on the left side as is so 

 commonly supposed but very 

 nearly in the middle line, with 

 the upper part inclined to the 

 right, and the lower (which 

 may be more easily felt beating 

 hence the common belief) to 

 the left. 



The Membranes of the 

 Heart. The heart does not lie 

 bare in the chest but is sur- 

 rounded by a loose bag com- 

 posed of connective tissue and FlG 99 ._ The hcart and blood-vessels 



called the pericardium. This diagrammatically represented L, lung; 



. , M, intestine; P, liver; dotted lines repre- 

 like the heart, IS COniCal sent lymphatic vessels. 



