POSITION OF THE HEART. 203 



must do so by oozing 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 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 

 t lie capillaries full and reneAving the blood within them. 

 It is in the capillaries alone that the blood does its phy- 

 siological work. 



The Position of the Heart. The heart (h, Fig. 1) lies 

 in the chest immediately above the diaphragm and oppo- 

 site 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, however, 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 easier 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 surrounded by a loose bag composed 

 of connective tissue and called the pericardium. This bag, 

 like the heart, is conical but turned the other way, its broad 

 part being lowest and attached to the upper surface of the 

 diaphragm. Internally it is lined by a smooth serous mem- 

 brane like that lining the abdominal cavity, and a similar 

 layer (the visceral layer of the pericardium) covers the out- 

 side of the heart itself, adhering closely to it. Each of the 

 serous layers is covered by a stratum of flat cells, and in the 

 space between them is found a small quantity of liquid 

 which moistens the contiguous surfaces, and diminishes the 

 friction which would otherwise occur during the movements 

 of the heart. 



Internally the heart is also lined by a fibrous membrane, 



