290 THE HUMAN BODY 



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 down- 

 wards, projects to the left of that bone, where it may be felt beat- 

 ing 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 com- 

 monly 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 surrounded by a loose bag composed of connect- 

 ive 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 membrane like that lining the ab- 

 dominal cavity, and a similar layer (the visceral layer of the 

 pericardium) covers the outside 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, covered 

 with a single layer of flattened cells, and called the endocardium. 

 Between the endocardium and the visceral layer of the pericar- 

 dium the bulk of the wall of the heart lies and is made up mainly 

 of striped muscular tissue (myocardium) differing from that of 

 the skeletal muscles; but connective tissues, blood-vessels, nerve- 

 cells, and nerve-fibers are also abundant in it. 



Note. Sometimes the pericardium becomes inflamed, this af- 

 fection being known as pericarditis. It is extremely apt to occur 

 in acute rheumatism, and great care should be taken never, even 

 for a moment, except under medical advice, to expose a patient 

 to cold during that disease, since any chill is then especially apt 

 to set up pericarditis. In the earlier stages of pericardiac inflam- 

 mation the rubbing surfaces on the outside of the heart and the 

 inside of the pericardium become roughened, and their friction 



