THE CIRCULATION. 151 



upwards and towards the right. As the beat of the heart 

 may readily be felt where the apex touches the chest, this 

 has given rise to the popular statement that the heart is on 

 the left side. As far as actual material is concerned, prob- 

 ably as much of it lies on the right side as on the left. 



2. Coverings. It does not, however, hang loose in the 

 chest, but is supported by a double membrane covering it, 

 called the pericardium. The heart does not really lie inside 

 the pericardium, although at first appearance it seems to 

 do so. It is entirely on the outside of it, and the interior 

 of the pericardium is really the space filled by the peri- 

 cardial fluid between the two membranes. If an ordinary 

 foot-ball only loosely distended be pressed down over a fist 

 so as to surround the fist, we should have an analogous ex- 

 ample. It is evident in this case that the fist is not in the 

 collapsed foot-ball at all. This pericardium is attached to 

 the walls of the chest, and in this manner the rather heavy 

 heart is supported. The pericardium is really but a con- 

 tinuation of the pleura. 



3. Cavities. As pointed out in a previous chapter, the 

 muscular tissue of which the heart is composed is of a 

 peculiar kind, called the cardiac muscle. This muscle is 

 thrown into such folds and sheets as to form the four cav- 

 ities of the heart. Of these, the upper cavities are small, 

 and their walls quite thin. In a cut-out heart they are usu- 

 ally collapsed, and look more like little muscular append- 

 ages than large chambers. On account of their flap-like 

 appearance they are called " auricles;" that is, little ears. 



The two lower cavities comprise nearly all the substance 

 of the heart, are entirely separate from the upper cavities 

 as seen from the outside, have comparatively very thick 

 walls, the left being much the thicker, and are called "ven- 

 tricles." It is the contraction of the ventricles that forces 

 the blood through the arteries. 



4. Vessels Arising from the Heart. A number of ves- 

 sels are connected with the heart, carrying blood to it (the 



