THE HEART. 615 



being much larger than this organ, allows it free motion 

 within its walls. The pericardium consists of two layers, 

 an external or fibrous, and an internal or serous. 



The fibrous layer is attached below to the cordiform 

 tendon of the diaphragm, to which it is strongly bound by 

 compact cellular substance; laterally it is embraced by 

 the two pleuraB ; in front it corresponds to the anterior, 

 and behind to the posterior mediastinum. Above it is 

 traced upon the great vessels, proceeding from the heart, 

 upon the aorta as high as the arch, upon the pulmonary 

 artery, upon the superior cava for an inch before entering 

 the right auricle, upon the inferior cava, and upon the 

 pulmonary veins. It is prolonged upon the sheath of these 

 vessels, being insensibly lost upon, and becoming identified 

 with their external coat. In structure it is like the dura 

 mater, though thinner. It is also white, inelastic, and 

 semi-transparent. 



The internal layer of the pericardium is seen by opening 

 this bag, when, like all serous membranes, it presents a 

 smooth, polished, delicate, transparent surface, and forms 

 a shut sack. It lines the interior of the fibrous coat, and 

 is reflected thence upon the great vessels to the heart, 

 whence it is traced over this organ, covering its anterior 

 and posterior surfaces. It is connected to the heart by 

 cellular substance, having frequently interposed a quantity 

 of adipose matter. 



Function. The pericardium, by its fibrous coat, is of use 

 in fixing and retaining the heart in its natural position, and 

 preventing over-distention of its several cavities; while, 

 by its serous layer, a fluid is furnished, which both lubri- 

 cates and facilitates the motions of this organ. 



The heart, as already stated, is a hollow muscle, and 

 contains four cavities, (Fig. 198,) two of which are upon 

 the right side, and anterior, the other two upon the left 

 side, and posterior. The two upon the right being sepa- 

 rated by partitions from those on the left, constitute the 

 heart a double organ, which is distinguished into a right 

 and left heart. Two of these cavities occupy the superior 



