108 THE ANATOMY OF THE HORSE. 



THE PERICARDIUM (PLATES 22 AND 26). 



The pericardium is the bag that contains the heart. It occupies a 

 position about the centre of the thorax, and between the right and left 

 layers of the mediastinum. The sac is fibrous in structure, and is lined 

 internally by a serous membrane. Like the organ which it encloses, 

 the pericardium has a conical form, the point of the cone being fixed 

 to the floor of the sternum from about the third chondro-sternal joint to 

 within an inch of the insertion of the diaphragm across the ensiform 

 cartilage. A bove the sac is pierced by the large vessels of the heart, 

 and there its fibrous texture blends with the outer coat of the vessels. 

 Its outer surface is overspread by the mediastinal pleura, which can 

 easily be stripped off. 



Directions. — The pericardium should be pinched up, and slit from its 

 apex to near the base of the heart. 



The pericardium is considerably larger than the heart which it con- 

 tains, but this disposition is not very evident until it is opened, when 

 the sac can be pulled away from the heart and a considerable cavity left 

 between them. The inner surface of the bag and the outer surface of the 

 heart are overspread by a serous covering — the serous membrane of the 

 pericardium. The 'parietal division of this membrane is that which 

 lines the sac ; the visceral division covers the heart and the roots of the 

 great vessels at its base, investing the aorta and pulmonary artery in a 

 common tube. The visceral portion is also known as the epicardium, 

 and around the base of the heart it is continuous with the parietal 

 division, The free surface of this, as of other serous membranes, is 

 exquisitely smooth, and is formed by a single layer of endothelial cells. 

 Its object is to facilitate the movements of the heart in its sac ; and, for 

 this purpose, it is kept moist by a minute quantity of serous fluid. As 

 with the pleura, the cavity of the serous sac is only a potential one ; but 

 when inflammatory or other effusions are poured out by the membrane 

 ft becomes an actual cavity, and the parietal and visceral layers of the 

 membrane may be pushed widely apart. In old and emaciated subjects 

 this is not infrequently the case, the cavity containing a few ounces of 

 watery, dropsical fluid. 



THE HEART (PLATES 23 AND 24). 



Directions. — The pericardium may now be slit transversely, and the 

 heart should be tilted out by introducing the hand beneath its apex. 

 This will permit the examination of the exterior of the heart without 

 destroying any of its connections ; and afterwards, in order to observe 

 accurately its position, it should be restored within the pericardium. 



Exterior of the Heart. The heart is a hollow, muscular organ, 

 and acts the part of a force-pump in maintaining the circulation of the 



