106 THE ANATOMY OF THE HORSE. 



Directions. — The pericardium should be pinched up, aud 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 puUed 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 se7-ous memhrane of the 

 2}ericardium. The pa7-ietal division of this membrane is that which 

 lines the ^c ; 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 efl^isions are poured out by the membrane, 

 it 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 considerable 

 quantity 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 tlie hand beneath its apex. 

 This will permit the examination of the exterior of the heart withoiit 

 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 blood. 

 In its interior there are four cavities — two auricles and two ventricles, 

 the auricle of each side being placed above the ventricle. This sub- 

 division of the interior of the heart into cavities is indicated on its 

 exterior by certain grooves. Thus, the auriculo-ve utricular groove runs 

 around the heart like a belt, and marks off the auricles from the 

 ventricles. Although this groove is carried quite roimd the heart, it 

 is not very evident in front, being concealed there by the origins of the 

 aorta and pulmonary artery. Two other grooves, one on the right side, 

 the other on the left, descend from the base of the heart, and become 

 continuous a little in front of the apex. These grooves correspond to 

 the edges of the septum which separates the cavities of the right side 

 from those of the left. They are much more distinctly marked on the 

 ventricular portion of the heart, where they are termed the ventricular 



