500 THE CIRCULATORY APPARATUS. 



of the blood canal that is the veins : it is named the auricle. The inferior 

 gives origin to the divergent or centrifugal part of the same canal, and is 

 designated the ventricle. 



The cavities of the heart are distinguished into right or anterior, and 

 left or posterior, because of their relative positions. There are, then : a 

 right auricle and ventricle, the two dark-blood pouches ; and a left auricle 

 and ventricle, situated on the track of the red-blood canal. 



Situation. The heart, enclosed as it is in a fibro-serous sac, named the 

 pericardium, is placed in the chest between the two layers of the mediastinum, 

 opposite the third, fourth, fifth, and sixth ribs ; in front of the diaphragm, 

 which separates it from the abdominal viscera ; above the sternum, which 

 appears to support it ; and beneath the vertebral column, to which it is 

 suspended by means of the large vessels. (Between the middle of the 

 anterior border of the heart, in front, and the entrance to the chest, is an 

 interval of about four inches ; and behind, at the same level, this organ is 

 at a similar distance from the diaphragm. It is distant from the fifth and 

 six-tit dorsal vertebrae, from which it is suspended, about 4J to 5 inches 

 in an average-sized Horse.) 



Form and direction. The heart presents the form of an inverted cone, 

 slightly depressed on each side, and whose axis, directed obliquely 

 downwards and backwards, deviates a little to the right at its superior 

 extremity. 



Volume. In a middle-sized Horse, the greater axis of the heart is about 

 10i inches in length ; its antero -posterior diameter, measured near the base, 

 is equivalent to about 7^ inches. Its lateral diameter does not exceed 

 from 5 to 5J inches. 



Capacity. It is very difficult, if not impossible, to obtain the exact 

 capacity of the heart's cavities. From reasoning, one is led to think that 

 the two hearts have exactly the same capacity, and that this capacity is 

 equivalent to an average of 1 to 1J pints. The amount obtained by 

 measurement is much more considerable ; but then the heart is distended 

 to a greater degree than in its physiological state. 



Weight. The weight of the heart varies with the size of animals, and 

 that to a considerable degree. Its average is about 6J pounds. (The volume 

 and weight of the heart are very much greater in well-bred than in under- 

 bred Horses. Its dimensions and capacity are greater in the living than 

 the dead animal ; as, after death, its cavities contract, particularly the aortic 

 ventricle, whose walls are the thickest. This ventricle will then scarcely 

 contain more than 3-4ths to lith gills ; the pulmonary ventricle, which 

 is not so thick, and consequently less contracted, may usually receive double 

 that quantity; while in animals experimented on when expiring, it was 

 observed that these two ventricles were much more capacious, and that each 

 contained at least from 1J to 1J pints.) 



External Conformation of the Heart. (Figs. 258, 259.) 



The cone represented by the heart is divided by a horizontal groove into 

 two unequal portions : the one superior, comprising the auricles or auricular 

 mass ; the other inferior or principal, formed by the ventricles or ventricular 

 mass. 



A. VENTRICULAR MASS. It is this which determines the conical shape 

 of the heart, and constitues its largest portion. Owing to the slight flatten- 

 ing which depresses the organ in a lateral sense, it may be considered as 



