584 CIRCULATORY APPARATUS. 



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 cavities ; and a left auricle and ventricle, 

 situated on the track of the red-blood canal. 



Situation. — The heart is enclosed in a fibro-serous sac, named the pericardium, 

 and 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 sunilar distance from the diaphragm. 

 It is distant from the fifth and sixth dorsal vertebrae— from which it is suspended 

 — about 4| or 5 inches in an average-sized Horse.) 



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

 depressed on each side, the axis of which, directed obliquely downwards and 

 backwards, deviates a Uttle to the right at its superior extremity. 



Volume. — In a medium-sized Horse, the greater axis of the heart is about 10^ 

 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 

 5^ inches. 



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

 of the heart's cavities. From reasoning, it might be supposed that the two hearts 

 have exactly the same capacity, and that this is equivalent to an average of from 1 

 to 1:^ pints. The amount obtained by measurement is much more considerable ; 

 but then the heart is distended to a greater extent than in its physiological state. 



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

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

 and weight of the heart are very much greater in w'ell-bred than in common-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, 

 which has the thickest walls. This ventricle will then scarcely contain more 

 than from f to I5 gills ; the pulmonary ventricle, which is not so thick, and con- 

 sequently less contracted, may usually receive double that quantity ; while in 

 animals experimented on when expiring, it has been observed that these two 

 ventricles were much more capacious, and that each contained at least from 

 1:^ to 1^ pints.) 



2. External Conformation of the Heart (Figs. 349, 350). 



Preparation. — Remove the heart, and with it a certain length of the vessels belonging to it ; 

 fill its cavities with tow, and free the furrows from the adipose deposited in them. 



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

 unequal portions : the one superior, comprising the auricles or auricidar 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 constitutes its largest portion. Owing to a slight flattening of the 

 organ in a lateral sense, it may be considered as having a light and left face, an 

 anterior and posterior border, an apex, and a base. 



The right face, smooth and rounded, is traversed by a vascular f m'row (ri^ht 



