THE HEART. 513 



described a thin muscle, nine inches long, situated between, the pericardium 

 and the diaphragm of the Horse.) 



6. Action of the Heart* 



The function of the heart is to maintain the circulation of. the blood, by 

 the rhythmical contractions of its two pouches. The right pouch sends 

 that fluid to the lungs, whence it returns to the left pouch, and from 

 this it is thrown into all parts of the body, and is brought back again to the 

 right heart. These contractions take place simultaneously in the two cardiac 

 compartments. 



In taking the heart at the moment when it is in a state of repose : that 

 is, in the intervals between the two contractions, we find that its two 

 pouches are being rapidly filled with the blood brought to it by the venous 

 openings. When sufficiently replete, the auricles slightly contract and 

 push a portion of the fluid they contain into the ventricles : these contracting 

 immediately after, to propel the blood into the arterial ramifications. This 

 passage of the blood into the arteries is a necessary consequence of the 

 contraction of the ventricles, as at the moment of this contraction the 

 auriculo-ventricular valves are raised, and so prevent the reflux of the 

 blood into the auricles. This fluid is then forced to enter the arterial 

 orifices, whose valves are separated under the impulsive effort communicated 

 to the column of blood. When the heart returns to a state of repose, these 

 valves fall down, preventing the return of the blood into the ventricular 

 cavities ; while the mitral and tricuspid valves subside against the walls of 

 these cavities, and thus again allow the passage of blood through the 

 auriculo-ventricular openings. 



By the term systole is designated the contraction of the heart's cavities, 

 and by diastole, the repose or relaxation of its tissue. For each revolution 

 of the heart there is, therefore : 1, The general diastole of the organ, during 

 which the two cardiac cavities are filled by the afflux of venous blood; 

 2, The systole of the auricles, the effect of which is the repletion of the 

 ventricles ; 3, The systole of the ventricles, propelling the blood into the 

 arterial systems ; after which comes another period of general diastole. 



DIFFERENTIAL CHARACTERS IN THE HEART OF OTHER THAN SOLIPED ANIMALS. 



In the Ox, Sheep, and Goat, the ventricular mass of the heart is more regularly 

 conical than in Solipeds ; it has three longitudinal furrows, one of which is accessory 

 and passes behind the (left) ventricle. 



In the Ox two small bones, named bones of the heart, are found in the substance of 

 Ihe aortic zone. The largest is in the right side, at the point where the arterial ring is 

 approximated to the auriculo-ventricular zones : the other, situated in the left, is perhaps 

 not constantly present. The first is triangular in shape, curved to the right and its 

 base directed upwards. The right face lies against the auriculo-ventricular opening ; 

 the left is covered by the walls of the aorta at its commencement. It is about an inch in 

 length. (The Ox's heart averages from about 3| to 4i Ibs., that of the Sheep from 5J to 

 7 oz. It is more elongated and pointed in Ruminants than in the Horse or Pig. The 

 large bone in the Ox's heart is elongated from before to behind, flattened laterally and 

 curved to the left ; its surtace is roughened, and its length is sometimes about 2 inches. 

 The left, or small bone, is usually flattened on each side and triangular, one of its points 

 is directed forwards, another backward, and a third inferiorly ; its length is about three- 

 quarters of an inch when fully developed. Besides the Ox, a small cross-shaped bone is 

 found in the heart of the Sheep, Pig, Camel, Deer, Giraffe, and fometimes in the Horse. 

 Eemak found in Ihe pericardium of the Ox, at the border of the- left auricle, a row of 

 villi similar to those discovered in the border of the chicken's heart.) 



The heart of the Pig resembles that of the Horse ; its direction is a little more oblique, 



