598 CIRCULATORY APPARATUS. 



part of the venae cavas — particularly the anterior — spreads over the insertion of the 

 pulmonary veins, and then descends on the auricles and ventricles. The free face 

 of this layer is in contact with that of the parietal layer ; the adherent face is 

 applied to the tissue of the heart or that of the large vascular trunks, except at 

 the horizontal and vertical grooves, where it rests on the coronary vessels, and on 

 the mass of adipose tissue constantly accumulated on their track. 



In the living animal, the cavity of the pericardium is never entirely filled by 

 the heart, so that the movements of that organ are allowed much more liberty. 

 Othenvise, as it does not contain any gas, nor a sensible proportion of fluid,^ 

 its walls are immediately applied to the surface of the heart. 



The two layers of the pericardium are covered by a layer of endothelial 

 polygonal cells. 



Blood reaches the pericardium by the mediastinal arteries. Its walls receive 

 some sympathetic nerve-fibres. 



(The epicardium is composed of a fine network of connective tissue and elastic 

 fibres, as well as bundles of non-striped muscular fibres ; these resist the distension 

 that occurs when the heart contracts and a great strain is thrown upon the 

 endocardium. Gurlt, in 1867, described a thin muscle, niue 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 cavities. The right side propels that fluid 

 to the lungs, whence it returns to the left side, and from this it is thrown into 

 all parts of the body, after which it 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 veins. When suffi- 

 ciently replete, the auricles slightly contract and push a portion of the fluid they 

 contain into the ventricles — these contracting irumediately after, to propel the 

 blood into the arteries. This passage of the blood into the arteries is a necessary 

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

 occurs 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, 

 the valves of which 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 ventricles ; 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 gmieral diastole of the organ, during which the 

 two cardiac cavities are filled by the afflux of venous blood. 2. The systole of 



' With Horses in health, the fluid exhaled into the pericardium is barely sufficient to 

 moisten and lubrify the free surface of its serous membrane. But in those worn out and 

 enfeebled by age, privations, or disease, it is not rare to see it accumulated in greater or less 

 quantity. To verify this, however, an examination ought to take place immediately after 

 death, as tlie accumulation of fluid in the serous cavities by cadaveric exhalation is common in 

 all animals. 



