DISSECTION OF THE BACK AND THORAX. 133 



prevented from regurgitating into the auricle, is forced out of the 

 savity along the great systemic artery — the common aorta, which 

 springs from the right-anterior part of the base of the ventricle. 

 The aortic orifice is guarded by a three-segmented semilunar valve. 

 These segments are stronger than those at the mouth of the pul- 

 monary artery, which they otherwise exactly resemble. Opposite to 

 aach a large sinus of Valsalva is developed on the wall of the artery, 

 :ind from two of these spring the right and left coronary arteries of 

 the heart. 



STRUCTURE OF THE HEART. 



In structure the heart consists of a muscular wall, an external serous 

 investment — the epicardium, and an internal serous lining — the endocar- 

 dium. The valves are folds of the endocardium, strengthened with 

 Sbrous connective-tissue, to which are added some elastic fibres. The 

 muscular tissue is of the striped variety (although not under the control 

 3f the will), and its fibres are grouped in bundles separated by fibrous 

 5onnective-tissue. Connective-tissue occurs also in large amount in the 

 aeighbourhood of the auriculo-ventriciilar and arterial openings, where 

 ■i is aggregated in the form of rings, or zones. These rings give to the 

 Jrfices that firmness which is necessary for the efiicient working of the 

 ralves, and at the same time afford origin to some of the muscular fibres. 

 The tissue of which they are composed is mainly fibro-cartilaginous. 

 [n the heart of the ox, and rarely also in the horse, a bone — the os 

 :ordis — is developed in the angle between the aortic ring and the two 

 luriculo-ventricular rings. 



Directions. — The arrangement of the muscular tissue in the wall of 

 ;he heart is exceedingly complex, and cannot be studied except in a 

 leart specially prepared. A heart from any of the domestic animals, 

 Dut preferably from the horse, should be procured, and boiled for about 

 in hour. This will favour the dissection of the fibres, by making them 

 irm and softening the connective-tissue between them. The epicardium, 

 at, and vessels have been cleaned off the surface of the heart, the 

 luricles .should be first examined. 



The auricles have the muscular fibres of their walls distinct from 

 ;hose of the ventricles. Moreover, the fibres are arranged in two layers 

 —a deep set proper to each auricle, and a superficial set common to 

 )oth, some of the fibres of the latter stratum being carried into the 

 luricular septum. In the deep stratum some of the fibres run 

 ibliquely in the wall, while others are arranged as rings around the 

 .uricula and the different venous orifices, the latter playing an 

 mjjortant part in preventing regurgitation into the veins when the 

 .uricle contracts. 



Directions. — Separation of the auricles from the ventricles should next 



