DISSECTION OF THE BACK AND THORAX. 133 



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

 cavity 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 

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

 and 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 

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

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

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

 connective-tissue. Connective-tissue occurs also in large amount in the 

 neighbourhood of the auriculo-ventricular and arterial openings, where 

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

 orifices that firmness which is necessary for the efficient working of the 

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

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

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

 cordis — is developed in the angle between the aortic ring and the two 

 auriculo-ventricular rings. • 



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

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

 heart specially prepared. A heart from any of the domestic mammals, 

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

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

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

 fat, and vessels having been cleaned off the surface of the heart, the 

 auricles should be first examined. 



The auricles have the muscular fibres of their walls distinct from 

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

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

 both, some of the fibres of the latter stratum being carried into the 

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

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

 auricula and the different venous orifices, the latter • playing an 

 important part in preventing regurgitation into the veins when the 

 auricle contracts. 



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



