130 THE ANATOMY OF THE HORSE. 



conuective-tissue, to which are added some elastic fibres. The muscuhxr 

 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 neighbour- 

 hood 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 give 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 animals, 

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

 an horn-. 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 

 diff'erent venous orifices, the latter playing an important part in pre- 

 venting regurgitation into the veins when the auricle contracts. 



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

 be effected by cutting the auriculo-A-entricular fibrous rings, which form 

 the bond of connection between the auricular and ventricular fibres. 

 By combined cutting and teasing the following facts may be observed. 



Over the whole exterior of the ventricles the fibres have an oblique 

 direction. Thus, on the left side the fibres pass obliquely downwards and 

 backwards, and on the right side downwards and forwards. At the left 

 ventricular furrow many of the fibres dip into the septum; but on the 

 right side the fibres of the left ventricle pass across the furrow, and are 

 directly continued on the right ventricle. At the apex of the heart 

 the fibres turn inwards in a whorl-like manner and disappear from view. 

 If a thin stratum of these superficial fibres be now removed, they will 

 be found to cover others having a less oblique course ; and further 

 dissection will show that the fibres become less and less oblique until 

 the centre of the wall is reached, where the fibres are approximately 

 horizontal. On peeling off" these horizontal fibres, a deeper set will be 



