The Heart. 49 



Its base is uppermost, its apex nearly touches the sternum, 

 and it occupies a position corresponding to the third, fourth, 

 fifth, and sixth ribs ; it is between the fifth and sixth ribs, 

 just at their sternal insertion, where the impulse of the 

 heart may be felt in the horse. Its other relations are with 

 the diaphragm which is some five or six inches behind it, 

 but with which it has no connection. On its right side is 

 the right lung, and on its left part of the left lung. There 

 is a triangular notch in the left lung of the horse which 

 exposes the left ventricle, and allows it to make its impulse 

 felt against the chest wall. The anterior face of the heart is 

 formed by the right auricle and ventricle, the posterior by 

 the left auricle and ventricle. 



The heart, though an involuntary muscle, does not con- 

 form histologically with the involuntary muscular fibre 

 met with in other parts of the body. It is red in appearance, 

 its fibres are short, striated, possess no sarcolemma, freely 

 anastomose, and contain a nucleus. The network formed 

 by the fibres of the heart is a most distinctive feature. In 

 some animals, sheep and ox in particular, cells of a peculiar 

 kind are found beneath the endocardium, they are poly- 

 hedral in shape, containing protoplasm and a nucleus, and 

 are surrounded by striated fibres. They are called the cells 

 of Purkinje. 



The arrangement of the fibres of the heart is peculiar ; 

 those of the auricle are quite distinct from the ventricle, 

 and both are arranged in layers. Two layers have been 

 described in the auricle, transverse and longitudinal, with 

 circular fibres around the entrance of the veins, whilst in 

 the ventricle no less than seven layers have been de- 

 scribed. Owing to the peculiar direction in which these 

 run a somewhat spiral arrangement results, but this is not 

 fully accepted. 



The heart is lined by the endocardium which is reflected 

 over the valves. The lining membrane of the left auricle of 

 the horse is naturally of a peculiar gray colour. 



Certain fibrous rings are found in the heart where the 

 valves are situated, and to which these obtain a firm attach- 



4 



