THE BACK. 197 



backwards and slightly to the left, surrounding the left 

 auriculo-ventricular opening ; the third to the left slightly 

 forwards, from which superiorly the elastic layer of the 

 aorta commences ; while outwardly it is continued to form 

 a ring for the commencement of the pulmonary artery 

 (this bemg the weakest of them all). To these rings is 

 attached the muscular structure of the heart, consisting of 

 fibres all arranged in a more or less oblique manner. Each 

 cavity possesses fibres peculiar to itself, while the two 

 auricles are connected together by common fibres, and the 

 two ventricles also by common fibres, the auricles thus 

 being separable from the ventricles without division of 

 muscular fibre. Thus the auricles first contract together 

 (synchronously), then the ventricles contract simultaneously. 

 The muscular walls of the heart are smooth externally, 

 but present ^prominences and depressions internally. The 

 right auricle is situated at the supero -anterior part of the 

 heart. It has thin walls, which collapse when the cavity is 

 empty, and is divided into two parts, sinus and apiJendix. 

 It is termed auricle from the likeness of the corresponding 

 part in the human subject to a dog's ear ; in the horse the 

 appendix portion assumes this figure, and is inclined to 

 the left side, where it terminates in a point. The inner 

 surface of this part presents prominences of muscular sub- 

 stance arranged in a reticulated manner, musculi i^ectinati, 

 separated by dej^ressions, foramina Thehesii, into the latter 

 small blood-vessels are supposed to open. The sinus portion 

 presents five openings. At its supero-anterior part it 

 receives the anterior vena cava, with which vessel its 

 muscular walls are structurally continuous. At its supero- 

 posterior part the posterior vena cava, below which is the 

 double opening of the cardiac or coronary veins. The walls 

 of the sinus present but few muscular prominences ; a broad 

 ridge separates the opening of the anterior from that of the 

 posterior vena cava, this is tuherculum Loweri ; below it may 

 be seen the remains of the foramen ovale (fossa ovalis and 

 annulus ovalis). In the human subject the anterior margin 

 of this opening presents the crescent-shaped Eustachian 

 valve ; we have not seen it in the ass. Between the coro- 

 nary opening and the termination of posterior vena cava is 

 a thin double layer of endocardium, with a small quantity 

 of muscular structure intervening, the valve of Thehesius. 

 The inferior part of the auricle presents the right auriculo- 



