STRUCTURE OF THE HEART. 395 



the aortic opening. Between the aortic ring and auriculo-ven- 

 tricular opening is a fibro-cartilaginous mass, connected with the 

 several rings, to which muscular fibres are attached, and which in the 

 Ruminant is replaced by a triangular bone or os cordi*. A second 

 bone is sometimes found between the pulmonic and auriculo- 

 ventricular rings. The art'. /'/"I rtngs gird the arterial openings, 

 and serve for the attachment of the vessels, semilunar valves, and 

 some of the ventricular muscular fibres. They seem to consist 

 of three semilunar portions united, which gives them a festooned 

 border, to which is attached the middle coat of the artery. The 

 other edge is straight, affording attachment to the muscular 

 fasciculi of the ventricles. 



The mass of the heart is composed of muscular fibres, con- 

 nected by filamentous areolar tissue. These fibres are involuntary, 

 and of a deep-red colour, differing from other involuntary muscles, 

 the oesophagus excepted, in being transversely and longitudinally 

 striated. They are smaller than voluntary fibres, and are anasto- 

 rnatic. 



The muscular fibres of the auricles are separable into super- 

 ficial and deep ; the former common to 

 both auricles, the latter proper to each. 

 The superficial, common, or transverse 

 fibres suiTound the base of the auricles, 

 and are most abundant on their anterior 

 part. They cross from one auricle to the 

 other, diverging to enclose the appen- 

 dices and the openings of the large veins. 

 They are also prolonged into the inter- 

 auricular septum, to assist in forming the 

 circular band which surrounds the fossa FlG - 15 - 



ovalis. The deep fibres proper to each 

 auricle consist of two sets, looped 

 and annular, which are superficial where the external circular 

 fibres are deficient The looped fibres are attached by their 

 extremities to the auriculo-ventricular tendinous rings ; passing 

 over, they embrace each auricle, forming right and left loops. 

 The annular fibres are arranged in rings, forming a continuous 

 layer. They abound most in the appendices, and around the 

 coronary sinus and pulmonary veins, extending some distance 

 along the course of the latter, and of the cavae. 



Like the muscular fibres of the auricles, those of the ventricles 



