588 



HEART. 



in the heart of the ox and horse after a little 

 dissection. The following description is drawn 

 up from numerous dissections of these parts 

 made on the human heart. The tendinous ring 

 surrounding the aortic opening is stronger and 

 thicker than that surrounding the orifice of the 

 pulmonary artery. Both of them are stronger 

 than the auriculo-ventricular rings. Each of 

 the arterial rings appears as if composed of 

 three semilunar portions placed on the same 

 plane, the convexities of which are turned 

 towards the ventricles and the concavities to- 

 wards the vessels (fig. 266, a a).* Each of 



Fig. 266. 



Fig. 267. 



Appearance of tendinous ring at the origin of the 

 pulmonary artery. In slitting open the artery, 

 one of the three projecting extremities of the ten- 

 dinous ring has been divided. 



these semilunar portions has its projecting extre- 

 mities intimately blended at their terminations 

 with the corresponding projecting extremities 

 of those next to it, (Jig. 266, 6 b,) so that the 

 three form a complete circle, with three trian- 

 gular portions projecting from its. upper edge. 

 The semilunar portions approach fibre-carti- 

 lage in their structure, and have the intervals 

 left between their convex edges filled with a 

 texture more decidedly fibrous, (fig. 266, d,) 

 and which is considerably weaker than the se- 

 milunar portions, more particularly on the left 

 side of the heart.f The thinness of the ten- 

 dinous structure filling up these intervals has 

 led some anatomists erroneously to describe 

 these portions of the heart as protected only 

 by the two serous membranes. The right ten- 

 dinous zone is broader than the left and very 

 thin, particularly at its inner margin, at which 

 part in both sides of the heart it assumes more 

 of the tendinous than of the fibro-cartilaginous 

 structure. These tendinous rings are placed 

 obliquely from without inwards and from above 

 downwards, so that the outer edge is on a plane 

 superior to the inner. The sigmoid valves are 

 attached to the inner edge of the upper surface, 

 (Jig. 267, a,) and the tendinous fibres placed 

 in the fixed margins of these valves contribute 

 to the thickening of the ring at this part ; the 

 middle coat of the arteries is connected to the 

 outer edge of the same surface, and to the an- 

 terior part of the projecting extremities, (Jig. 

 267, b ;) while the muscular fibres of the ven- 

 tricles (fig. 266, /; fig. 267, /J are attached 

 to the lower surface of the projecting portion 

 of the convexity, and to the lower margin of 

 the fibrous tissue filling up the space between 

 the convexities of the projecting ends, (fig. 



* These tendinous festoons are represented 

 stronger in the woodcut than they are naturally. 



t These intervals are occupied by muscular nbrcs 

 in the heart of the ox and horse. 



Pulmonary artery slit open at its origin, its internal 

 membrane stripped off, and two of the sigmoid valves 

 completely removed. 



a a a, tendinous festoons. 



b b, muscular fibres of the right ventricle. 



c c c, middle fibrous coat of the artery after the 

 internal serous membrane has been stripped off. 



g, small portion of one of the semilunar valves 

 left to show its attachment to the inner edge of the 

 upper surface of the tendinous festoon. 



267, d.) There is, however, this difference 

 between the right and left arterial openings 

 with respect to the attachment of the muscular 

 fibres; on the right side the muscular fibres 

 arise from the projecting portion of the con- 

 vexity of the whole three tendinous festoons, 

 (Jig. 268, c, c t ) while in the left side the mus- 



Fig. 268. 



cular fibres are attached only to one and part 

 of a second, (fig. 269, b b,) as the larger lip 

 of the mitral valve (Jig. 269, a) is suspended 



Fig. 269. 



