Royal Society. 307 



of the left side round to the posterior part, where it more or less 

 nearly joins the extremity of the larger bone. Thus the lateral and 

 posterior portions of the aortic orifice are surrounded by firm bony 

 arches meeting posteriorly in the centre. From the large bone, a 

 small process usually passes backwards for some distance into the 

 muscular substance of the septum between the ventricles, and is 

 gradually lost in the dense fibrous tissue found in this part, sur- 

 rounding the right border of the left auriculo-ventricular aperture ; 

 and from the convex surface of the smaller portion, a thin process of 

 dense fibrous tissue is continued round the left margins of the auri- 

 culo-ventricular orifice. These heart-bones are intimately connected 

 above with the middle coat of the aorta, on the inner surface with 

 the base of the adjacent arterial valves, and posteriorly with the an- 

 terior mitral valve ; while at the sides, to their external and inferior 

 surfaces, the muscular fibres of the ventricle are attached. They 

 may be seen and felt in the base of the pouches formed by the two 

 posterior aortic valves, and no doubt greatly assist in sustaining the 

 " force_of the reflux." They occupy tlxe position of the two posterior 

 festoons of the aortic valves. In the human heart, in the situation 

 corresponding to the position of these heart-bones, the tissue com- 

 posing the festooned rings is thicker and denser than elsewhere, 

 offering to the knife, in some cases, almost the resistance of bone. 

 The processes of dense fibrous tissue found in the anterior portion 

 of the border of the ventricular septum, &c, and extending round 

 the right and left margins of the auriculo-ventricular orifice, are in- 

 timately connected with the thickened portions of the adjacent 

 festoons. 



Among the tissues entering into the structure of the arterial valves, 

 elastic fibres are described. They exist not only in the corpus 

 arantii, but delicate fibres of elastic tissue are found throughout the 

 valve ; most abundantly in the thicker portions, but even in the 

 thinner portions (lunula?) a few delicate but well-marked elastic 

 fibres may be seen, particularly after the addition of acetic acid, 

 which of course assists greatly in bringing them into view. 



Muscular fibres have not been found in the arterial valves. 



The structure and connexions of the auriculo-ventricular valves 

 are next examined by means of vertical sections. In tracing down 

 the muscular wall of the auricle, it is observed to pass on to the 

 inner surface of the ventricular border, and if minutely examined is 

 seen to terminate by two attachments. The external portion, which 

 is considerably the larger, is closely connected with the fibrous 

 structure forming the " auriculo-ventricular ring," while the thinner 

 internal portion is continued forwards for a very short distance be- 

 tween the surfaces of the valve, and terminates more or less abruptly 

 by an attachment to its tendinous tissue. This is generally best 

 seen in urn of the tricuspid valves, where, in a vertical section, the 

 muscular fibres may be observed terminating beneath its upper sur- 

 face immediately beyond its attachment t<> Die ring. In the po U rior 



mitral valve the muscular fibre- ietrate so far forwards, 



and this appears to result, when a section of the parts is examined, 



X2 



