136 



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

 whicli of course assists greatly in bringing them into view. 



Muscular fibres have not been found in the arterial valves. 



The structure and connections 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 attachm.ent to its tendinous tissue. This is generally best 

 seen in one 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 to the ring. In the posterior 

 mitral valve the muscular fibres seldom penetrate so far forwards, 

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

 from the much greater thickness and density of the lining membrane 

 of the left auricle. 



The connections of the anterior mitral valve, being peculiar, re- 

 quire a separate consideration. In dissecting down between the 

 anterior wall of the left auricle and the posterior surface of the 

 aorta, it is seen that the central fibres of the auricular wall are closely 

 attached to the adjacent wall of the vessel. A little further dissec- 

 tion on either side will show that the muscular substance of the left 

 ventricle is deficient between these parts. At the sides indeed it is 

 found, but is gradually lost at some distance from the mesial line. 

 Hence these tv/o orifices (the aortic and left auriculo-ventricular} 

 are not separated as the others are by the intervention of the muscular 

 fibres of the ventricle. The structure and connections of the ante- 

 rior mitral valve are examined by means of a vertical section, in- 

 cluding the posterior wall of the aorta and the anterior wall of the 

 left auricle. If the lining membrane of the auricle be traced down- 

 w^ards, it is found to be directly continued on to the posterior surface 

 of the valve, and the membrane on the anterior surface of the valve 

 is continued upwards over the tendinous festooned ring of the aorta, 

 on to the under surface of its semilunar valves. The anterior mitral 

 valve lies beneath a portion of the two posterior arterial valves. 

 The muscular wall of the auricle is observed to terminate by two 

 distinct insertions. The anterior (the larger) division of fibres is 

 attached to the posterior surface of the aorta, opposite to, and below 

 the festooned ring, while the posterior portion is continued directly 

 downwards for a short distance into the valve, and terminates by an 

 attachment to its fibrous tissue. The posterior wall of the aorta 

 descends nearly vertically. Suddenly becoming much thinner oppo- 

 site the upper border of the semilunar valve, it is continued down 

 to the festooned ring, or in other words, it here becomes blended 

 with the base of the semilunar valves. Below this a dense layer of 

 fibrous tissue (which exists belov/, and fills up the spaces between 



