538 



Prof. A. F. S. Kent. 



flap, and then run uninterruptedly forward, forming the upper portion of 

 the valve for a distance of about one-half of the total length shown in the 

 section. For two-thirds of this distance the thickness of the muscular 

 tissue is considerable. At its lower extremity the muscle is thinned out and 

 appears to be inserted into the connective tissue forming the upper layer of 

 the flap. 



Between the muscle and the underlying fibrous tissue of the valve is 

 a well-marked layer of very loose connective tissue, the presence of which 

 would produce the effect of an insertion of the muscle at a considerable 

 distance from the base of the flap. 



In fig. 2, Plate 16, taken from the heart of an adult cat, the auricular 

 muscle is seen to reach the auriculo-ventricular junction, and then to pass on 

 as a continuous sheet into the valve, reaching in the figure almost to the 

 extremity of the thicker basal part of the flap. The muscle fibres are thick 

 and robust even, at some distance from the base of the valve. 



In fig. 3, taken from the heart of a child, the auricular muscle, in the 

 neighbourhood of the auriculo-ventricular junction, is seen to be arranged 

 as a series of bundles cut across in the specimen, and as some smaller masses 

 running towards the point of attachment of the base of the valve. The 

 muscular fibres enter the valve and run parallel with the axis of the flap. 

 They pass down for about a quarter of the length of the flap shown in the 

 specimen, lying in the upper portions of its thickness, and finally ending in 

 relation with the tissues of its upper lamella. 



In fig. 4, taken from the heart of an adult man, the auricular muscle is 

 seen to approach the auriculo-ventricular junction as two sheets, the inner 

 having a somewhat circular course, the outer having a direction more nearly 

 parallel with the axis of the valve segment. 



The muscle fibres come to an end in a mass of connective tissue forming 

 the upper part of the segment, the actual point of insertion being at some 

 distance from the point of attachment of the valve. 



Thus in all the four photographs reproduced the same essential points of 

 structure are shown. The auricular muscle passes in considerable mass into 

 the basal portion of the auriculo-ventricular valve, it takes up a position in 

 the auricular part of the segments, and it finally ends by becoming inserted 

 into the fibrous connective tissue of the valve substance. As might have 

 been anticipated both mitral and tricuspid valves show the structure 

 described. 



Muscle in the situation described may well have an important function in 

 connection with the closure of the auriculo-ventricular valves. In accounting 

 for the closure of these valves, authors have commonly relied upon the floating 



