OF THE VALVES OF THE VASCULAR SYSTEM IN VERTEBRATA. 789 



slight admixture of true muscular fibres.* As the tendinous expansions of the 

 half of the segment described, bifurcate or split up, and run into similar expan- 

 sions from the other or remaining half of the same segment, to become strongly 

 embraced in the mesial line ; a complete segment may be described as consisting 

 of a reduplication of the endocardium or lining membrane, enclosing in its fold 

 certain muscular fibres, and eighteen or more tendinous expansions; the chordae 

 tendinese, on which these expansions are situated, proceeding from the anterior and 

 posterior musculi papillares equally (Plate XXVIII. fig. 28 a c), and pursuing diffe- 

 rent directions, to meet in the mesial line and form angles (Plate XXVIII. fig. 32) 

 which become more and more obtuse in a direction from above downwards. When, 

 therefore, a segment is examined by being held against the light, or by the aid 

 of a dissecting lens or microscope, it is found to consist of tendinous striae run- 

 ning transversely, obliquely, and more or less vertically ; the striae of opposite 

 sides being so disposed that they mutually act upon and support each other; an 

 arrangement productive of great strength, and one which secures that the seg- 

 ments shall be at once tightened or loosened, by the slightest contraction or 

 relaxation of the musculi papillares. The intermediate accessory segments of 

 the mitral valve, resemble the principal ones, in structure and general configura- 

 tion. They are, however, comparatively speaking, very thin; and the chordae 

 tendineae inserted into them differ from those inserted into the principal seg- 

 ments, in having a more vertical direction, and in being longer and more feeble. 

 The description given of the bicuspid valve, applies, with trifling alterations in 

 particular instances, to the tricuspid, if allowance be made for an additional large 

 segment, and three or more accessory segments. With regard to the smallest, of 

 the three large segments forming the tricuspid, I have to observe, that in all pro- 

 bability, it is simply an over-developed accessory segment ; the so-called tricuspid 

 valve being in reality a bicuspid one (Plate XXVIII. fig. 34 mn). Nor is this to 

 be wondered at, when it is stated! that the right ventricle, is a segmented portion 

 of the left, and partakes of its bilateral symmetry even in matters of detail. The 

 opinion here advanced is by no means new, but it appears to me that the point has 

 not been sufficiently investigated, and we are in want of statistics regarding it. 

 In ten human hearts which I examined for this purpose, no less than four 

 had well-marked bicuspid valves in both ventricles; and on looking over a 

 large collection of miscellaneous hearts in the Museum of the Royal College 

 of Surgeons of England, I found that nearly a third of them had the pecu- 

 liarity adverted to ; if indeed that can be called peculiar which seems to me 

 to be typical. When two principal segments, with two or more accessory 

 segments, occlude the right auriculo-ventricular orifice, as happens in Plate 



* According to Mr Savory's observations, the muscular fibre is found more particularly at the 

 •upper or attached border of the valves. 



t Phil, Trans., vol. cliv., pp. 464-67. 



VOL. XXIII. PART III. 10 B 



