OF THE VALVES OF THE VASCULAR SYSTEM IN VERTEBRATA. 787 



consist of a reduplication of the endocardium, or lining membrane of the heart, 

 containing within its fold, large quantities of white fibrous tissue, and, as was 

 pointed out by Mr W. S. Savory, and after him, by Professor Donders,* of a 

 moderate amount of yellow elastic tissue. The white fibrous tissue greatly pre- 

 ponderates, and is derived principally from the chordae tendinese, which split up 

 into a vast number of brush-shaped expansions, prior to being inserted into the 

 segments. The fibrous or tendinous expansions, which assume the form of bands, 

 may consequently be regarded as prolongations of the chordae tendinese. They are 

 analogous, in many respects, to similar bands in the semilunar valves ; the only 

 difference being, that in the semilunar valves, the bands referred to, instead 

 of being free, as in the present instance, are involved in the valvular substance. 

 (Compare fibrous bands marked in Plate XXVIII. fig, 20, with chordae tendinese 

 and brush-shaped expansions, marked s in Plate XXVIII. fig. 33). As each 

 of the segments composing the bicuspid or mitral valve, like the left ventricle 

 itself, is bilaterally symmetrical, it will be convenient, when speaking of 

 these structures, to describe, in the first instance, only the half of one of 

 the segments; and in order to do this the more effectually, it will be neces- 

 sary to consider each musculus papillaris, as essentially consisting of two 

 portions ;| a superior and external ijortion (Plate XXVIII. figs. 28 and 33 «), 

 which gives off tAvo, usually three chordae tendinese (s) to that half of the anterior 

 segment of the mitral valve {ni) which is next to it ; and an inferior and internal 

 portion (5), which also gives off" three tendinous chords ; these being inserted into 

 the adjacent half of the posterior segment in). The three tendinous chords which 

 proceed from the superior external portion (Plate XXVIII. fig. 31 r), subdivide, 

 and are inserted, by the brush-shaped expansions spoken of, into the half of 

 the anterior segment posteriorly, in nine different places. | Of these, three are 

 inserted into the mesial line of the segment (Plate XXVIII. figs. 31, 32, and 

 33, r\ viz., into the base {g)^ central portion (/), and apex {e) ; three into the 

 basal, central, and apicial portion of the free margin (Plate XXVIII. fig. 33 s) ; 

 and three into intermediate points between the mesial line and the margin 

 (/). On some occasions, as in the mitral valve of the whale, a slightly different 

 arrangement prevails ; three chorda3 tendinete being inserted into the mesial line 

 of the segment at the base, at the centre, and at the apex ; an additional chord 

 going to the free margin near the apex ; three into intermediate points between 

 the mesial line and the margin ; a second additional chord going to the central 



* Professor Donders describes the yellow elastic tissue as being most abundant in the upper 

 surface of the segments. 



] The musculi papillares in the human and other hearts (Plate XXVIII. figs. 28, 31, and 33 

 ab, cd) either bifurcate, or show a disposition to bifurcate at their free extremities, so that the division 

 of the chordae tendinese into two sets is by no means an arbitrary one, 



I The number of insertions vary in particular instances. In typical hearts, however, it is re- 

 markably uniform. 



