788 DR PETTIGREVV ON THE RELATIONS, STRUCTURE, AND FUNCTION, 



2)ortion of the margin. A third and independent chord, goes to the base of the 

 margin. In the whale, as will be observed, the arrangement is virtually the 

 same as that first given ; the insertions being nine in number, — three into the 

 mesial line, three into the free margin, and three into intermediate points. 

 The tendinous chords pursue different directions prior to insertion ; the three 

 which are inserted into the mesial line of the segment, and are the longest 

 and strongest (Plate XXVIII. figs. 31, 32, and 33 r), being less vertical 

 than those which are inserted nearer the margin (r') ; these in turn being 

 less vertical than the ones inserted into the margin, which are the shortest 

 and most delicate. The basal chord of each set, on the contrary, is more 

 vertical than that beneath it, or nearer the apex {x) ; the apicial chords being 

 more or less horizontal. As there is a disposition on the part of the higher 

 and more central chordae tendinese (Plate XXVIII. fig. 31 g') to overlap, by their 

 terminal brush-shaped expansions, those below and to the outside of them, the 

 segment is found to diminish in thickness from the base (2;) towards the apex {x) 

 and from the mesial line towards the periphery or margin ; the basal and central 

 portions of the segment being comparatively very thick, the apicial and marginal 

 portions very thin (Plate XXVIII. fig. 32 vx)\ so thin, indeed, that in some 

 hearts, particularly in the right ventricle, they present a cobwebbed appearance. 

 As the marginal portions form the counterparts of the lunula3 in the semilunar 

 valves, and are those parts of the segments which come into accurate apposition 

 when the valve is in action, they are, from this circumstance, entitled to con- 

 sideration. When a perfectly healthy mitral valve from an adult, or, still 

 better, from a foetus at the full time or soon after birth, is examined, the 

 portions referred to are found to be of a more or less crescentic shape {tide 

 that part of the mitral valve to which the chordae tendinese marked r', fig. 

 33, Plate XXVIII. , are distributed), and so extremely thin, that the slightest 

 current in the fluid in which they are examined causes them to move like 

 cilise. The physiological value of this delicacy of structure, and consequent 

 mobility, is very great ; as the most trifling impulse causes the marginal parts 

 of the segments, which are naturally in juxtaposition, to approach towards 

 or recede from each other. The half of one of the segments of the mitral 

 valve may be regarded as consisting of a reduplication of the endocardium or 

 lining membrane of the heart, supported or strengthened in all directions by nine 

 or more tendinous brash-shaped expansions; these expansions being arranged 

 in three vertical rows of three each (Plate XXVIII. fig. 33), with much pre- 

 cision, and according to a principle which is seldom deviated from. In addition 

 to the reduplication of the lining membrane and the tendinous expansions re- 

 ferred to, Lancisi,* SENAC,f and Kurschneb, :j: have ascertained that there is a 



* De Motu Cordis. f Traite de la Structure du Coeur, livre i. p. 76. 



J Wagner's Handvvbrterbucli, art. " Herzthajtigkeit." 



