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Show 530 DR. G. E. DOBSON O N T H E [June 20, the lines of flexion indicating the position of the joints of the toes, and the longitudinal cleft dividing the heel to the groove along which the tendons of the flexor muscles enter the sole of the foot; but the great depth and extent of these grooves evidently allow of very free motion in the sole during flexion and adduction, the importance of which will be seen when we investigate the mechanism on which the adhesive power of the foot depends. There is nothing therefore in the structure of the sole of the foot alone which can explain the extraordinary power of clinging to smooth surfaces which we know the species of Hyrax possess. In dissecting the flexor muscles of the forearm the comparatively very large size of the palmaris longus is particularly noticeable (Fig. B, P.l). This muscle, arising from the internal condyle of the humerus and from the intermuscular fascia, forms a broad and strong tendon which, passing into the sole of the foot, spreads out into the palmar fascia, which contains a fibro-cartilaginous disk. From the superficial and deep palmar fascia, and from this fibro-cartilaginous disk, arises a very peculiar muscle (first described by Messrs. Murie and Mivart*), the flexor brevis manus (Fig. B, F.b.mi), which is inserted by three tendons into the second, fourth, and fifth toes. The tendon going to the second, or inner toe, is inserted on the inner side of the tendon of the flexor profundus, while that going to the fifth or outer toe is inserted on the outer side of the tendon of the same muscle; the middle tendon forms, with the corresponding tendon of the flexor sublimis, the perforated tendon of the fourth toe; and as this toe is so much longer than the second and fifth toes, the positions of the insertions of the three tendons of the flexor brevis manus occupy nearly the same line across the foot. If we take a line drawn along the middle of the fore foot from the centre of the carpus to the extremity of the third toe as the centre line of the foot, we find the insertions of the flexor brevis manus on either side of this line, two of the three tendons being inserted at points on opposite sides of the foot furthest from the centre. The action of this muscle must therefore be, not only to flex the outer and inner toes, but also to rotate them slightly and draw inwards both sides of the foot towards the centre line. In this action the tendency to cup the sole of the foot is evident. Further, as the flexor brevis manus arises from the palmar fascia, it follows that this muscle can only act effectively when its point of origin is fixed; and this is accomplished by the action of the strong palmaris longus, which, as we have seen, terminates in the palmar fascia. In the coordinate action of these two muscles on such a foot provided with a soft elastic sole and united toes we have, I believe, all that is necessary to produce the remarkable power of adhesion of the foot to any smooth surface on which it may be placed, which has been observed in most species of Hyrax. The palmaris longus, in fixing the palmar fascia, removes the * L. c. p. 341. Messrs. Murie and Mivart (with the sanction of the Council of the Society) have kindly permitted m e to make use of one of the wood blocks (Fig. B ) from which the illustrations accompanying their paper were printed. |