1865.J Mr. J. Wood Varieties in Human Myology. 391 



interosseus of the thumb (6) exposed by the division of the abductor 

 indicis, and lying upon the flexor brevis, with the deep fibres of which it 

 is usually confounded. 



The insertions of these muscles are invariably (as usually described, and 

 as seen in the sketch) divided between the base of the phalanx (where it is 

 blended with the capsular investment of the joint derived from the extensor 

 aponeurosis) and the sides of the extensor tendon, passing with the fibres 

 from the lumbricalis, partly to the middle, and chiefly to the ungual 

 phalanx. 



In the foot, the same occasional reference to the type occurring in the hand 

 is found, in the origin of the first plantar interosseus. This muscle is 

 sometimes a double penniform, arising from the adjacent second and third 

 metatarsals on the plantar aspect of the second dorsal, and, like it, per- 

 forated by the communicating artery. In both the hand and foot where 

 these irregularities are found, the respective digits to which the muscles 

 are attached seem somewhat larger in proportion than is usual, the 

 size and extent of attachment of the muscles appearing to be determined 

 by the size and uses of the corresponding digit. The foregoing abnor- 

 malities of the interossei reflect some light upon the differences in the 

 normal arrangement in the upper and lower extremities, which have often 

 perplexed anatomists. The terms dorsal and plantar or palmar, referring 

 to position only, and not to the action of these muscles, have apparently 

 somewhat obscured the homologies of the separate muscles. 



In the hand, the middle digit being the most bulky, has a double or 

 dorsal interosseous muscle for each of its divaricators. Its divaricator to 

 the pollex excludes from the third metacarpal the divaricator from the 

 pollex of the second digit, and obtains origin for itself from the dorsal part 

 of the second metacarpal, so becoming a dorsal muscle. The transverse 

 convexity of the back of the hand gives a dorsal prominence to the middle 

 metacarpal and its digit over the rest. This explains the circumstance 

 of this muscle assuming a dorsal position over the palmar interosseous of 

 the index. 



In the foot, the first and second metatarsals and their digits attain a 

 greater proportionate size and dorsal prominence, to fulfil their chief func- 

 tion of sustaining and propelling the body. Here we find the divaricator 

 to the pollex of the second digit (the first palmar interosseous of the 

 hand) becoming developed into a double penniform muscle, with a dorsal 

 position, excluding the divaricator to the pollex of the third digit (the 

 second dorsal of the hand) from attachment to the second metatarsal, and 

 itself acquiring an origin from the third metatarsal. 



An occasional recurrence of one to the type of the other might have 

 been expected under peculiar conditions of development. Mr. Huxley 

 informs the author that he has found, almost invariably, that the inter- 

 osseous muscles in the foot are inserted entirely into the bases of the pha- 

 langes, and are not, as in the hand, prolonged by a tendinous expansion in 



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