564 DH DAVID HEPBURN ON THE 



coupled with the general acceptance of the current text-book descriptions of the dorsal 

 interosseous muscles of Man's hand, led me, in the first instance, to overlook its presence. 

 In this explanation we may also find the reason why different observers have recorded 

 so much variation in the number of palmar interosseous muscles seen in different dis- 

 sections of the hand of the Chimpanzee. In the Orang-utan and Gorilla the conditions 

 closely approximate to those found in Man. With care it can be shown that each dorsal 

 interosseous muscle carries upon its palmar aspect one head of a short flexor, so closely 

 fused as readily to escape observation, although capable of separation by fair dissection. 

 In the Gibbon, the difficulty of separating the dorsal interosseous muscles from those 

 heads of short flexors fused with them is extremely great, and I cannot claim to have 

 been successful. The difficulty is undoubtedly due to the cylindrical nature of the 

 shafts of the metacarpal bones rather than to the absence of the muscular slips, because 

 in every case muscular fibres, which took no part in forming the bipennate muscle, 

 arose from the palmar surfaces of the shafts of the metacarpal bones. The three so- 

 called palmar interosseous muscles were distinguished without any trouble, because their 

 insertions are independent, and are not obscured by fusion with contiguous muscles, as is 

 the case with certain heads of the short flexors which are inserted in common with the 

 individuals of the dorsal or abductor series. 



Conclusions. 



1. The shaft of each metacarpal bone, with the exception of the first, presents two 

 triangular areas, a larger on the dorsal aspect and a smaller on the palmar aspect, neither 

 of which affords origin to muscular fibres. It follows, therefore, that the palmar aspects 

 of the various metacarpal bones are more fully occupied by muscles than the dorsal 

 aspects. 



2. The dorsal interosseous muscles — which are Abductors in function — are smaller 

 than current descriptions lead us to believe. This is quite in accordance with the 

 comparatively feeble nature of the abductor movements. 



3. Each digit is provided with a short flexor muscle presenting radial and ulnar 

 heads which are capable of acting independently, and thereby producing a certain amount 

 of abduction and adduction according to their position with regard to the middle line of 

 the hand. 



4. Every muscle of the Dorsal or Abductor series is inserted in common with one of 

 the heads of a short flexor muscle, and, in consequence of their close fusion, the line of 

 separation between them is somewhat obscured, and has been overlooked. 



5. The members of the Palmar or true Adductor stratum have all disappeared from 

 the human hand, with the exception of the Adductor pollicis obliquus et transversus, 

 hence this action has been thrown upon certain heads of the short flexors, and in con- 



