INTEROSSEOUS MUSCLES OF THE HUMAN HAND. 563 



M. flexor brevis annularis consists of a radial head (Second Palmar Interosseous) 

 and an ulnar head, separable from Abductor annularis. These are inserted into 

 opposite sides of the base of the first phalanx of the ring finger. 



M. flexor brevis minimi digiti consists of a radial head (Third Palmar Inter- 

 osseous) and an ulnar head, which at present appropriates the entire name. These 

 are inserted into opposite sides of the base of the first phalanx of the little finger. 



From this description it will be seen that each Abductor in the dorsal stratum is 

 intimately associated at its insertion with one of the heads of a short flexor, and it is 

 to this association that I attribute the fact that these portions of the short flexors have 

 not hitherto received due recognition. Undoubtedly there are cases when the fusion is 

 more complete than at other times, and hence, under favourable circumstances, additional 

 palmar interosseous muscles have been recognised and recorded as abnormalities. In 

 my endeavour to prove the existence of a flexor muscle situated in a position where it 

 is more or less fused with an abductor muscle, I regard it as of great importance that, 

 even by current descriptions, a flexor action has always been claimed for an abductor 

 muscle, situated in an interosseous space, and that the flexor portions, which I consider 

 capable of separation from abductor muscles, always occupy the palmar aspect of the 

 hand. There has never been any difficulty in recognising those portions of the short 

 flexors which possess an independent insertion. From this statement we must except 

 the ulnar head of the short flexor of the Pollex, which has become overshadowed, and in 

 some cases suppressed, by the Adductor pollicis muscle. 



It is important to note that there are three nerve twigs sent into each interosseous 

 space for distribution to the three separate portions of muscle which I consider each 

 space contains. 



These views regarding the strata of intrinsic muscles in Man's hand have been 

 impressed upon me by many careful dissections, and they have led me to re-examine 

 my earlier dissections of the corresponding muscles in the hands of the Anthropoid 

 Apes, especially the so-called palmar interosseous muscles, with the results that I shall 

 now briefly indicate. 



In the Chimpanzee the intermediate stratum of short flexor muscles is found at its 

 fullest development, so far as regards the fingers, for, as already mentioned, the Flexor 

 brevis pollicis possessed only a rudimentary ulnar head in my dissection. In this ape, 

 the short flexors separate with perfect ease from the dorsal interosseous muscles, 

 so that each finger is provided with a short flexor presenting a radial and an 

 ulnar head of origin. In the original description of my dissection,* I recorded this 

 condition of parts as existing in the Medius, Annularis, and Minimus digits, but not in 

 the Index. A renewed examination of the dissection, however, convinces me that the 

 radial head of M. flexor brevis indicis is also present. At the same time, it was 

 more closely adherent to the palmar aspect of the Abductor indicis than any of the 

 other short flexors were to their respective dorsal interosseous muscles.t This fact, 

 * Hepburn, loc. cit. t Vide Dwight quoted ante. 



