INTEROSSEOUS MUSCLES OF THE HUMAN HAND. 561 



they act. In the hands of Man and the Anthropoid Apes there is recognition of certain 

 of these facts in current anatomical descriptions, so far as the Pollex and Minimus digits 

 are concerned. Thus we have M . flexor brevis pollicis and M. flexor brevis minimi 

 digiti, while those other muscles regarded as the incomplete members of this stratum 

 are classified under the title Palmar Interosseous, and to them is relegated the action of 

 Adductors. 



I shall now discuss those members of this stratum for which the name " Short Flexor " 

 is definitely accepted, and then show how the other individuals of the same group deserve 

 a similar descriptive term in place of " Palmar Interosseous." 



M. flexor brevis pollicis nominally possesses two heads, — radial and ulnar, — yet 

 much variation is known to exist, and the ulnar head is not always present in Man, in 

 whom its development and growth may be retarded or altogether prevented by the 

 relatively large proportions and unrestricted energy of the M. adductor pollicis 

 obliquus et transversus. Long before the true nature of this ulnar head was clearly 

 established, its presence was regarded as an abnormality, and it was described as the 

 Interosseous primus volaris of Henle. Indeed, its recognition by modern text-books is 

 usually confined to a short paragraph under the foregoing designation. 



Among the Anthropoid Apes,* the same muscle is subject to similar variation. In 

 my memoir already referred to, I have recorded the presence of both the radial and 

 ulnar heads in the Gibbon and Orang-utan ; the rudimentary condition of the ulnar 

 head in the Chimpanzee, and its absence in the Gorilla. My dissection of the Gorilla 

 dealt with the right hand, but I have since dissected the left hand of the same animal, 

 and feeble rudiments of the ulnar head were present. In a recent dissection of a Chim- 

 panzee by Dr DwiGHT.t no trace of this ulnar head of the Flexor brevis pollicis was 

 found. 



The M. flexor brevis minimi digiti, both in Man and in all the Anthropoid Apes, is 

 described as presenting only one head of origin, viz., its ulnar head ; while the radial 

 head, although really present as a constant structure, is artificially detached from its 

 morphological position by a nomenclature which assigns to it the term Third Palmar 

 Interosseous muscle. Such a mode of description could only have arisen from regarding 

 the function of a palmar interosseus muscle as quite distinct from that of a short flexor. 

 That the interosseous position does not debar a muscle from being a short flexor is 

 evident from the position of the ulnar head of Flexor brevis pollicis, and therefore there 

 is no serious obstacle in the way of restoring the innermost palmar interosseous muscle 

 to its proper place, and speaking of it as the radial head of the short flexor of the little 

 finger. 



Palmar interosseous muscles are therefore presented in their correct anatomical 

 positions when they are considered as members of the Intermediate stratum of short 



* Hepburn, loc. cit. 



t Dwight, " Notes on the Dissection and Brain of the Chimpanzee, ' Gumbo,' " Memoirs of the Boston Society of 

 Natural History, vol. v. number ii. p. 38, 1895. 



