INTEROSSEOUS MUSCLES OF THE HUMAN HAND. 559 



very unequally distributed between them, for, whereas the palmar muscle is com- 

 paratively powerful, the dorsal head is almost membranous in its dimensions in all the 

 inter-metacarpal spaces except the first ; and, since the dorsal muscles — as at present 

 described — project as far towards the palm as the palmar muscles, their difference in 

 size is not so pronounced as one would suppose from an examination of the surface 

 which accommodates a palmar interosseous muscle and one of the heads of a dorsal 

 interosseous muscle. From such a remarkable disparity in the size of the two heads of 

 a dorsal muscle as the foregoing facts elucidate, one might naturally entertain doubts 

 as to whether the larger head was in reality all it is credited with being, viz., dorsal 

 interosseous muscle. These and other reasons have led me to make careful dissections 

 of all the dorsal interosseous muscles of which I could avail myself. Quite recently I 

 have dissected the first half-dozen hands* — representing as many subjects — that were 

 available, in order to test my earlier results, and in every instance my earlier dissections 

 have been verified. These results I shall now proceed to specify in detail. 



The feature of a Dorsal Interosseous muscle which I regard as its most distinctive 

 character is the bipennate arrangement of its muscular fibres, an appearance best seen 

 from its dorsal aspect, from which point of view it is usually figured. In this respect 

 it presents a marked contrast to the more or less longitudinal nature of the fibres 

 of a Palmar Interosseous muscle. If, however, a careful dissection be made of each 

 Dorsal Interosseous muscle from its palmar aspect, then we shall find that each member 

 of the group possesses no small share of longitudinal fibres which do not coincide with 

 the general bipennate arrangement ; and although the two sets of fibres are continued to 

 a common insertion, yet the longitudinal fibres maintain their palmar position even at 

 the point of insertion into the first phalanx. In this way, the First Dorsal Interosseous 

 muscle (Abductor indicis) yields a substantial muscular mass whose fibres take origin 

 from the palmar aspect and radial side of the radial lateral surface of the shaft of the 

 second metacarpal bone, just as the First Palmar Interosseous muscle arises from corre- 

 sponding areas on the ulnar lateral surface of the same bone. Similarly, the Second 

 Dorsal Interosseous muscle (Abductor medii digiti, radiad) yields a set of longitudi- 

 nal fibres arising from the palmar aspect and radial side of the radial lateral surface of 

 the third metacarpal bone. In the same manner the Third Dorsal Interosseous muscle 

 (Abductor medii digiti, ulnad) and the Fourth Dorsal Interosseous muscle (Abductor 

 annularis v. quarti digiti) yield longitudinal masses which arise from the palmar aspects 

 and ulnar sides of the ulnar lateral surfaces of the third and fourth metacarpal bones 

 respectively. In each instance these longitudinal bundles maintain their palmar positions, 

 although each fuses with and is inserted in common with the dorsal interosseous muscle 

 in front of which each is found, and of which these longitudinal fibres have been 

 regarded as forming an integral part. 



At first sight, these detached longitudinal muscular masses appear to increase the number 



* My heartiest thanks are due to Sir William Turner for placing his private collection at my disposal for the 

 purpose referred to. 



