1909 - 10 .] Short Muscles of the Hand of the Agile Gibbon. 215 
palmar interosseous muscles. But I bring forward the arguments men- 
tioned below in favour of the view that, although changing its position, the 
muscle retains its primary function of flexion, and really belongs to the 
layer of short flexors. 
It has, I submit, migrated across the nerve the better to act as a flexor 
of the digit to which it belongs. 
In the first place, the muscle so obviously corresponds in function and 
arrangement to the flexor brevis pollicis, that it is natural to conclude that 
they are both derived from the same layer. 
Secondly, the flexor brevis minimi digiti cannot possibly be considered 
to represent the inner contrahens muscle in the hand of the gibbon, as that 
muscle is itself present in a well-developed condition (Plate II. fig. 2). 
Thirdly, the relationship of the muscle to the deep division of the ulnar 
nerve is not an infallible guide to the morphological position of the muscle. 
In this ape the muscle is pierced by the nerve which passes between its two 
heads of origin, and the bulk of the muscle is on a deeper level than the 
nerve. The muscle is here seen in process of migration across the nerve, 
and the fascial expansion of the anterior annular ligament over the vessels 
and nerve forms the bridge over which the muscular origin travels. The 
abductor minimi digiti, a muscle of the third layer, is in an exactly 
analogous position in the hand of the gibbon. The call for flexion in the 
hand of this animal causes the abductor minimi digiti to migrate at its 
insertion so as to become a flexor of the little finger, and, the better to act 
as such, it has migrated also at its origin so as to be placed, like the flexor 
brevis minimi digiti, partially superficial to the deep division of the ulnar 
nerve. No one could, however, class the abductor minimi digiti as belong- 
ing to any but the third or abducting layer. 
Another good instance of a muscle travelling for functional reasons 
across a nerve is to be found in the relationship the supinator radii brevis 
muscle bears to the posterior interosseous nerve. Professor Hepburn * 
drew attention to the varying positions of these two structures in the series 
of the anthropoid apes and man. He writes : “ The posterior interosseous 
nerve of the chimpanzee deserves special attention because it affords some 
explanation of the position of this nerve in the substance of the supinator 
brevis muscle. As the nerve passes from the anterior to the posterior 
aspect of the forearm, it is never hidden altogether from view, being merely 
covered by a very thin aponeurotic fascia on the surface of the supinator 
brevis, and it can be readily understood how an increase in the size of the 
muscle and in the amount of its fibres taking origin from this investing 
* Journ. Anat. and Physiol ., 1893. 
VOL. XXX. 
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