1909-10.] Short Muscles of the Hand of the Agile Gibbon. 213 
palmar aspect of the hand. In sections through the foetal hand the meta- 
carpal bones are found pressed tightly together with the muscles lying on 
their anterior surfaces. It is only as development advances that the bones 
separate and the muscles become pressed back into the position which they 
occupy in the adult. 
Having found that the first and the third layers are completely repre- 
sented in the hand of the gibbon, we can now proceed with more confidence 
to relegate the remaining muscles to their true morphological position and 
function. 
The second or intermediate layer is typically represented by a double- 
headed or paired muscle to each digit. All these ten muscles are flexors. 
In the hand of man there exist six of these ten muscles. They are the two 
heads of the flexor brevis pollicis, the three palmar interossei, and the flexor 
brevis minimi digiti muscles (see over page). The remaining four have 
completely disappeared (Plate II. figs. 1, 3, and 4). 
In the hand of man, flexion is so well performed by the long flexors of 
the forearm, which have taken on increased development, that these short 
and relatively weak muscles are superfluous. In the 3rd digit they are 
quite gone. In the 2nd, 4th, and 5th digits one head becomes the palmar 
interosseous muscle. By becoming palmar interossei, both their position 
and function are altered. Whether the change of position and function was 
the cause or the effect of the disappearance of the adductor (contrahentes) 
muscles to these digits it is quite impossible to say. But certain it is that 
the migration and development of the one are proportionate to the degene- 
ration in the other. This is well demonstrated in the hand of the gibbon, 
where the size of the muscles of the one layer bears an inverse ratio to the 
size of the muscles in the other. In this animal only two palmar interossei 
are present (Plate II. fig. 2, P.I.l and P.1.3), but there are in addition the 
muscles we have termed musculi accessorii interossei. We will therefore 
consider the digits seriatim, to decide the position of each muscle. 
The 1st digit has both heads represented by the two heads of the flexor 
brevis pollicis. 
In the 2nd digit there is a strong palmar interosseous muscle to produce 
adduction, and the contrahens or true adductor is only rudimentary. This 
palmar interosseous is the inner of the two heads of the flexor brevis muscle 
of this digit. Musculus accessorius interosseus 1, which lies to the outer 
side of the first palmar interosseous, must therefore represent the outer head 
of the flexor brevis muscle. 
The 3rd, being the middle digit, has no palmar interosseous muscle, and 
the abducting dorsal interossei serve to bring the finger back to the middle 
