1909-10.] Short Muscles of the Hand of the Agile Gibbon. 211 
European brachio-radial index is only 74 while that of the gibbon is 116, 
the great elongation of the animal’s forearm will readily be appreciated. 
The hand and fingers participate in this elongation. The hand measures, 
from the crease in front of the wrist to the tip of the middle finger, 15 \ cm., 
while at its widest part it does not exceed 3J cm. in breadth. The agile 
gibbon is largely arboreal in its habits, and the hands are used as hooks by 
which to suspend its body from the branches. The animal progresses by 
swinging its body to and fro by its long arms until sufficient impetus 
is gained to project it through space to the next branch. Distances of 
40 feet are cleared in this manner with apparently little effort. It is this 
peculiar facility of flying through space that gains for this animal the 
distinctive title of Agile. The fingers lie parallel to one another, and are 
kept half bent into the palm. They show little tendency to oppose the 
thumb. The use to which this hook -like extremity is put calls for 
great strengthening of the muscles which produce flexion of the fingers. 
Flexion is needed not only at the metacarpo-phalangeal joints, but also 
at the proximal interphalangeal articulations, so as to complete the hook- 
like attitude. Increased flexion at the distal interphalangeal joints is 
uncalled for. 
It will have been noticed that nearly all the above muscles exhibit a 
tendency to wander down the phalanges. This is a mechanical gain, for the 
muscles act to better advantage when inserted well down the shaft of the 
long phalanx than when only attached to the base of the bone. Some 
muscles have even passed down as far as the middle of the 2nd phalanx. 
Many find partial attachment to the dorsal extensor expansion. The dorsal 
extensor expansion is rather a misnomer in this case, for it has a double 
function to perform. The long extensor tendons of the forearm acting 
through the middle of the dorsal expansion produce extension in the 
ordinary way ; but the margins of the expansion extend so far round the 
sides of the fingers that the lateral portions, into which the short muscles 
are inserted, produce, when pulled upon, flexion not only of the metacarpo- 
phalangeal joints, but also of the proximal interphalangeal articulations. 
Indeed, as far as the short muscles are concerned the chief function of the 
dorsal expansion seems to be the production of flexion at these two joints. 
Moreover, the flexor brevis minimi digiti and musculus accessorius interos- 
seus 1 actually pass far enough down to gain attachment to the shaft of 
the middle phalanx. As the fibrous pulp of the front of the finger serves 
as a partial attachment for these same two muscles, the action is well trans- 
ferred from the dorsal to the palmar aspect of the digit. The lumbrical 
muscles, though not included in this description, are inserted as far down 
