1909-10.] Short Muscles of the Hand of the Agile Gibbon. 209 
Musculus Accessorius Interosseus 3. 
This muscle is situated on the inner aspect of the metacarpal and 
proximal phalanx of the 3rd digit. Like the last muscle, it is partly 
segmented into two parts, one of which, however, is very minute. 
Origin. — The larger, anterior half arises from the inner and front 
aspects of the lower half of the 3rd metacarpal, and slightly from the layer 
of fascia which gives origin to the contrahentes. The posterior part arises 
from the lower part of the same bone behind the anterior portion. 
Insertion . — The anterior portion is inserted into the inner edge of the 
dorsal expansion of this digit near the first interphalangeal joint. The 
posterior part finds attachment to the inner side of the proximal phalanx 
near its base. 
Nerve Supply. — Fibres from the median by way of the deep division of 
the ulnar. 
Structure. — The anterior half is nearly 6 cm. long, and quite hides the 
posterior part, which only measures about 2J cm. 
The muscle at its origin lies on a slightly posterior plane to the adductor 
of the 4th digit (contrahens 3). Behind it lies the 3rd dorsal interosseous 
muscle. 
Musculus Accessorius Interosseus 1^. 
This muscle is situated on the inner side of the 4th metacarpal bone and 
first phalanx of the 4th digit. 
Origin. — From the front and inner surfaces of the 4th metacarpal in 
its lower three-quarters. 
Insertion. — Into the inner margin of the first phalanx and into the 
inner side of the dorsal expansion of the 4th digit. 
Nerve Supply. — The same as the muscle just described. 
Structure . — This muscle resembles the immediately preceding in shape 
and size, but is unsegmented. Behind the muscle lies the 4th dorsal inter- 
osseous muscle, while to the inner side and rather in front is the adductor 
of the little finger (contrahens 4). 
Fourth Layer: Dorsal Interossei or Abductores. 
(See Plate II. fig. 6.) 
These muscles resemble the corresponding muscles in the hand of man. 
They all abduct the digits on which they act from a line which passes 
through the centre of the middle digit. 
