1909-10.] Short Muscles of the Hand of the Agile Gibbon. 20 7 
1st Palmar Interosseous. 
Origin. — From the front and inner aspect of the 2nd metacarpal, and 
very slightly from the base of the 3rd. 
Insertion . — Into the inner side of the base of the proximal phalanx of 
the index, and into the dorsal extensor expansion on that bone. 
Nerve Supply. — Fibres from the median by way of the deep division of 
the ulnar nerve. 
Structure. — The muscle is strong and well developed. The fibres are 
arranged in a penniform manner, and end in a small tendon. This muscle 
lies in front of the second dorsal interosseous and behind the adductors of 
the thumb and index (contrahentes 1 and 2). 
The 3nd Palmar Interosseous Muscle is absent, and its function is taken 
by the strong contrahens of this digit. 
3rd Palmar Interosseous. 
Or gin. — From a small part of the outer and front surface of the 5th 
metacarpal, behind the contrahens of this digit. 
Insertion. — Into the outer side of the base of the proximal phalanx of 
the 5th digit, and partly into the ligaments of the joint. 
Nerve Supply. — Ulnar fibres from the deep division of the ulnar nerve. 
Structure. — This muscle was very rudimentary; it had a small belly 
and a thin, weak tendon. 
The 3rd palmar interosseous being completely hidden by the contrahens 
of this digit, appears to be absent till the latter muscle is pulled aside. 
Third Layer : Musculi Interossei Accessorii. 
This is a group of four muscles which pass from the palm to the fingers 
(Plate I., M.A. 1, 2, 3, 4). They all lie posterior to the deep branch of the 
ulnar nerve, and therefore belong to the two groups of muscles which lie 
deeper than that nerve, namely the palmar and dorsal interossei muscles. 
Their exact morphological position will be settled later when the general 
morphology of the muscles is considered. 
Musculus Accessorius Interosseus 1. 
Origin. — From the front of the lower half of the 2nd metacarpal bone, 
between the palmar and dorsal interossei muscles of this digit, and closely 
associated with them. 
Insertion. — Into the outer surface of the middle phalanx of the index, 
