208 Proceedings of the Royal Society of Edinburgh. [Sess. 
the base and also the outer margin of the shaft. In addition, some 
tendinous processes pass to the pulp of the finger. 
Nerve Swpply. — Fibres from the median by way of the deep division of 
the ulnar. 
Structure. — This is the largest muscle of the group, measuring 9 cm. in 
length. It is slightly thicker than is depicted in Plate I. The shape of 
the muscle is that of a cylinder tapered towards both extremities. The 
origin is fleshy ; the insertion is by means of tendon which appears opposite 
the first interphalangeal joint. The muscle lies, to begin with, on the front 
of the metacarpal bone, and then gradually passes towards the outer side of 
the finger. An interosseous muscle lies on each side of its origin. The 
tendon of the first lumbrical passes between the muscle and the outer side 
of the first phalanx. 
Musculus Interosseus 2. 
This is partly segmented into two bellies placed the one in front of 
the other. 
Origin. — The posterior part arises from the outer side of the head and 
shaft of the 3rd metacarpal, in front of the dorsal interosseous, through 
which it may have some slight attachment to the proximal half of the 2nd 
metacarpal. The anterior part arises from the front of the head and the 
lower half of the 3rd metacarpal on its outer aspect. 
Insertion. — The two bellies are inserted almost together. The posterior 
half is inserted partly into the outer side of the proximal phalanx of the 
middle digit, but mostly into the dorsal extensor expansion. The anterior 
half is inserted partly with the tendon of the posterior half, and partly by 
fleshy fibres, into the dorsal expansion below that tendon. 
Nerve Supply. — Fibres from the median nerve by way of the deep 
division of the ulnar. The nerve appears from under cover of the contra- 
hentes, and before entering the muscle gives off a twig which runs down 
the front of the metacarpal bone to the proximal phalanx, where it probably 
forms a communication with the digital branch of the median, which 
supplies the skin in this region. 
Structure . — As already stated, the muscle is in two parts. The anterior 
half is the smaller, measuring only 4 cm., while the posterior measures 
fully 6 cm. in length. Each half is pointed at both ends, arises fleshily 
from the bone, and is for the most part inserted by tendon. The two 
bellies lie between the adductors of the pollex and index in front, and 
the 2nd dorsal interosseous muscle behind. The 1st palmar interosseous 
lies to the outer side. 
