206 Proceedings of the Royal Society of Edinburgh. [Sess. 
Nerve Supply. — From the deep division of the ulnar nerve — the fibres, 
however, come from the median by a communication in the forearm. The 
nerve supply to both muscles is the same. 
Structure. — The origin is the thinnest and widest part of the muscle 
which becomes thicker and narrower as it passes outwards. The muscle is 
twisted on itself so that the fibres which have the highest origin have the 
lowest insertion, and vice versa. 
Contrahens 3. 
Origin. — From the front of 3rd metacarpal close to its base, and from 
the fascia over the metacarpals. 
Insertion. — Into the inner side of the base of the proximal phalanx of 
the index. 
Contrahens 3. 
Origin. — From the front of the bases of the 3rd and 4th metacarpals, 
proximal three-quarters of the shaft of the latter bone, and fascia over them. 
Insertion. — Into the outer side of the base of the proximal phalanx of 
the 4th digit, and into the dorsal extensor expansion. 
Contrahens 4 1 
Origin. — Chiefly from the front of the base of the 4th and slightly 
from the front of the base of the 3rd metacarpals ; a small slip from the 
inner lumbrical and the fascia over the metacarpals. 
Insertion. — Into the outer side of the base of the proximal phalanx of 
the 5th digit, into the dorsal extensor expansion in common with the inner 
lumbrical, and into the sesamoid bone on the front of the 5th metacarpo- 
phalangeal joint. 
Nerve Supply. — This has already been mentioned in referring to the 
1st. In the case of the others the nerve fibres come from the median by 
way of the deep division of the ulnar, but I am not quite certain of this in 
the case of the 4th. 
Structure. — As regards size, the 1st is the largest, next comes the 3rd, 
then the 4th, and the smallest is the 2nd. The inner three have small 
tendons, which in the case of the 3rd and 4th are inserted partly into the 
dorsal extensor expansion just below the lumbricals. 
Second Layer: Palmar Interosseous or Flexores Breves. 
Of this group only two are found ; they belong to the index and little 
fingers, corresponding to the 1st and 3rd in man. 
