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Proceedings of the Royal Society of Edinburgh. [Sess. 
fascia, would cause a submergence of the nerve, and produce the charac- 
teristic appearance of the nerve piercing the muscle.” 
I believe that the increased need for flexion has caused both the origins 
of the abductor and the flexor brevis minimi digiti partially to cross the 
nerve in the hand of the gibbon, and the widening of the sphere of action 
and opposability have caused the latter muscle to cross the nerve completely 
in the hand of man. 
This view leaves the adductors of the thumb the only representatives 
in man of the first or adductor layer of muscles. 
No mention has up till now been made of the opponens pollicis and the 
opponens minimi digiti muscles. The former is plainly a segmentation 
from the outer head of the flexor brevis pollicis, from which indeed it is 
scarcely differentiated in the gibbon. The opponens minimi digiti is poorly 
developed, but suffices to clothe the front of the 5th metacarpal bone. It 
is closely associated with the flexor brevis minimi digiti at its origin, and 
with the third palmar interosseous at its insertion. According to the view 
adopted by Quain’s Anatomy * these two muscles belong originally to 
different layers, and the opponens is represented as being composed of 
portions from each layer. I have above endeavoured to prove that these 
two muscles in question are derived originally from the same layer. Ruge 
has shown by sections through the developing foot that the opponens is a 
segmentation from off the short flexor. In the ape the migration of the 
short flexor across the nerve has carried the opponens along with it. If the 
position of the nerve is to be looked upon as an infallible guide to the 
morphological position of the muscle, the opponens, flexor brevis, and 
abductor minimi digiti muscles must all be regarded as the representatives 
of the already present contrahens 4. The position of the opponens is 
therefore an additional reason for rejecting such a view. The view has, 
however, been adopted by the editors of Quains Anatomy : in dealing with 
the opponens in the hand of man they represent the muscle as being seg- 
mented partly from the adductor and partly from the short flexor layer. 
The diagram is stated to be “based on Cunningham,” though I have 
not found in the writings of that author sufficient to warrant the idea 
that this is the position he attributes to this muscle. I believe the 
opponens and flexor brevis minimi digiti belong solely to the second or 
flexor brevis layer. 
* Quairts Anatomy , vol. ii. pt. ii. p. 276. 
