212 Proceedings of the Royal Society of Edinburgh. [Sess. 
as the first interphalangeal joints, and, like the other muscles attached to 
the lateral parts of the dorsal expansion, produce flexion of the joints. 
We must now pass to the discussion of the true morphological position 
of the muscles, and to do this with any hope of success we must first call 
to mind the muscular arrangement of the typical mammalian manus. 
To Professor Cunningham* belongs the credit of pointing out the three 
primitive layers of muscles in the typical mammalian manus. These 
primitive layers are : — 
(1) A palmar layer of adductores. 
(2) An intermediate layer of flexores breves. 
(3) A dorsal layer of abductores. 
Between the first and second layers runs the deep division of the ulnar 
nerve. These three layers correspond to the contrahentes, the palmar, and 
the dorsal interossei muscles of Halford 
The first or palmar layer consists of a group of four adductor muscles, 
one being supplied to each digit, with the exception of the third. The third 
digit has no need of an adductor, for the others are adducted towards a line 
which runs down its centre. The two abductors supplied to the third digit 
serve to bring it back to the middle line. In man this group disappears, 
with the exception of the adductor of the thumb, which becomes greatly 
developed with the growth of importance and opposability of this digit. 
(According to Quain the flexor brevis minimi digiti is a representative of 
this group of muscles, but strong reasons are adduced later to show that 
this muscle really belongs to the intermediate or flexor group.) In order 
to cope with the increased amount of work that opposability involves, the 
adductor to the thumb becomes segmented into two, the adductor obliquus 
and the adductor transversus pollicis. 
In the gibbon all the muscles of this layer are found present. The 
adductor to the thumb is the largest, and its partial segmentation has already 
been noted. 
It is convenient next to consider the third or dorsal layer. This layer 
in the typical manus consists of six muscles, of which two are attached to 
the third and one to each of the other digits. They all abduct from a line 
which runs down the centre of the third digit. These muscles are fully 
represented both in the hand of man and in that of the gibbon by the 
abductor pollicis, the abductor minimi digiti, and the four dorsal interossei 
muscles. The unequal appearance given to them in Plate II. fig. 2 is intended 
to represent the greater attachment they possess to the digit on which they 
act. Ruge has shown that these muscles are primarily developed on the 
* Challenger Reports. 
