PRIMROSE : THE ANATOMY OF THE ORANG OUTANG 
79 
The dorsal layer of muscles is represented by a full series ; the four 
dorsal interossei and with them the abductor pollicis and the abductor 
minimi digiti. It is interesting here to note the suggestion that the 
abductor of the thumb and the little finger each arise from marginal 
structures which have been looked upon as possibly praepollex and 
postminimus rudiments, viz., the ridge (or margin) of the trapezium and 
the pisiform bone. From this standpoint they have been regarded as 
interossei muscles which have persisted after the suppression of the 
osseous elements of the additional digits. I have already noted the 
relation of the extensor ossis metacarpi pollicis to the praepollex, and 
in this connection one may note an interesting variation of that 
muscle which 1 found in the dissecting room of the University of 
Toronto. The extensor ossis metacarpi pollicis split into two distinct 
tendons at a point 5 cm. above the wrist joint ; one tendon was inserted 
Into the base of the metacarpal bone in its palmar aspect, and the other 
passed on to join the abductor pollicis and could be traced as a distinct 
tendon extending 2 cm. along the deep aspect of that muscle, where it 
blended with its muscular fibres. 
We have thus in the hand the following muscles in the dorsal layer. 
1. Abductor pollicis. 
2. Abductor minimi digiti. 
3. The Dorsal interossei. 
In the foot Cunningham adds a fourth muscle occasionally present, 
a rudiment of which I have described in my Orang, viz. : 
4. The occasional Abductor ossis metatarsi minimi digiti. 
The second layer of muscles, the intermediate layer, is made up of 
those muscles which Cunningham has termed the Flexores breves in 
the marsupials. These in man, as in the Orang, include the plantar 
interossei and the short flexors of the thumb and of the little finger. 
Cunningham^ at first was inclined to place the plantar interossei in the 
plantar group of adductors, but he subsequently placed them in the 
intermediate layer for the following reasons : 
1. In quadrumana he has traced the gradual disappearance of the 
adductor muscles except those belonging to the great toe. 
2. We find plantar interossei not only in those apes which have 
I D. J. Cunningham, “The Intrinsic Muscles of Thylacine, Cuscus and Phascogale.” Journ. Anat. 
and Phys., Vol. XII, 1878, p. 434. 
