46 
PRIMROSE : THE ANATOMY OF THE ORANG OUTANG 
flexor brevis pollicis is also well developed, probably better than in man, 
and thus flexion of the first phalanx can be well accomplished. The 
inner head of the flexor brevis resembles that in man in being very feebly 
developed, and in being completely pressed into the deeper parts and 
covered over by the adductor obliquus, and according to Bischoff^ a 
similar condition is found in the Gibbon. In the Gorilla, Bischoff^ was 
unable to separate the outer head of the flexor brevis from the opponens, 
or the inner head from the adductor obliquus. It would appear from 
reference to the records of various authorities, that whilst great varia- 
tions exist among apes in the degree of development of the short mus- 
cles of the thumb, they are nevertheless all represented in the different 
species. Even in the spider monkey (Ateles) according to Huxley, in 
which the thumb is functionless, being wholly rudimentary and buried 
under the skin, all the muscles, abductors, adductors, short flexors and 
opponens are present, the long flexor alone of the muscles usually 
found in this situation being absent. A variation sometimes occurs 
according to Fick^ and others, in the insertion of certain fibres of the 
adductor into the shaft of the metacarpal bone of the thumb, thus con- 
stituting a second opponens. 
The Interossei (plate V, figs. 6 and 7). The first dorsal interosseous 
muscle arose from the ulnar side of the base of the first metacarpal by 
one head, whilst the other head arose from the dorsal, radial and palmar 
surfaces of the second metacarpal bone. The second dorsal interos- 
seous arose from the ulnar half of the dorsal surface of the second 
metacarpal bone, and from the dorsal, radial and palmar aspects of the 
third metacarpal. The third dorsal interosseous arose from the radial 
half of the dorsal surface of the fourth metacarpal, and from the dorsal, 
ulnar and palmar aspects of the third metacarpal. The fourth dorsal 
interosseous arose from the radial half of the dorsal surface of the fifth 
metacarpal, and from the dorsal ulnar, and palmar aspects of the fourth 
metacarpal bone. 
The palmar origins of the dorsal interossei, above described, were in 
all instances particularly well developed, forming in fact larger fasciculi 
than those arising from the dorsal region. On the palmar aspect of the 
third metacarpal bone the palmar origins of the second and third 
interossei meet together over the proximal half of the bone. The 
muscles were inserted as in man. 
1 Loc. cit. I, p. 215. 
2 Loc. cit. 2, p. 16. 
3 Loc. cit. 2, p. 302. 
