114 THE STRUCTURE OF MAN 
is always innervated by the ulnar nerve. According to W. Gruber 
(St. Petersburg), it was found in about 34 per cent, but, accord- 
ing to Wood (London), in only 8 per cent, of bodies examined— 
a want of agreement which may perhaps be indicative of a racial 
difference. This muscle must be referred back to a time when 
a transverse shifting of the ulna was possible in the ancestors of 
Man, as it now is, to some extent, in many lower animals; and it 
would appear that after the movements of this bone had become 
hmited almost entirely to flexion and extension, the muscle — 
gradually degenerated and disappeared. 
Finally must be mentioned the levator clavicule and the 
ischio-femorals or gluteus quartus, which occasionally occur in 
Man. ‘The latter muscle is constantly present in Anthropoids 
[as the so-called scansorius]. 
3. PROGRESSIVE MUSCLES 
Attention has already been drawn to the fact (ante, p. 97) 
that in certain regions progressive and retrogressive variations 
may occur simultaneously; and this is nowhere so conspicuous 
as with the facial muscles. Some of these which are in various 
stages of degeneration have already been referred to (ante, p. 
109). All the other mimetic muscles (ze. by far the greater 
number) appear to be progressively developing, in correlation with 
the increase of the intellect and the correspondingly advanced 
functional activity of their associated nerves. This advancing 
specialisation 1s indicated in the aberration of certain parts, and 
the formation of new layers of muscle. These changes have 
brought about striking differences between these muscles in Man 
and the homologous tracts in the Lemuroidea, where they are 
simple and comparatively easy to understand. We are thus able 
to demonstrate for the mimetic musculature very great variations 
of form and size in both a progressive and retrogressive direction, 
as indeed is the case in all organs which are in the act either of 
suppression or of differentiation, ze. are not in a definitive 
state. 
Progressive development is especially shown in the muscles 
round the eyes, the mouth, and the nose, and also in those of the 
sub-zygomatic region. 
Ruge expresses himself upon the tendency to further develop- 
ment and completion of the human facial muscles, very aptly, 
as follows :— 
