114 THE STRUCTURE OF MAX 



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 

 limited almost entirely to flexion and extension, the muscle 

 gradually degenerated and disappeared. 



Finally must be mentioned the levator claviculse and the 

 ischio-femoralis or glutseus 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 (i.e. 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 is 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, i.e. 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. 



Euge expresses himself upon the tendency to further develop- 

 ment and completion of the human facial muscles, very aptly, 

 as follows : 



