Supplement to ''Nature,'' August i8, 1923 



261 



them, when fully differentiated, to perform effectively 

 their respective functions. The influences which con- 

 trol their movements and regulate their dispositions 

 we do not know as yet. But whatever the nature of 

 these regulating forces may prove to be, we can see 

 that they are exactly of the same kind as those which 

 control the differentiation of facial musculature in 

 monkeys and anthropoids. The degree of differentia- 

 tion of the facial musculature of man is but the final 

 stage of a continuous series of evolutionary forms to 

 be traced in the faces of monkeys and anthropoids. 

 The elaboration of the facial musculature runs more 

 or less parallel to the elaboration of the brain. 



The manner in which muscular adaptations arise 

 may be better exemplified if we take a muscle which 

 is concerned in purely mechanical actions, such as the 

 latissimus dorsi. This muscle is concerned in pulling 

 the upper arm backwards as in rowing. It works in 

 the human body from a wide firm base, placed in 

 the lower part of the back — one which extends from 

 the sixth dorsal spine to the crest of the ilium. As 

 points of origin it also utilises the lowest three or four 

 ribs, and occasionally also the lower angle of the scapula. 

 This muscle, occupying the lower half of the back, 

 makes its appearance in the human embryo in the lower 

 part of the neck, just below the embryonic shoulder- 

 blade. By the end of the sixth week of development 

 the army of cells which compose the muscle have 

 extended or migrated downwards as far as the fourth 

 rib, reaching the twelfth rib about the seventh week, 

 and the iliac crest by the time the human embryo is 

 two months old.^'' The success with which the develop- 

 ing muscle cells reach their ultimate destinations is one 

 of surprising accuracy ; they may take hold of a spine 

 or a rib too far up or too low down, but the total result 

 is always one which makes the whole muscle into an 

 effective mechanical engine. Such variations may 

 make the muscle a Httle less or a little more useful to 

 the individual. The young muscle cells, when they 

 have reached their definitive sites, arrange themselves 

 in serried ranks, each rank hitched directly or indirectly 

 to the lever through which the collective army of cells 

 exerts its strength. 



Now, this muscle has almost the same attachments 

 in the gorilla and chimpanzee as in man ; there is a 

 greater range of individual variation in its points of 

 origin ; the marksmanship made by the migrating 

 myoblasts is less accurate than in man. In the orang 

 this muscle obtains no direct origin from the ribs, 

 while in the gibbon five or six ribs are seized. In the 

 gil)bon, however, there is no direct muscular origin 

 from the crest of the ilium. In the old-world monkeys, 



" Warren H. Lewis, Keibcl and Mall's Manual of Human Embryology, 

 1910, vol. i. pp. 454-522. 



and also in their American cousins, the origin of the 

 latissimus is restricted to the lower three or four dorsal 

 spines ; the origin from the iliac crest is slight or in- 

 direct ; while the fibres rising on the side of the thorax 

 are not directly attached to the ribs. Very occasionally 

 one sees fibres rising from the lower angle of the 

 scapula of monkeys, a variation in attachment which 

 has become very common in man. In these variations 

 of attachment we are seeing evolution at work, and 

 its machinery lies in the forces which regulate or control 

 the migratory movements of the young muscle cells. 



Influence of Nerve-connections. 



It is true that nerve fibres have entered, and formed 

 a union with, the muscle mass in the neck before 

 migration has set in ; the nerves are carried along 

 by the migrating muscle horde ; differentiation of the 

 muscle fibres begins at the point at which the nerve 

 enters the muscle mass. When muscular fibres are 

 fully differentiated they depend on their union with 

 nerve fibres for a continuance of their health and life. 

 But the migratory impulse, be that impulse what it 

 may, lies not in the nerve union but in the muscle 

 elements themselves, for Ross G. Harrison ^^ found, if 

 the limb of a developing tadpole were deprived of its 

 nerve supply, the muscles still became duly differentiated 

 in their usual stations. 



Adaptations may appear first as occasional 

 Variations. 



Let us take another example to illustrate the manner 

 in which a new muscular feature has been evolved in 

 man's body. The muscles of the calf of man's leg 

 have taken on an enormous growth to raise the heel 

 in walking. The structure of the deeper muscle of 

 man's calf, the soleus, has taken on an extremely 

 complex and efficient arrangement of fibres ; its 

 origin from the posterior aspect of the bones of the 

 leg is particularly extensive. In all dog-like or prono- 

 grade apes this muscle has a narrow origin from the 

 smaller bone of the leg, the fibula, and this is also 

 usually the case in the orthograde apes, or anthropoids. 

 In man the origin of the muscle has undergone an 

 extension, a large part migrating from the fibula and 

 obtaining an extensive attachment to the tibia. But 

 this extension to the tibia which is constant in man 

 occurs as a frequent variation in all the anthropoids. 

 Out of 8 gorillas examined, 3 had a tibial origin for 

 this muscle ; this was also the case in 2 out of 12 

 chimpanzees, i out of 8 orangs, and 2 out of 12 gibbons. 

 In the anthropoids there is a tendency for the soleus 

 to extend its origin to the tibia ; in man this tendency 



" Anat. Record, 1908, vol. 2, p. 145; Atnerkan Joum. of Anal., 1906 

 vol. 5, p. 121 ; Jott¥». of Experim. Zool., 1907, vol. 4, p. 239. ' 



