326 PRESENT PROBLEMS IN EVOLUTION AND HEREDITY. 



You have noticed the recent discovery that the grasping power of 

 infants is so great tliat the reflex contraction of the fingers upon a 

 slender crossbar sustains their weiglit; this power and the decided 

 inward rotation of the sole of the foot and mobility of the toes are per- 

 sistent adai)tations. Our grasping uniscle, the pal maris longus, is 

 highly variable and often absent; like the plantaris of the calf, it has 

 been re])laced by other muscles, and its insertion has been withdrawn 

 from the metapodium to the palmar fascia. In negroes we freipiently 

 find the palmaris reverting to its former function of Hexing the fingers 

 by insertion in the metacarpals. 



The rise of muscular specialization by degeneration is beautifully 

 shown in the extensor indicis, which, while normally supplying the 

 index only, reverts by sending its former slips to the thumb, middle, 

 and even to the ring finger. Testut* believes that the extension power 

 of the middle and ring fingers has declined, as the cases of reversion 

 point to greater mobility; the extensor minimi digit! is distinct and 

 highly variable (Wood), often sending a slip to the ring finger. 



The entire flexor group of the hand, excepting the palmaris, is appa- 

 rently specializing. The demonstration by Windlef and Bland Sutton, 

 that the origin of the flexors and extensors is shifting downward from 

 their original position, is evidence of an ada])tation to the short special 

 contractions required of them. 



The abductor pollicisj is also i>rogressiv^e and variable (Wood); the 

 reduplication of its inferior tendon, which is sometimes provided with 

 a distinct muscle, apparently points to the birth of a. second abductor. 

 The opponens of the thumb is well established and constant. Yarial>il- 

 ity seems to characterize both the developing and degenerating nuis- 

 cles; tlie latter are apt to be absent; it is rare that an important muscle, 

 sucli as the extensor indicis, is absent, but such cases are reported. 



It is interesting to note that the lost muscles of the body are almost 

 exclusively in the trunk or shoulder, and pelvic arches, and not in the 

 limbs. It will be remembered that the human shoulder joint is excej)- 

 tionally rigid, whereas in the quadrupedal state it was a factor in pro- 

 gression. Some of the muscular reversions in this quadrupedal region 

 are the h^vator clavieuhe (1 to GO, Macalister), tracln^lo-clavicularis, 

 scalenus intermedins, acromio-basilaris (Ohampneys), transversns 

 imcha^ (Gegenbaur). Api)arently associated with the former swinging 

 of the body by the fore limb in the arboreal life are the atavistic coraco- 

 brachialis-brevis (Testut), the epitrochlco-dorsalis (Testut), and ])ecto 

 ralistei'tius (Testut). § 



Centers of rariahiliti/. — As the literature is so readily acc<'ssible I 

 will not multiply illustrations of the innumerable (congenital variations 



* Sur les Anomalies Musculaires, -p. 564. 

 UToitriud of Auaiomi/ and rhysiolofjn, 1890, p. 72. 

 t Or extensor ossis inetacari)! pollicis. Spi^. Testut, ]>. 553. 



\\ Qiiiiiu describes sovcMify .■inomaloiis uniscles (./»/., vol. t.). Testut iN-seribes ;i 

 still larger iiiiinber. * 



