292 



HUMAN EMBRYOLOGY AND MOEPHOLOGY. 



the axis of the body, (2) to rotate the leg outwards so that the 

 extensor surface of the knee is directed upwards, with the great 

 toe in front and the little toe behind. (3) The arm, on the 

 other hand, would require to be rotated inwards to bring the 

 elbow (extensor surface) into the dorsal position. The rotation 

 which brings the embryonic limbs into the adult position occurs 

 at the junction of the limb girdle with the trunk. 



Rotation at the Limb Girdle. — To understand the extent of 

 this rotation it is best to compare the scapula and ilium and pick 

 out their corresponding points. The extensors of the thigh and 

 arm may be taken as guides. The long head of the triceps and 

 rectus femoris of the quadriceps manifestly correspond ; their 

 points of origin — the anterior border of the ilium and axillary 



neural 



•ost. root gang. 



muscle plate 

 sp. nerue 



basis of head 

 of humerus 



nerue of exten. asp. 

 nerves of flex. asp. 



{:} nerve to somatopleure 

 somatopleure 



Fig. 237.— Section of the Arm Bud of a human embryo at the end of the 4th week. 

 (Alex. Low.) 



border of the scapula — may be regarded as homologous points. 

 The other corresponding points are shown in Fig. 236. The 

 sacral articular surface of the ilium corresponds to part of the 

 supra-spinous fossa. To restore the limb girdles to their primitive 

 and corresponding positions, the scapula has to be rotated so that 



