312 HUMAN EMBRYOLOGY AND MORPHOLOGY. 



sciatic ligament is mainly derived from the great median sheet, 

 out of which the middle layer of the lumbar fascia is also formed. 

 Parsons regards the short head of the biceps as a derivative of 

 the tenuissimus, while others regard it as part of the muscular 

 sheet which forms the peroneal muscles. Amongst Primates, it 

 is found only in man, the anthropoids, and some South American 

 apes. The short head of the biceps corresponds to the brachialis 

 anticus in the arm, and is supplied by the external popliteal 

 nerve. 



(16) The psoas parvus is also vestigial. It acts primarily as 

 a flexor of the pelvis on the spine. It begins to disappear with 

 the assumption of the erect posture. 



(17) The scansorius is a separated segment of the gluteus 

 medius and minimus. It rises from the anterior border of the 

 ilium and passes to the great trochanter. It corresponds to the 

 teres minor. It is not constant in any animal. 



(18) The flexor brevis digitorum to the little toe and the 

 adductor transversus of the great toe are often fibrous. 



Vessels of the Limbs. — The Vas Aberrans. — This vessel gives 

 rise to a number of anomalous arrangements of the brachial 

 artery. If a newly-born child be well injected, a number of 

 branches derived from the axillary, brachial and main arteries of 

 the fore-arm will be found to form an anastomatic chain along the 

 superficial aspect of the median nerve. The upper end of the 

 chain is formed by a branch from the axillary ; the lower end is 

 completed by a branch of the ulnar or radial at the elbow. 

 In South American apes this anastomosis always opens up to 

 form a supplemental brachial artery — the vas aberrans — ending 

 usually as the radial. From the manner of its origin and dis- 

 position in front of the median nerve, it will be seen that the 

 vas may arise from the axillary or upper part of the brachial, 

 and terminate in the radial, ulnar or brachial, according to the 

 branches which participate. It may be so large as to simulate 

 a large branch of the axillary or a division of the brachial. Such 

 a condition is spoken of as high division of the brachial. 

 The vas may supplant the brachial artery altogether, which 

 is then represented by a trunk which ends high up in the 

 arm by giving off the superior and inferior profunda arteries. 

 The brachial vessel formed from the vas is recognised by its 



