THE HAND 95 



" line of pull " falls behind the point of contact of the phalanx 

 with the head of the metacarpal. As soon as manipulations 

 have succeeded in bringing the line of pull in front of the point 

 of contact, hyper-extension is replaced by a slight degree of 

 flexion. 



Difficulty in reduction may be caused (i) by the head of the 

 metacarpal being caught between the flexor pollicis brevis and 

 the adductor pollicis, (2) by the tendon of the flexor pollicis 

 longus slipping over to the ulnar side of the metacarpal, or (3) 

 by the interposition of the volar accessory ligament between the 

 two bones. Reduction is brought about by traction and hyper- 

 extension in order to allow the torn end of the volar ligament 

 to pass over the head of the metacarpal before the thumb is 

 flexed to the palm. 



Development of Fingers. During the sixth week of foetal life, four linear 

 furrows appear on the distal part of the upper limb. These deepen and 

 finally become clefts, which separate the digits from one another. The 

 furrows may be incomplete, giving rise to webbing of the ringers (syn- 

 dactylisni) ; or extra furrows may develop and produce supernumerary digits 

 (polydactylism). In the former case the furrow may be so shallow that the 

 bones of contiguous digits are fused together. 



THE BRACHIAL PLEXUS AND ITS 

 BRANCHES. 



The brachial plexus is formed by the anterior rami (anterior 

 primary divisions) of the lower four cervical and the first thoracic 

 nerves, assisted by small twigs from the fourth cervical and 

 the second thoracic nerves. The twig from the fourth cervical 

 joins the fifth, which then unites with the sixth, constituting the 

 Upper Trunk of the Plexus. The seventh cervical alone forms 

 the Middle Trunk. The eighth cervical and the first thoracic 

 unite to form the Lower Trunk. Each of these trunks divides 

 into an anterior and a posterior branch, and the three posterior 

 branches join to form the Posterior Cord of the Plexus. The 

 anterior branches of the upper and middle trunks join to form 

 the Lateral Cord, while the anterior branch of the lower trunk 

 forms the Medial Cord (Fig. 34). 



