THE REGION OF THE ELBOW 



49 



tion of the limb at an early stage of development provides a 

 useful explanation. 

 At the fourth week 

 of embryonic life 

 the upper limb pro- 

 jects from the side 

 of the neck region 

 as a small bud, 

 which possesses a 

 ventral and a dor- 

 sal surface, a ceph- 

 alic (pre-axial) and 

 a caudal (post- 

 axial) border. The 

 anterior rami (ant- 

 erior primary divi- 

 sions) of the lower 

 four cervical and 

 the first thoracic 

 nerves grow out 

 into the bud ; C. 5 

 supplies the pre- 

 axial border, and 

 T. i the post-axial. 

 As the limb length- 

 ens, C. 6 and C. 8 

 reach the respec- 

 tive borders of the 

 bud, but C. 7 be- 

 comes situated 

 deeply in its sub- 

 stance and reaches 

 the surface only at 

 the distal part of 

 the limb. 



When wounds 

 or incisions on the 

 lateral aspect of 

 the forearm heal, 

 they may include twigs from the musculo-cutaneous nerve 

 in the cicatrix. The irritation of the nerve reflexly stimulates 

 the segments of the spinal medulla (C. 5 and 6) from which the 



FIG. 15. Schematic Representation of the Distribu- 

 tion of the Spinal Nerves to the Skin on the 

 Dorsal Aspect of the Upper Limb. 

 D.A.L. Dorsal axial line. 



