294 



HUMAN EMBRYOLOGY AND MORPHOLOGY. 



nerves encounter the condensed skeletal inesoblast at its base and 

 divide into a dorsal or extensor set and a ventral or flexor set 

 (Figs. 237 and 238). 



The nerve supply assists to indicate the body segments from 

 which the arm is developed (Fig. 239). The 4th cervical is the 



post root ganglia.—^^_ C4 



^[j— c.e 



Fig. 239.— The Distribution of the Posterior Roots of the Spinal Nerves on the Plexor 

 Aspect of the Arm. 



most anterior, the 3rd dorsal, sometimes it is the 2nd, is the most 

 posterior segment. Hence the arm is produced from seven, or 

 more commonly eight segments in all. Each segment contributes 

 from its nerve, its muscle plate and probably also its artery. 

 The typical distribution of a segmental nerve to the limb bud is 

 shown diagrammatically in Fig. 238. Each segmental nerve, as is 



