126 



NEKVOUS SYSTEM 



fifth week between the ganglia, and the dorsal roots are by that time completed. 

 Meantime the spinal nerve-roots have been formed by the union of fibres from the 

 ganglia with the motor root-fibres. During the fourth week, at a time when the 

 limb-buds are still small and undivided, the segmental nerves begin to be con- 

 nected by anastomosis (fig. 169). The connecting filaments between the nerve-roots 

 from the fourth cervical to the first thoracic, arid again from the second lumbar to 

 the second sacral, constitute the future limb-plexuses (fig. 173). Each segmental 



ad 



tr.b. 



FIG. 170. TEANSVEKSE SECTION OF A HUMAN EMBBYO AT THE END OF THE FIFTH 

 OK BEGINNING OF THE SIXTH WEEK. (T. H. BryCC.) 



n.c., neural canal; n.a., neural arch; sp.g., spinal ganglion; s.r,, sensory root; m.r., motor root of 

 spinal nerve; d.b., dorsal branch; v.b., ventral branch; sy, visceral branch of spinal nerve, with 

 sympathetic ganglion ; st, stomach; sp, spleen; ad, adrenal ; g.g., genital gland ; w.b. t Wolffian body; 

 p } pancreas ; I, liver ; a, aorta. 



nerve early gives off a dorsal branch which passes through the myotome, and a 

 ventral branch which passes along the inner side of its extended ventral part. 

 From this a short visceral branch passes to the rudiment of the sympathetic 

 ganglion (fig. 170). The ventral nerve extends quickly into the neighbourhood of 

 the Wolffian ridge, and thence much more slowly round the body-wall. All the 

 limb-nerves as they extend into the growing limb-bud divide into primary dorsal 

 and ventral branches. 



