THE NERVOUS SYSTEM. 



457 



of the efferent fibers are the same as for the afferent and are further dealt 

 with later (pp. 462-465). 



The majority of the efferent root fibers pass to the differentiating somatic 

 muscles which they innervate, forming specialized terminal arborizations (the 

 motor end plates). The fibers to the dorsal musculature form, together with 

 the afferent fibers (p. 460), the dorsal branch of the peripheral spinal nerve; 

 others form part of the ventral branch which sends a branch mesially toward 

 the aorta. Some of the fibers of the mesial branch take a longitudinal course. 

 This mesial branch is the white ramus communicans and terminates in the 

 various sympathetic ganglia which are later formed along its course (p. 461). 



IG. 389. Diagram (lateral view) of the brain of a 10.2 mm. human embryo (during the fifth week), 



showing the roots of the cranial nerves. His. 

 Ill, Oculomotor; IV, Trochlear; V, Trigeminus (m, efferent root, s, afferent root) ; VI, Abducens; 

 VII, Facial; VIII, Acoustic (c, cochlear part, v t vestibular part); IX, Glossopharyiigeus; 

 X, Vagus; XI, Spinal accessory; XII, Hypoglossus. ot., Auditory vesicle; Rh.l., rhombic 

 lip. The two series of efferent roots (medial and lateral) are clearly shown. 



[Comp. Figs. 225, 227, 394 and 36^.) The fibers to the sympathetic ganglia 

 ire the visceral (splanchnic) fibers of the ventral root. There are a few other 

 fibers which grow dorsally from neuroblasts in the ventro-lateral walls of the 

 cord and thence out via the dorsal root (Fig. 392). They also are probably 

 visceral. 



In the cord the splanchnic fibers, with the exception above noted, issue with 

 the somatic fibers in a common ventral root. In the epichordal segmental brain, 

 however, there is a differentiation of the efferent neuroblasts of the basal plate 

 into two series of nuclei, a medial and a lateral. The medial series consists of 



