DEVELOPMENT OF THE SPINAL NEEVES. 



679 



splanchnic system serves to) collect and transmit to the spinal medulla impulses from the 

 viscera, and to distribute efferent fibres to vessels in the splanchnic area, and to glands 

 and involuntary muscle-fibres. 



DEVELOPMENT OF THE PERIPHEKAL NERVES AND 

 SYMPATHETIC SYSTEM. 



DEVELOPMENT OF THE SPINAL NERVES. 



I. Origin of the Spinal Nerve Roots. The process of development of the spinal 

 nerves commences by means of the outgrowth of posterior and anterior roots from the 

 medullary tube. The two roots take origin in pairs in quite different ways. 



FIG. 602. DEVELOPMENT OF THE SPINAL NERVES. 



A, Formation of nerve roots. 



D.R, Posterior root. 

 V.R, Anterior root. 

 N.T, Neural tube. 

 No, Notochord. 



C, Formation of nerves. 



So, Somatic division. 

 Vi, Visceral branch. 

 P, Posterior ramus. 



Al.C, Alimentary canal. 

 Ao, Aorta. 

 V, Cardinal vein. 

 M.P, Muscle plate. 



D, E, Formation of subordinate 

 branches. 



Lat, Lateral, and 



Ant, Anterior, branches. 



B, Formation of nerve trunk (N). 



D.G, Spinal ganglion. 

 Sy, Sympathetic trunk. 

 W.D, Wolffian duct. 

 Co, Coelom. 



Formation of nerve trunks in relation 

 to the limb : dorsal and ventral 

 trunks corresponding to lateral and 

 anterior trunks in D and E. 



The posterior root is the first to appear, before, during, or after the union of the 

 medullary plates and the formation of the neural tube. 



It takes origin as a ganglionic crest, forming a continuous lateral unsegmented 

 band, on the dorsal surface of the medullary tube. It may arise in one of three ways : 

 (1) from the junction of the medullary plate and surface epiblast, before the closure of 

 the medullary groove ; (2) from a neural crest, a median ridge on the dorsum of the 

 completed tube ; or (3) as a direct outgrowth from the dorsal surface of the medullary 

 tube. The ganglionic crest becomes completely separated from the medullary tube, and 

 secondarily its cells (neuroblasts) rapidly become spindle-shaped, and by the end of the 

 fourth week give rise to two sets of processes : (1) a central series, which grow centrally 



44 I 



