458 TEXT-BOOK OF EMBRYOLOGY. 



nerves enter the dorsal part of the lateral walls of the tube, while the efferent 

 nerves leave the ventral part of the lateral walls, their neurone bodies lying in 

 this ventral part. The effect of this upon the structural arrangements within 

 the tube is the production in the tube of two columns of neurone bodies, a dorsal 

 gray column for the reception of the dorsal or afferent roots and a ventral 

 gray column containing the efferent neurone bodies. 



Another important differentiation arises apparently from the important 

 physiological difference in general character between the activities of what may 



FIG. 404. Transverse section through the body of a typical Vertebrate, showing the peripheral 



(segmental) nervous apparatus. Froriep. 

 Small dots, afferent visceral neurones; coarse dots, afferent somatic neurones; dashes, efferent 



visceral (ventral root and sympathetic) neurones; lines, efferent somatic neurones. 

 Darm, gut; Ggl. spin., spinal ganglion; Ggl. vert., vertebral sympathetic ganglion; Ggl. mesent., 



mesenteric sympathetic ganglion. The peripheral sympathetic ganglionic plexuses (Auer- 



bach and Meissner) are not shown. Muse., muscle; Rad. dors., dorsal root; Rad. vent., 



ventral root; R. comm., white ramus communicans. 

 Two sympathetic neurones are represented as intercalated in the visceral efferent pathway. It is 



doubtful if there should be more than one. 



be termed the internal (visceral or splanchnic} and the external (somatic) struc- 

 tures. Internal activities are to a certain extent independent of activities 

 which have to do more with the reactions of the organism to the external world, 

 and consequently their nervous mechanisms have a more or less independent 

 character, forming what is often called the autonomic (sympathetic) system. 

 This independence is exhibited structurally by the intercalation in the per- 

 ipheral pathway of additional neurones, whose bodies form visceral ganglia 



