472 TEXT-BOOK OF EMBRYOLOGY. 



horn) ; (b) the specialized somatic vestibular nerve (descending or spinal VIII 

 and various terminal nuclei) and also the cochlear nerve and its various termi- 

 nal nuclei; (c) the splanchnic afferent nerves (tractus solitarius and its 

 terminal nuclei). (2) The separation of the efferent neurone bodies lying in the 

 neural tube into two main longitudinal series of nuclei (a) the somatic efferent 

 nuclei, occupying a more medial position, their axones emerging from the neural 

 tube as medial ventral nerve roots ; (b) the splanchnic efferent nuclei occupying 

 a more lateral position, their axones emerging laterally and forming mixed 

 roots with the incoming splanchnic afferent fibers (Fig. 407). 



FIG. 407. Diagram of a transverse section through the lower human medulla showing the origin of 

 the X and XII cranial nerves. Schdfer, 



g, Ganglion cell of afferent vagus sending central arm (root fiber) to solitary tract (f.s.) and col- 

 lateral to the nucleus of the solitary tract (f. s. n.). It is not certain that the axones of the 

 cells of this terminal nucleus take the course indicated in the figure, n. amb., nucleus am- 

 biguus and d. n. X, dorsal efferent nucleus of the vagus, both of which send out axones as the 

 efferent root fibers of the vagus. These two represent the lateral or splanchnic efferent nuclei 

 of this region, n. XII, nucleus of the hypoglossus the axones of which pass out medially as 

 efferent root fibers of the XII. This nucleus represents the medial or somatic efferent nuclei 

 of this region, f. s.. tractus solitarius or descending roots of vagus, glossopharyngeus and 

 facial; d. V., descending spinal root of the trigeminus; r., restiform body; o., inferior olivary 

 nucleus (''olive "); pyr. pyramid. 



The intermediate neurones of the epichordal segmental brain, as well as 

 of the cord, fall into two general systems. One of these is the system of 

 inter segmental neurones, connecting various segments of the segmental brain 

 and cord. This system may be collectively termed the ground bundle (of the 

 cord) and reticular formation (of the brain) . These neurones may be regarded 

 as not only furnishing the various reflex communications between the afferent 

 and efferent cerebrospinal peripheral neurones, but as also forming a system 

 upon which the descending neurones from the higher coordinating centers 

 (suprasegmental structures) act, before the efferent peripheral neurones are 

 reached. This system may thus be regarded in general as more closely associ- 



