II02 



HUMAN ANATOMY. 



Fig. 



fibres are also early represented by bundles which grow centrally from the ganglion of the vagus 

 towards the developing medulla, upon whose surface, opposite the junction of the dorsal and 

 ventral zones, they appear as a flattened oval bundle (fasciculus solitarius). For a time super- 

 ficial and loosely applied, this bundle gradually becomes more deeply placed in consequence of 

 the extension, ventral folding, and final fusion of the rhombic lip with the remainder of the dorsal 

 zone. Subseciuently the fasciculus solitarius becomes still farther removed from the surface by 

 the ingrowth of tracts of nerve-fibres from the neuroblasts of the rhombic lip and from other 



sources until, finally, the bundle comes to lie 

 beneath the ventricular floor where its position 

 permanently indicates the junction between the 

 original dorsal and ventral zones of the medul- 

 lary wall. In a similar manner the sensory fibres 

 of the trigeminal nerve are applied to the sur- 

 face of the developing pons ; since, however, 

 the bundle is attached after consolidation of the 

 dorsal zone of the medulla has begun, the 

 descending trifacial fibres retain the relatively 

 superficial position characterizing the spinal root, 

 while the descending root (fasciculus solitarius) 

 of the glosso-pharyngeo-vagus lies more deeply 

 placed. Subsequent to the invasion of the medulla 

 by the sensory parts of this nerve, the outgrowth 

 of the axones from the neuroblasts constitut- 

 ing the nucleus of origin provides its motor root- 

 fibres. 



The rhombic lip is a region of much impor- 

 tance, since from the neuroblasts which appear 

 within it are derived the cells of the reception 

 nuclei (substantia gelatinosa) of the sensory 

 cranial nerves, of the nuclei of the posterior col- 

 umns, of the inferior and accessory olivary nuclei 

 and of the arcuate nucleus. From the neuro- 

 blasts many axones grow medio-ventrally, pierce 

 the median spongioblastic septum derived from 

 the primary floor-plate, which later becomes the 

 median raphe, and gain the opposite side and 

 thus establish the systems of arcuate fibres. Other 

 axones grow dorsally and take part in even- 

 tually producing the fibre-tracts connecting the 

 olivary, dorsal and arcuate nuclei with the cere- 

 bellum. It is evident that the development of the 

 myelencephalon primarily contributes the nerv- 

 ous substance that becomes the dorsal part of the 

 medulla and underlies the fourth ventricle. Later 

 the closed part of the' medulla, which at first 

 is wanting, as well as the conspicuous pyramidal 

 tracts, are added as the strands of ascending 

 and descending fibres grow into the medulla 

 from the spinal cord and from other parts of the 

 brain. In this manner the important tracts of 

 the posterior columns and the spinal constitu- 

 ents of the restiform body and of the brain-stem 

 are added and, still later, the bulky pyramids 

 take form when the cerebro-spinal paths are 

 established. 



In accord with the falling apart and thick- 

 ening that affect the lateral walls of the mye- 

 lencephalon and lead to the production of the 

 medulla, the roof-plate of the brain-vesicle 

 becomes flattened and laterally expanded to keep pace with the increasing width of the ventricular 

 floor. In consequence, the roof-plate is converted into a rhomboidal sheet of great delicacy, the 

 primary velum, which histologically consists of little more than the layer of ependymal cells. 

 These, however, soon come into close relation with the overlying mesoblastic tissue from which 

 the pia is differentiated. During the third month a transverse fold, the plica chorioidca, appears 

 in the roof-sheet, near the posterior limit of the developing cerebellum (Fig. 955, B). Into this 



Transverse sections of hind-brain of human embryos, 

 showing three sta^res in development of medulla; A, 

 about four and a half weeks ; 5, about six weeks; C, 

 about eight weeks; rp, roof-plate; r, raphe; d, ?', 

 dorsal (alar) and ventral (basal) lamince ; >7, rhombic 

 lip; />-, lateral recess; /j, fasciculus solitarius; cr, 

 restiform bodv ; .r//, hypoglossal nerve ; sv, spinal 

 root of trigeminus; ;o, inferior olivary nucleus. (His.) 



