738 



NEUROLOGY 



the head. The lateral walls of the brain-tube, like those of the medulla spinalis, 

 are divided by internal furrows into alar or dorsal and basal or ventral lamime 

 (Fig. 646). 



The Hind-brain or Rhombencephalon. The cavity of the hind-brain becomes 

 the fourth ventricle. At the time when the ventral cephalic flexure makes its 





For 



Roof-plate 





FIG. 646. Diagram to illustrate the alar and 

 basal laminae of brain vesicles. (His.) 



Alar lamina 



Furrow between 

 alar and basal 

 laminae 



Basal lamina 



Vagus nerve 

 Hypoqlossal nerve 

 Floor-plate 



FIG. 647. Transverse section of medulla oblongata of human 

 embryo. X 32. (Kollmann.) 



appearance, the length of the hind-brain exceeds the combined lengths of the other 

 two vesicles. Immediately behind the mid-brain it exhibits a marked constriction, 

 the isthmus rhombencephali (Fig. 650, Isthmus), which is best seen when the brain is 

 viewed from the dorsal aspect. From the isthmus the anterior medullary velum 

 and the superior peduncle of the cerebellum are formed. It is customary to 



Rhombic lip 



Roof-piate, 



Alar lamina 

 Basal lami 



Tractus 

 solitarius 



Floor-plate 

 FIG. 648. Transverse section of medulla oblongata of human embryo. (After His.) 



divide the rest of the hind-brain into two parts, viz., an upper, called the meten- 

 cephalon, and a lower, the myelencephalon. The cerebellum is developed by a 

 thickening of the roof, and the pons by a thickening in the floor and lateral walls 

 of the metencephalon. The floor and lateral w 7 alls of the myelencephalon are 

 thickened to form the medulla oblongata; its roof remains thin, and, retaining to 



