HIND-BRAIN 



107 



ipe when viewed from the surface. Its apex corresponds to the cervical flexure ; 

 its base is marked off by a thickened band, which afterwards develops into the 

 cerebellum. The hind-brain is broadest at the level of this transverse thickened 

 band ; in front it is connected with the mesencephalon by a thinner portion called 

 the isthmus (His) ; behind it gradually tapers to its junction with the spinal cord 

 at the cervical flexure. 



In early stages, as shown for the human embryo by Broman and by Peter 

 Thompson, 1 the rhombic brain shows slight constrictions marking off a series of 

 segments or neuromeres, seven in number. 



FIG. 146. SECTION OF THE HEAD OF A HUMAN EMBRYO OF 30 MM. (BEGINNING OF THIRD MONTH). 



Photograph. (T. H. Bryce.) 



IV, cavity of rhombencephalon or fourth ventricle ; in the roof of the ventricle the paired cerebellar 

 plate ; the lateral enlargements are the rhombic lips cut where they pass into the cerebellar plate. N 

 is placed in the naso-pharynx still continuous between the two palatal folds with the buccal cavity (Af). 

 The Eustachian tubes are seen opening into the naso-pharynx on each side ; above them in the cranial 

 base are the auditory labyrinths ; in the floor of the mouth is the tongue. Below the tongue are seen 

 the distal ends of Meckel's cartilages; above on each side their proximal ends. IAI! U 



Pari passu with the development of the pontine flexure the lateral walls of the 

 hind-brain are opened out, and the epithelial roof becomes greatly expanded 

 (figs. 142, 143, 144). This seems to be the mechanical effect of the forces causing 

 the ventral folding ; the behaviour of the tube has been compared to that of a 

 split indiarubber tube which is bent on itself. The thickened lateral walls are 

 divided, just as in the spinal portion of the neural canal, by lateral grooves into 



1 Jour. Anat. and Phys. xli. 



