198 



HUMAN EMBRYOLOGY AND MOEPHOLOGY. 



(2) The Pons Varolii. 



(3) The Cerebellum. 



The Fourth Ventricle. — The cavity or neural canal of the hind 

 brain becomes the fourth ventricle. In its floor are developed, 



roof plate (inf. med. velum) 



inner zone 



middle zone 



outer zone 



ependyma 



taenia 

 alar lamina 

 basal lamina 



Fig. 162. — Section across the Hind Brain of a Human Embryo in the 5th week. 



out of the ventral and dorsal parts or laminae (Fig. 162) of the 

 medullary plates, the pons and medulla. In its roof are developed 

 the cerebellum, superior and inferior medullary vela. 



Inferior Medullary Velum. — When a section is made across the 

 posterior half of the hind brain of a fifth week human embryo 

 (Fig. 162), the same parts are seen as in a section of the cord. 

 The only difference is that the roof plate, which in the cord is 

 narrow, is here very wide and thin. It will also be noticed that 

 each medullary plate, shows, as in the spinal cord, a dorsal or 

 alar lamina and a ventral or basal one. The two laminae of each 

 side meet at an angle. In the roof plate over the anterior half 

 of the 4th ventricle is developed the cerebellum and superior 

 medullary velum ; over the posterior half, the roof plate forms the 

 inferior medullary velum. 



As shown in Fig. 163, the velum is continuous with the 

 cerebellum above and the roof of the central canal of the cord 

 below. In the posterior margin of the cerebellar rudiment are 

 developed : (1) the nodule, (2) the flocculus, (3) the peduncle of 

 the flocculus between 1 and 2 (Fig. 164). Hence the inferior 

 medullary velum ends above in these structures. 



The obex and ligula, thickenings found on the margins of the 

 lower angle of the 4th ventricle, mark the attachment of the roof 



