THE BRAIN AND SPINAL CORD. 



199 



late or velum to the alar laminae of the medullary plates, 

 hey represent the attached margin of the velum. The velum is 

 Iso attached to the restiform body which is developed in the 

 pper margin of the alar lamina. 



rudiment of cerebellum 



inf. med. uel. 



ceph. flex. 



ligula 



obex 

 nuchal flex. 



olf. lobe 



restiform body 



Fig. 163. — Lateral view of the Cephalic Part of the Neural Tube in a 5th week human 

 embryo. (After His.) 



The velum is to be regarded as a specialized part of the neural 

 ibe, which has been arrested at an early embryonic stage and 

 serialized for the secretion (and absorption ?) of cerebro-spinal 

 uid. The foramen of Majendie and openings at the lateral 

 scesses of the 4th ventricle, where the velum is produced into 

 loroid villi or cornucopia, may be caused by the absorption of a 

 art of the velum or in some cases they may be produced in the 

 imoval of the brain from the skull. In the early embryonic 

 mdition, at least, the ventricle is quite closed. As will be 

 sen from Fig. 163, the neural tube is bent with its convexity 

 irwards at the pons. His has suggested that this bend may 

 ive something to do with the production of the wide roof plate 

 ' the 4th ventricle, for if a piece of tubing be bent so, the part 

 . the concavity of the bend becomes widened out. 



Cerebellum. — The condition of the cerebellum in a 4th week 

 jman foetus resembles that of the Frog (Figs. 165 and 166). 

 . is then merely a thickened transverse band in the anterior 

 irt of the roof plate of the 4th ventricle. The vermis or median 

 be is the first part to be developed, the lateral lobes form in the 



