110 



NERVOUS SYSTEM 



plicated (fig. 151). Between the plaits of the membrane vessels are formed ii 

 the included mesenchymatous tissue, and thus is produced the choroid plexus of 

 the fourth ventricle (plica choroidea inferior). If we follow the ependymal lamell 

 forwards we find it to be continuous with the cerebellar band already mentioned. 

 Just below this it retains its more primitive characters, and forms the inft 

 medullary velum. In the region of the isthmus the roof-plate forms similarly th< 

 superior medullary velum (valve of Vieussens), while laterally it is thickened 

 produce the superior cerebellar peduncles. The cerebellar band proper becom< 

 enormously increased in thickness to form the cerebellum. The thickening oj 

 the roof -plate is at first bilateral, so that two plates are laid down, joined by 

 thin intermediate band, and separated by a cleft which communicates with the 



cavity of the rhombic vesicle (fig. 146). 

 This cleft is afterwards obliterated 

 the fusion of its lips, but a part ma}; 

 for some time persist as a small cere- 

 bellar ventricle (Blake). The central 

 plate forms the central lobe or vermis ; 

 the lateral lobes or hemispheres appeal 

 *./ --^p j ater ag roun d e( j enlargements of ii 



lateral portions. During the earlier 

 ip* I stages the cerebellum has a smootl 



surface, but by the end of the thi] 

 month it begins to be folded owii 

 to the increase of its surface area 

 (fig. 147). Fissures appear which 

 separate certain definite areas or 

 primitive lobes from one another. 

 The first partition involves the lateral 

 and posterior borders, which are cut 

 off by lateral fissures (fioccular fissures) 

 to form the flocculus and paraflocculus. 



hemisphere 



pineal body 



fl occu i ar fi ssu res meet on the 

 vermis, and mark off an area which 

 becomes the nodule. A deep fissi 

 (fissura prima of Stroud) also appears 

 on the vermis which separates the 

 future culmen and clivus monticuli, 

 and extends on to the hemispheres. 

 Later two other fissures mark off the 

 pyramid from the tuber valvulce (sulcus 

 prepyramidalis ; fissura secunda, Elliot 

 Smith) on the one side and the uvula 

 on the other. These four fissures constitute the main dividing furrows of the 

 vermis, but during the fourth and fifth months various furrows appear on the 

 hemispheres which further subdivide its surface. The morphology of the fissures 

 and lobes of the cerebellum will be considered in the volume of this work 

 devoted to Neurology, but it may be here stated that the great horizontal fissure 

 is of late appearance, and is not of morphological importance, being produced 

 merely by the great growth of the hemispheres in the region of the lobus clivi and 

 the lobus pyramis, which is so special a feature in the human embryo (Bradley). 1 



FIG. 150. MODEL OF THE BEAIN OF A HUMAN 

 EMBRYO OF 13'6 MM. VIEWED FBOM ABOVE. (His.) 



The hemisphere-vesicles are laid open ; in the 

 left the choroid plexus has been left in position, 

 in the right it has been removed to show the 

 corpus striatum. For description, see text. 



1 For the literature of the cerebellum, see Hertwig's Handbuch, loc. cit. and Bradley, Journ. of Anat. 

 and Phys., vol. xxxviii. ; on the mammalian hind-brain, see also the same author, Journ. of Anat. and 

 Phys. vol. xli. 



