Blake, Roof of the Fourth Ventricle. 91 



the body of the fourth ventricle, the other those of the lateral 

 recesses. In their course they follow that of the arteriae post- 

 cerebellaris. 



In the lower animals the plexus in the lateral recess reach- 

 es the greatest development ; in man the development in both 

 regions is about the same. 



In man, however, the plexus in the body of the ventricle 

 has a greater proportional length and may reach caudad on the 

 vermis as far as the pyramis. 



I was at first led to believe that this development of the 

 plexus took place beyond the limits of the ventricular epithe- 

 lium but am now convinced that it does not. 



In the early embryos the lateral extensions of the choroidal 

 plexus are situated a little ventrad to the middle of the caudal 

 and lateral wall of the lateral recess. Figs. 4, 8 and 13. La- 

 ter because of the greater growth of the portion of the roof 

 situated caudad to the plexus, and possibly partly on account of 

 partial fusion of the roof cephalad of the plexus to the cerebellum, 

 this portion of the roof (velum medulare posterius) becomes 

 much abbreviated in comparison with the remainder. This rela- 

 tion holds true throughout life and the plexus is involuted along 

 a line which closely borders the peduncle of the flocculus. Figs. 

 15, 16 and 29. 



That the plexus forms a true part of the roof is evinced by 

 the continuity of the epithelium over it. 



Having consideied the development of the margins of at- 

 tachment of the roof and choroid plexus, it becomes necessary 

 to follow the changes which take place in the roof as a whole, 



THE ROOF AS A WHOLE. 



In early embryos the opening between the body of the 

 ventricle and the lateral recess is relatively wide, Fig. 18, but 

 soon the constriction becomes more marked, so that the descrip- 

 tion of the ventricular cavity becomes naturally divided into that 

 of the lateral recess and the body of the ventricle. 



The constitution of the roof of the body of the ventricle dif- 

 fers considerably in man from the condition found in the lower 



