Blake, Roof of the Foufth Ventricle. loi 



The primary fold which has been called the choroidal fold 

 becomes continuous with the fold between the secondary lip 

 and the oblongata, thus the entire lateral recess is cephalad to 

 it. Fig. 19. If we ascribe a limiting value to this primary 

 fold, we will have to place the lateral recesses entirely in the 

 epencephalic segment. If sagittal sections through the region 

 of the lateral recess are examined in embryos of man, sheep, 

 pig, dog or cat, the cerebellar laminae will be seen to entirely 

 form its roof. Fig. 20. Its caudal wall, caudal portion of floor 

 and extremity in these stages are the only membranous por- 

 tions. The major portion of its floor is formed by the oblongata 

 and its lip (rhomboidal lip). Figs. 26, 33, 34, 35 and 36. 



As will be seen directly, the enlargement of the membran- 

 ous portions constitutes the major portion of the lateral recess 

 in adult stages. 



As growth proceeds there seems to be a distension of the 

 recesses which is more pronounced in the lateral portions. The 

 walls bulge as if from some some internal hydrostatic pressure. 

 Their extremities now approach the cranial walls and come to 

 lie in close relation with the recessus labyrinthi. Figs. 4 and 8. 

 In man, as growth proceeds, they apparently recede from the 

 cranial wall (Figs. 29 and 30) and fold over the sides of the ob- 

 longata, where they are finally limited by the roots of the nerves. 



In the pig, dog, cat and sheep their extremities seem to 

 fuse with the dura. Fig. 38. 



Coincident with their lateral extension they grow caudally. 

 In the lower animals the caudal bulging is not so marked as in 

 man, where their caudal limits extend as far as a point oppo- 

 site the apex of the ventricle. Fig. 28. 



The entrances to the recesses share but little in the general 

 growth, and here the original embryonic condition is nearly 

 preserved, namely, the formation of the roof by the cerebellum. 

 Figs. 33, 34 and 35. 



As growth proceeds further the lateral recess loses its 

 distended character and the roof and floor in many cases come 

 in contact, except where separated by the choroid plexus, so 

 that its cavity appears as a slit. Figs. 33, 34 and 35. 



