422 



By this groove the isthmus is divided into two parts, the one ad- 

 jacent to the cerebellum, the other to the mesencephalon. That part 

 which lies near the cerebellum forms the valvula cerebelli ( V. c. Fig. 2), 

 whilst that which is adjacent to the mesencephalon, is characterized 

 by the rudiment of the nucleus of the fourth nerve. This nucleus is 

 situated at a little distance from the mesial plane (His, Kuithan) and 

 consists of a group of neuroblasts originating in the matrix-layer of 

 the isthmus-region. 



Another deep groove, the fossa rhomboidea, separates the cere- 

 bellum from the myelencephalon. The direction of this groove is 

 chiefly antero-posterior. The tela chorioidea which forms the roof of 



the fossa rhomboidea 

 is attached along the 

 myelencephalic (i. e. 

 the inferior) border of 

 the cerebellum. From 

 there the line of attach- 

 ment goes over upon 

 the antero - superior 

 border of the myel- 

 encephalon. At the 

 place where the in- 

 ferior border of the 

 cerebellum passes over 

 into the myelincepha- 

 lon, the tela forms a 

 little recessus, the top 

 of which is directed 

 towards the mesence- 

 phalon. This recessus 

 forms the rudiment of 

 the later recessus late- 

 ralis of the fourth ven- 

 tricle. The tela itself 

 is folded in by a trans- 

 verse fold, the fissura 

 chorioidea (His, Stroud, Bolk). The fissura has its greatest depth in the 

 median plane (Fig. 1), becomes shallow a little more laterally (Fig. 2) and 

 then disappears (Fig. 3, 4). By this fold the tela is doubled, the superior 

 sheet being turned towards the cerebellum, the inferior one towards 

 the myelencephalon. The superior sheet covers the postero-inferior 



Fig. 1. Frontal section at a distance of 0.4 mm 

 from the posterior border of the cerebellum. 15/1. Out- 

 line of photograph, details partly with the camera lucida. 

 C. cortex-layer. C.d. dorsal commissure. F.R.m. fissura 

 rhombomesencephalica. I. intermediate layer. 31. matrix- 

 layer. 3Ies. mesencephalon, ßlyel. myelencephalon. P.ch. 

 fissura chorioidea. B. rafe. T, taenia. T.ch.q. tela 

 chorioidea of the fourth ventricle. V.mes. mesencephalic 

 ventricle. V.rh. fourth ventricle. 



