Blake, Roof of the Fourth Ventncle. 83 



NOMENCLATURE. 



Inasmuch as there are some apparent inconsistencies in the 

 nomenclature employed, it is well to call attention to them, so 

 that they will not be misunderstood. 



As generally accepted, the third cerebral vesicle is divided 

 into two encephalic segments, the epencephal and metencephal, 

 the corresponding cavities being the epicoele and metacoele. 



These terms have been used in several of the accompany- 

 ing drawings, and also the term fourth ventricle — Figs. 12, 13 

 and 14, for example. 



The writer is unable to accept, for reasons that will appear 

 later, the division of this part of the brain into two segments, 

 and has used the terms metacoele and epicoele to emphasize the 

 fact that they cannot be made to apply to a considerable por- 

 tion of the fourth ventricle. 



Objection may be made to the use of secondary rhomboidal 

 lip in figures of advanced embryos and adults, when that struc- 

 ture no longer exists as such. The reason for its employment 

 in these cases is to indicate the derivation of the structure thus 

 designated, its adult name being also added when it seemed 

 necessary. 



In order to fully comprehend the conditions found in the 

 adult fourth ventricle, it is necessary to follow the development 

 from early embryonic stages, i. e., before the institution of the 

 pons flexure. 



The roof at this period is comparatively simple, but soon 

 complications appear, due to change in the margins of the ven- 

 tricle or rhomboidal fossae, as it is generally termed. 



Later the involution of vessels to form the choroid plexus 

 appears, together with the pons flexure. 



Thus it is convenient to classify the study of the roof as 

 follows : 



I. The line of attachment of the roof. 

 II. The choroidal fold. 

 III. The roof as a whole. 



