72 THE FRIST CEREBRAL VESICLE. 



hemisphere vesicles, so that it happens that the former are thrown into folds 

 separated by sulci, and the surface loses its smooth appearance. Such relatively 

 rapid growth occurs during the second and third month in the human embryo, 

 resulting in the production of a number of infoldings of the surface, which are 

 mostly transverse to the (bent) axis of the brain, although one or two on the mesial 

 surface run parallel to that axis. These infoldings of the surface, which may be 

 termed temporary m primitive sulci, necessarily have a corresponding projection into 

 the cavity of the thin- walled hemisphere vesicle (fig. 75). During the fourth month, 

 probably owing to a relatively more rapid expansion of the cranium, most of these 

 primitive sulci become obliterated, and the cerebral surface is again almost smooth. 

 Three, however, of the primitive sulci remain as permanent fissures of the brain, 1 

 and since the fissure of Sylvius is also now formed, although in a somewhat different 

 manner, the hemisphere of the human foetus at the beginning of the fifth month is 

 marked by four well characterised sulci having corresponding projections into the 

 interior of its cavity. These permanent primitive sulci are the following : 



1. The hippocampal sulcus, corresponding with the projection of the cornu 

 ammonis (hippocampus) into the lateral ventricle. 



par. occ 



Fig. 82. FtETAL BRAIN OP THE BEGINNING OF THE EIGHTH MONTH. (MihalkovicS.) 



A. , from above ; B. , from the side ; C. , mesial surface. 



-So, Rolandic sulcus : Sy, Sylvian fissure ; par.ovc, parieto-occipital ; calc, calcarine ; pr.c, precentral , 

 pll, parallel ; int.par, intraparietal ; call.mar, calloso-marginal ; unc, uiicus. 



2. The parieto-occipital sulcus, corresponding with the bend of the posterior 

 cornu of that ventricle. 



3. The calcarine sulcus, corresponding with the projection of the calcar avis. 



4. The Sylvian fissure, corresponding with the curve of the lateral ventricle. 



To these may be reckoned the longitudinal infolding of the mesial wall of the 



1 It is, however, uncertain whether the temporary sulci develope into or whether they are replaced 

 by corresponding permanent sulci. See on this subject a paper by D. J. Cunningham in the Journal of 

 Anatomy, April, 1890. 



