

DEVELOPMENT OF THE NERVOUS SYSTEM 



747 



Parietal 

 operculum 



spheres, viz., the corpus callosum, the fornix, and anterior commissures, arise from 

 the lamina terminalis. About the fourth month a small thickening appears in 

 this lamina, immediately in front of the interventricular foramen. The lower 

 part of this thickening is soon constricted off, and fibers appear in it to form 

 the anterior commissure. The upper part continues to grow with the hemispheres, 

 and is invaded by two sets of fibers. Transverse fibers, extending between the 

 hemispheres, pass into its dorsal part, which is now differentiated as the corpus 

 callosum (in rare cases the corpus callosum is not developed). Into the ventral 

 part longitudinal fibers from the hippocampus pass to the lamina terminalis, and 

 through that structure to the corpora mamillaria; these fibers constitute the 

 fornix. A small portion, lying antero-inferiorly between .the corpus callosum and 

 fornix, is not invaded by the commissural fibers; it remains thin, and later a 

 cavity, the cavity of the septum pellucidum, forms in its interior. 



Fissures and Sulci. The outer surface of the cerebral hemisphere is at first smooth, 

 >ut later it exhibits a number of elevations or convolutions, separated from each 

 )ther by fissures and sulci, most of which 

 make their appearance during the sixth 

 or seventh months of fetal life. The 

 term fissure is applied to such grooves as 

 involve the entire thickness of the cere- 

 bral wall, and thus produce correspond- 

 ing eminences in the ventricular, cavity, 

 while the sulci affect only the superficial 

 part of the wall, and therefore leave no 

 impressions in the ventricle. The fissures 

 comprise the choroidal and hippocampal 

 already described, and two others, viz., 

 the calcarine and collateral, which pro- 

 duce the swellings known respectively 

 as the calcar avis and the collateral 

 eminence in the ventricular cavity. Of 

 the sulci the following may be referred 

 to, viz., the central sulcus (fissure of 

 Rolando), which is developed in two 

 parts; the intraparietal sulcus in four 

 parts; and the cingulate sulcus in two 



or three parts. The lateral cerebral or Sylvian fissure differs from all the other 

 fissures in its mode of development. It appears about the third month as a depres- 

 sion, the Sylvian fossa, on the lateral surface of the hemisphere (Fig. 658); this 

 fossa corresponds with the position of the corpus striatum, and its floor is moulded 

 to form the insula. The intimate connection which exists between the cortex 

 of the insula and the subjacent corpus striatum prevents this part of the hemi- 

 sphere wall from expanding at the same rate as the portions which surround it. 

 The neighboring parts of the hemisphere therefore gradually grow over and cover 

 in the insula, and constitute the temporal, parietal, frontal, and orbital opercula 

 of the adult brain. The frontal and orbital opercula are the last to form, but by the 

 end of the first year after birth the insula is completely submerged by the approxi- 

 mation of the opercula. The fissures separating the opposed margins of the oper- 

 cula constitute the composite lateral cerebral fissure. 



If a section across the wall of the hemisphere about the sixth week be examined 

 microscopically it will be found to consist of a thin marginal or reticular layer, a 

 thick ependymal layer, and a thin intervening mantle layer. Xeuroblasts from the 

 ependymal and mantle layers migrate into the deep part of the marginal layer and 

 form the cells of the cerebral cortex. The nerve fibers which form the underlying 

 white substance of the hemispheres consist at first of outgrowths from the cells of 



Temporal operculum 



Sylvian fossa 

 Frontal operculum 



FIG. 658. Outer surface of cerebral hemisphere of 

 human embryo of about five months. 



