124 



NERVOUS SYSTEM 



brought about by the free medullary edges of the margins of the hemispheres comii 

 together, and by the growth of fibres across the bridge so produced (His and others) 

 This takes place pari passu with the expansion of the pallium, and owing especially 

 to expansion of the frontal lobes the hippocampal commissure, which remains 

 small and retains its primitive position, is separated from the fore-part of the 

 corpus callosum, and the tissue of the trapezoid plates is stretched between them 

 to form the leaves of the septum pellucidum. 



The fate of the hippocampal formation is the same whichever interpretation 

 be adopted. The corpus callosum is formed within its arc, and causes by its 

 growth a stretching and atrophy of its forward moiety. The hippocampal fissure 

 of the temporal horn is represented by the cattosal fissure of the mesial surface. 

 The grey cortex sunk in the fissure becomes the hippocampus in the temporal horn, 



but is reduced to the gyrus cinguli &bovQ 

 the corpus callosum. The marginal 

 seam of grey matter becomes the 

 fascia dentata below, the indusium 

 above ; while the nervi Lancisii are 

 fibres arising from this wasted part of 

 the hippocampal formation, just as the 

 fornix-fibres spring from the hippo- 

 campus. The free edge of the white 

 matter becomes the fimbria, the 

 posterior pillar (cms), and body of the 

 fornix. The anterior pillar (column! 

 fornicis) is formed by a strand of fibres 

 early formed in the wall of the hemi- 

 sphere and connected with the thalamus 

 of its own side (fig. 166). The so-called 

 peduncle of the corpus callosum is a 

 strand (fasciculus prcecommissuralis, 

 Elliot-Smith, or gyrus subcallosus, 

 Zuckerkandl) formed on the trapezoid 

 plate (area paraterminalis). It is in- 



FlG. 168. VlEW OF THE INNER SURFACE OF THE 

 RIGHT HALF OF THE FO3TAL BRAIN OF ABOUT 



six MONTHS. (Reichert.) 



F, frontal lobe ; P, parietal ; 0, occipital ; 

 T, temporal ; J, olfactory bulb ; II, optic nerve ; 

 fp, calloso-marginal fissure; p, p', parts of the 

 parieto-occipital fissure ; h, calcarine fissure ; g, g, 

 gyrus fornicatus ; c, c, corpus callosum ; s, septum 

 pellucidum ; /, placed between the middle commis- 

 sure and the foramen of Monro ; v, in the upper part 

 of the third ventricle ; v', in the back part of the 

 third ventricle ; v", in the lower part of the third 

 ventricle above the infundibulum ; r, recessus 

 pinealis; p-v., pons Varolii ; Ce, cerebellum. 



timately connected with, indeed is the 

 anterior termination of, the hippo- 

 campal formation. When formed it 



closes-in the septum pellucidum, and separates it from the other derivatives 

 of the rhinencephalon below and in front of it. 



Formation of the fissures. We have already seen how the wall of the 

 vesicle is infolded to form the choroidal and hippocampal fissures. These are, in that 

 they involve the whole thickness of the wall, complete fissures. Two other such 

 fissures are produced the calcarine on the mesial aspect of the hinder part of the 

 vesicle, and the central portion of the definitive collateral. Corresponding to these 

 fissures, the calcar avis (fig. 159) and collateral eminence are found as projections 

 into the ventricle. From among the other fissures the fissure of Sylvius must be 

 placed in a category by itself. It is formed by the uprising of the edges of the fossa 

 of Sylvius until with the increasing expansion of the free portion of the pallium they 

 meet over the grey area on the surface of the hemisphere-stalk. This becomes the 

 island of Reil, and the meeting lips are named the opercula. The frontal operculum 

 is last formed (Cunningham), and owing to the development of a triangular field 

 upon it, the two small anterior limbs of the fissure of Sylvius are produced. The 

 remaining sulci, which begin to appear in the sixth month, are merely folds 

 produced during the progress of differentiation of the grey covering of the 



