122 NEKVOUS SYSTEM 



it is absent, and this thins away by a margin (tcenia) into the ependymal layer. 

 This portion of the hemisphere becomes much complicated by the development of 

 the fornix commissure and the corpus callosum. To make the matter clear, we 

 shall first imagine it developed without either fornix commissure or corpus 

 callosum. 



The margin of the hemisphere becomes folded early in the third month, to 

 form a fissure arching from the foramen of Monro to the temporal horn, parallel with 

 the choroidal fissure. The inflected area is continuous in front with the trapezoid 

 area (areaparaterminalis). The fissure is a ' complete' one, and has an elevation 

 within the vesicle corresponding to it. The thickened marginal seam of the grey 

 matter sunk in the fissure, lies at the lip of the choroidal fissure, and the edge of the 

 white matter is rolled inwards towards the ventricle, as it thins away into the 

 ependymal layer covering the choroid plexus (fig. 165); The projection into the 

 ventricle becomes the hippocampus, the marginal grey seam the fascia dentata, 

 and the white lip the fimbria. The primitive hippocampal formation thus con- 

 stituted extends from the front of the foramen of Monro to the temporal horn 

 arching round parallel with the choroidal fissure. It is continuous in front with 

 the trapezoid plate (area paraterminalis) and behind with the uncus. 



This marginal area of the cerebral cortex is of great morphological importance. In lower 

 forms, with a largely developed olfactory sense, it forms a prominent part of the brain, while 

 the rest of the hemisphere is relatively little expanded. In man it remains a diminutive 

 element, while the remainder of the cortex becomes enormously developed. Elliot-Smith, 

 seeing that it is in all probability related to the sense of smell, has included it in his 

 * rhinencephalon,' while to the rest of the pallium he gives the name neopallium. On ontogenetic 

 grounds it seems better to limit the term * rhinencephalon ' to that very distinct and early 

 isolated part of the hemisphere-rudiment to which the term has been applied in the foregoing 

 account. The later formed hippocampal formation is a part of the pallium in the sense defined 

 in the note on page 116. In respect that it is intimately related to the olfactory apparatus it 

 might perhaps be termed rhinopallium to distinguish it from the neopallium. 



The primitive hippocampal formation becomes profoundly affected by the 

 development of the commissures, hippocampal and neopallial (corpus callosum). 

 These are developed in intimate relationship with the lamina terminalis, which, as 

 we have seen, from the first connects rhinencephalon with rhinencephalon and 

 pallium with pallium. The lamina terminalis becomes thickened ; but opinion 

 is divided as to the manner in which this is effected. Some regard the increase 

 as due to interstitial growth, others believe that the opposed mesial surfaces of the 

 hemispheres become fused to form the plate in which the commissures appear. 

 The appearances seen in sections such as are here reproduced (fig. 164) are on the 

 whole in favour of the latter view. Immediately in front of the ependymal lamina 

 terminalis the faces of the trapezoid plates have fused to all appearance for a 

 certain distance. In the band of tissue thus produced the anterior commissure first 

 appears. It connects the corpora striata and temporal lobes; Both hippocampal 

 commissure and corpus callosum develop in the upper part of the thickened lamina 

 terminalis. The hippocampal fibres appear first, and the callosal later. They are 

 indistinguishable to begin with, but the callosal soon accumulate on the dorsal 

 aspect of the hippocampal, and form a short plate arching forwards over the 

 cleft between the two trapezoid plates (areae paraterminales). The cleft open 

 below is now the cavum septi the future fifth ventricle and the two trapezoid 

 plates will become the two leaves of the septum pellucidum. 



Once laid down, the corpus callosum becomes elongated ; there are two 

 divergent interpretations of the process. According to one view, the growth is 

 interstitial. The body is said to grow forwards and backwards, and is raised and 

 separated from the hippocampal commissure. As the original union between the 



