518 



THE HUMAN EMBKYO. 



area, along which the right and left hemispheres are fused 

 together, the dorsal border, c, becomes the corpus callosum, 

 while the posterior border gives rise to the anterior commissure, 

 and also to the longitudinal fibres which form the body of the 

 fornix. The anterior part of the corpus callosum, or genu, is the 

 first to be formed ; and as the hemispheres grow backwards over 

 the hinder part of the brain, the area of fusion extends back- 

 wards also, and so causes lengthening of the corpus callosum. 



The anterior pillars of the fornix develop early, as longi- 

 tudinal bauds of fibres, which form the upper lips of the foramina 

 of Monro, and then run round in the substance of the brain walls 



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FIG. 224. The brain of a Human Foetus of the fifth month. The brain is 

 bisected by a median sagittal section, and the figure shows the left half 

 from the inner surface. (From Kolliker.) Natural size. 



c, corpus callosum. cc, cerebellum, cm, middle or soft commissure, cr, crus cerebri. 

 fc, calcarine fissure, m, mid-brain, o, optic chiasma. ol, olfactory lobe, p, pons Varolii. 

 po, parieto-occipital fissure, pr, pyramid of the medulla oblongata. r, fissura arcuata. 

 sp, septum lucidum, forming lateral wall of fifth ventricle, u, temporo-sphenoidal lobe 

 of cerebral hemisphere. 



to its ventral surface, where they end in the corpus albicans, 

 which is at first single and median. The posterior pillars of the 

 fornix develop later, about the time the backward extension 

 of the corpus callosum is taking place. 



The convolutions of the cerebral hemispheres. In regard to 

 the sulci or fissures on the surface of the hemispheres, by which 

 the several convolutions are mapped out from one another, a 

 distinction must be made between (i) the primary sulci, which 

 appear at an early stage, and cause foldings of the entire 

 thickness of the wall of the hemisphere, but which ultimately 



