1190 



HUMAN ANATOMY. 



intergrowth of the fibre-tracts (later the white matter), which arise partly from the young 

 nerve-cells within its walls and partly from neuroblasts situated in other segments. An ad- 

 ditional factor of moment in the production of the bulky cerebral hemisphere is the special 

 mass of gray matter, the corpus striatum, which, with the increasing fibre-tracts, leads to 

 the reduction and conversion of the cavity of the pallium to the irregular lateral ventricle. Its 

 once wide communication (Fig. 1030) with the cavity of the fore-brain is retained as the 

 proportionately narrow foramen of Monro. The pallium expands in all directions save 

 directly downward, where increase concerns chiefly the rhinencephalon, but the lines of its 

 growth are particularly backward and downward, in consequence of which, in addition to 

 the production of a temporal and the distinctive occipital lobe, the other brain-segments 

 become gradually covered over and deposed from their original superior position toward the 

 basal surface of the brain. This process is already marked during the third month (Fig. 1031), 

 by the end of which period the pallium covers the diencephalon. By the beginning of the fifth 

 month the mid-brain is completely overlaid, and by the eighth month the entire upper surface 

 of the cerebellum is covered. 



Development of the Sulci and Gyri. The modelling of the surface of the cerebral hemi- 

 sphere begins towards the end of the fifth month of foetal life, by which time the occipital lobe 

 is well formed and the brain-case is separated from the cerebral surface by an intervening layer 



FIG. 1028. 



Choroid plexus 



Choroidal fissures 



Pallium 



Lateral ventricle 



Roof-plate of III 

 ventricle, with 1 

 choroid plexus 



Corpus striatum 



Thalamu 



Frontal section of brain of rabbit embryo showing invagination of mesial wall of hemisphere along hippocampal and 

 choroidal fissures ; thin roof-plate of third ventricle stretches between thalami. X 13. 



of yielding arachnoid tissue, which offers little opposition to the production of the convolutions 

 which now follows. Preceding this period, the outer surface of the young hemisphere is quite 

 smooth, with the exception of the crescentic Sylvian fossa (Fig. 982) which marks the position 

 of the later insula. This depression has been described (page 1137) in connection with the pro- 

 duction of the Sylvian fissure. The uncertain creases, the so-called " transitory fissures," some- 

 tinit s seen on brains of a much earlier period are without morphological significance and are 

 now usually regarded as artefacts (Ziehen, Hochstetter) . 



Long antedating the appearance of the fissures on the outer aspect of the pallium, the 

 ni'-si.il surface of the latter is early marked by two grooves, the choroidal and the hippocampol 

 jixsures. The first of these (Fig. 1031 ) appears by the end of the fifth week as an invagination 

 of the mesial wall of the pallium just above the position of the foramen of Monro. At tirst 

 small, the groove is carried backward and downward by the expansion of the pallium until, 

 finally, it is traceable along the inner wall of the inferior horn of the lateral ventricle as far as 

 its loui-r limit. Knterini; by means of this invagination, the mesoblastic tissue forces before 

 it the attenuated cerebral wall and expands into a voluminous mass, the choroid body, which 

 on becoming supplied with blood-vessels, forms a vascular complex that for a time almost 

 completely tills the early lateral \.-ntricle. \Vith the subsequent growth of the pallium 

 backward and downward, the choroidal tissnre and the contained vascular fringe are carried 

 from the foramen of Monro over and .inmiul the thalamus into the inferior horn of the later 



