MID-BRAIN AND FOBE-BRAIN 



111 



The mid-brain (mesencephalon) is marked off from the isthmus by the 

 crossing of the trochlear roots, while in front, in early stages, it is separated from 

 the fore-brain merely by a slight fold in the roof-plate which extends on to the 

 lateral wall (figs. 148, 149). The cavity is at first relatively large, and owing to 

 the expansion of the roof it is prolonged backwards over the isthmus (fig. 151). This 

 diverticulum is ultimately obliterated and the lumen of the vesicle is reduced to 

 a narrow passage the aqueductus cerebri (aqueduct of Sylvius). The roof-plate 

 shows at first a mesial ridge (fig. 150), which in later stages disappears except 

 at its posterior extremity where it persists as the frcenum of the valve of Vieussens. 

 It then becomes thickened on each side to form two lateral swellings, subsequently 

 divided again by a transverse groove into the corpora quadrigemina. The ventral 

 portion of the vesicle forms the pedunculi (crura) cerebri, of which the tegmentum 



cb 



FIG. 151. MEDIAN SECTION THROUGH THE BRAIN OF A TWO 

 AND A- HALF MONTHS FOETUS. (His.) Magnified 5 diameters. 



The mesial surface of the left cerebral hemisphere is seen 

 in the upper and right-hand part ot the figure; the large 

 cavity of the third ventricle is bounded above and in front by 

 a thin lamina ; below is seen the infundibulum and pituitary 

 body. Filling the upper part of the cavity is the thalamus 

 opticus ; in front and below this is the slit like foramen of 

 Monro. Behind the thalamus is seen another slit-like opening 

 which leads into the still hollow external geniculate body. 



olf, olfactory lobe ; p, pituitary body ; e.g., corpora quadri- 

 gemina; cb, cerebellum ; m o., medulla oblongata. 



FIG. 152. MEDIAN SAGITTAL SECTION 

 OF THE INFUNDIBULUM AND 

 PITUITARY DIVERTICULUM IN A 

 RABBIT-EMBRYO, AFTER THE 



OPENING OF THE FAUCES. (From 



Mihalkovics ) 



be, basis cranii with basilar 

 artery ; if, infundibulum ; tha, floor 

 of thalamencephalon ; py, pituitary 

 diverticulum, now closed; p', stalk 

 of original communication with the 

 mouth ; pli, pharynx ; ch, notochord 

 in the spheno-occipital part of the 

 cranial basis. 



is first laid down, while the crusta is, like the other portions of the pyramid tract, 

 a later formation. 



The fore-brain (prosencephalon) undergoes a much more complicated 

 series of changes, which result in the formation of the thalami, the geniculate bodies, 

 the pineal body and its peduncles, the optic nerves, chiasma, tracts, and retina, 

 tuber cinereum and cerebral part of the pituitary body, the corpora mamillaria, 

 and the cerebral hemispheres. 



The remarkable feature in the development of the human brain is the enormous 

 expansion of the hemispheres till they dominate all the other portions of the brain, 

 overlapping as they grow the part of the fore-brain from which they spring, then 

 the mid-brain, and ultimately also the hind-brain. 



At a very early stage, as has already been mentioned, the optic vesicles are 

 developed as hollow diverticula from the fore-brain. The mouths of the diverti- 



