THE BRAIN AND ITS MEMBRANES. 753 



giving off an inferior, and a posterior division. As these four divisions repre- 

 sent sufficiently accurately the four large divisions or lobes of the adult hemi- 

 sphere, it will be more convenient, in tracing their further development, to give 

 them the same names, thus : the anterior division will be called the frontal lobe ; 

 the superior, the parietal lobe ; the posterior, the occipital lobe ; and the inferior, 

 the temporal or temporo-sphenoidal lobe. Each of these has its portion of the 

 original cavity, all of which naturally intercommunicate. 



The frontal lobes are now closely approximated in front of the inter-brain, 

 while there is also to be noted the formation of the optic thalami, which are merely 

 the thickened sides of the inter-brain (prosencephalon and thalamencephalon). 



The parietal lobes are similarly approximated above the optic thalami, but they 

 have now enlarged, so that the inner aspect of each comprises two regions (see 

 Fig. 459) one, just mentioned, close to its fellow above the inter-brain ; the other, 

 lower region, lying external to the external surface of the optic thalamus. 

 Furthermore, the upper region is the inner wall of the cavity of the parietal lobe, 

 which also comes above the inter-brain, while the lower region is simply the inner 

 aspect of solid matter i. e. a downward thickening of the original wall. Along 

 the floor of that portion of the ventricle contained in the parietal lobe is now seen 

 a thickening which soon resolves itself into a bundle of fibres. This band of 

 fibres, when traced forward, is found to be continuous with a similar thickening 

 around the foramen of Monro, which in its turn is continued down through the 

 optic thalamus (anterior pillar of fornix ; see page 761). The curve of this band 

 is due to the marked antero-posterior flexure of the entire foetal brain which has 

 already taken place, while its transition in position L e. to the floor of the " pari- 

 etal cavity " is due to a certain twisting or rotation which the hemisphere 

 vesicle now undergoes. 



The approximation of the parietal lobes brings these bands very close together 

 as they curve upward, and, as neither one is developed in the cavity of the cor- 

 responding frontal lobe, each serves, just here, as a line of demarkation between 

 the inner wall of the frontal lobe in front and that of the parietal lobe behind. 

 Traced backward, these bands are necessarily found to lie dorsal to the inter- 

 brain, since each is in the floor of the corresponding cavity of the parietal lobe. 



Fornix. The next point to be noted is the absolute approximation of the two 

 frontal and of the " upper regions " of the inner aspects of the two parietal lobes. 

 As a result of the latter approximation, the two anterior pillars of the fornix, just 

 above the foramen of Monro, are brought together edge to edge, and an actual 

 union takes place between them. This union extends posteriorly for more than 

 half the length of the floor of each cavity of the parietal lobe, and is known as 

 the body of the fornix, or as " the fornix " in the adult brain. The anterior 

 pillars, however, as they curve downward, are not united, this slight separation 

 persisting in the adult brain. 



Anterior Commissure. Immediately anterior to and connected with each of 

 these pillars, just previous to its passage through the optic thalamus, is a portion 

 of the inner wall of the cavity of the frontal lobe, which is now in close contact 

 with the opposite one. Just at this point there now occurs an interchange of 

 fibres between the inner walls of each frontal cavity. These fibres (anterior com- 

 missure) run transversely across the front of the upper part of the anterior 

 boundary of the prosencephalon, and causes an absorption of its tissue, its epi- 

 thelial lining excepted. so that the latter comes to lie directly on the centre part 

 of the posterior aspect of the anterior commissure, in the interval between the 

 anterior pillars of the fornix. 



The lower part of the anterior boundary of the prosencephalon persists in the 

 adult brain as the upper limit of the lamina cinerea (lamina terminalis). 



Corpus Callosum. The frontal lobes having now grown well forward, and hav- 

 ing also curve*.! upward to form the parietal lobes, and the inner surface of each 

 frontal and the " upper region " of the inner surface of the corresponding parietal 

 lobe having met the same structures of the opposite hemisphere, there is formed 



