754 THE NERVOUS SYSTEM. 



a curved line of actual contact, all along \vbich occurs an interchange of fibres 

 running transversely from one hemisphere to the other. Another factor, besides 

 the upward curve of the parietal lobe, in causing this line of contact to be curved 

 is doubtless the antero-posterior flexure of the whole foetal brain, already re- 

 ferred to. This large transverse commissure, thus formed, curved anteriorly, is 

 the corpus callosum of the adult brain. 



This curved line, along which the above-mentioned interchange of fibres takes 

 place, has two extremities, an anterior and a posterior. 



The anterior extremity is immediately in front of, and in direct contact with, 

 the lateral part of the anterior commissure. Here the corpus callosum itself is 

 thin and conceals the anterior commissure from in front and below ; it is also 

 adherent to it and blends inferiorly with the lamina terminalis. 



The posterior extremity of the curved line is at the posterior part of the inner 

 wall of the cavity of the parietal lobe. Here the corpus callosum itself is very 

 thick and with a free posterior edge, beyond which project, posteriorly, the inner 

 surfaces of the occipital lobes, entirely separate from each other. 



This curved line or the cut surface of the corpus callosum, which is the same 

 thing will now be traced from the posterior to the anterior extremity. 



As it passes along anteriorly in the inner wall of the " parietal cavity" it is 

 quite near the floor, so that it soon reaches and becomes adherent to the corre- 

 sponding half of the "body of the fornix " immediately below it. As it goes 

 forward from this point it separates from its half of the fornix, which is now curv- 

 ing around the foramen of Monro. Continuing forward, the cut edge of the corpus 

 callosum is now on the inner wall of the "frontal cavity." It continues this 

 course for a distance, and then bends sharply downward, after which it runs lack- 

 ward until it reaches the anterior commissure, by which it is separated from the 

 anterior pillar of the fornix, just as the latter is about to run downward through 

 the optic thalamus. 



Septum Lucidum. There is thus formed a somewhat oval-shaped interval, 

 tapering posteriorly, bounded above and in front by the corpus callosum ; below, 

 by the corpus callosum (reflected part) and anterior commissure; behind and below, 

 by the anterior pillar of the fornix and body of the fornix, respectively. 



This interval is filled in by a lamina on each side, which is a portion of the inner 

 Avails of both frontal and parietal cavities. This lamina is necessarily in close 

 contact with the opposite one, and they both together constitute the septum lucidum 

 of the adult brain (Figs. 448, 461). 



The corpus callosum in addition to forming the commissure just described 

 spreads outwardly, also, in the frontal and parietal lobes, and, as it is now quite 

 thick, its under surface forms the roof of the "parietal and frontal cavities;" 

 its posterior surface (at the bend) is the anterior boundary of the " frontal cav- 

 ity," while the upper surface of its reflected portion is the floor of the frontal 

 cavity. 



Along the outer wall of both frontal and parietal cavities the corpus strict nm 

 is developed as a marked thickening, and close above it passes the corpus cal- 

 losum. All portions of the frontal and parietal lobes external to the corpus stri- 

 atum and above the corpus callosum, and those portions of the frontal lobe ante- 

 rior to and below the corpus callosum, develop into thick, solid matter and 

 project for a considerable distance, but without uniting with the opposite side, 

 beyond the corpus callosum in the corresponding directions. This solid mattrr 

 constitutes the bulk of the lobe. 



The "lower region " of the internal surface of the parietal lobe (internal cap*ul<' 

 of adult brain) is eventually closely united to the external surface of the optic 

 thalamus (Figs. 461, 460). 



The occipital lobe is the backward extension of the hemisphere. It is entirely 

 separate from the opposite one. Its cavity is roofed over by backward curved 

 prolongations from the corpus callosum. 



The temporal lobe (temporo-sphenoidoT) grows downward and forward, as 



