THE CEREBRAL HEMISPHERES 



935 



but little obscured by a thin, gray lamina the indusium which is thickened 

 longitudinally by two symmetrically situated fibre strands, the mesal (striae Lancisii) 

 and lateral longitudinal striae (teniae tectae\ already mentioned as rudiments of 

 the rhinencephalon. 



The best conception of the size and form of the corpus callosum is obtained from 

 a view of a mesal section. It is then seen to be a long, thick, somewhat flattened 

 arch which bends sharply upon itself fronted to form the genu (genu corporis 

 callosi), while its caudal end is rounded and somewhat folded closely upon itself 

 to form the splenium. The corpus callosum ranges in length from 7 to 10 cm.. 

 its cross-section area from 5 to 10 sq. cm., being longer and larger in heavier 

 brains and in those of the highly intellectual as compared with smaller and less 

 highly efficient brains. It extends to within 4 cm. of the frontal pole and to within 

 (} cm. of the occipital pole. The thickness of the "body" averages 5 mm.; of the 

 splenium, 9 mm. or more, while the maximum thickness of the genu is about 

 13 to 15 mm. The reflected portion or rostrum (rostrum corporis callosi) gradually 

 tapers into a very thin lamina, the copula (lamina baseos alba), which in turn 

 joins the lamina terminalis frontad of the anterior commissure. 



The splenium (splenium corporis callosi) projects as a rounded welt over the 

 mid-brain, but is separated from it by a pial fold the velum interpositum. 

 Farther frontad the fornix becomes fused to the ventral surface of the corpus 

 callosum for a short distance, to again leave it in its more arched course toward 

 the corpus albicans. Two thin lamina?, one on either side of the median plane. 

 but closely applied to each other and frequently partially fused, occupy the interval 

 between corpus callosum and the fornix of each side. The lamina? together 

 are termed the septum lucidum of the authors, each one being called a hemiseptum; 

 the enclosed cavity is called the fifth ventricle or pseudocele. though not derived 

 from the original neural cavity. 



HIPPOCAMPAL 

 COMMISSURE 



CORPUS CALLOSUM 



CORPUS 

 CALLOSUM 



'ANT. COMMISSURE 

 REGION OF 

 ANT. COM. 



ANT. COMMISSURE 



G. SUBCALLOSUS / 

 ANT. COMMISSURE 



LAMINA TERMINAL'S 



A 



FIG. 687. Schemata showing the development of the corpus callosum and its relations to hippocampus, 

 fornix, and anterior commissure. Lamina terminalis (terms) in heavy black, corpus callosum dotted. * Rep- 

 resents the attenuated indusium and longitudinal striae already described. 



Development. The corpus callosum develops as a mass of commissural fibres which grow 

 from side to side in the lamina terminalis (terma). The lamina terminalis serves as a matrix 

 for several commissural systems viz., the hippocampal commissure and the anterior commissure, 

 in addition to the corpus callosum. The last develops rapidly in higher mammalian brains, 

 thrusts aside the hippocampal margin of the pallium so that it atrophies in large part, and 

 stretches out within its sharply bent arch a portion of the precommissural wall of each 

 cerebral vesicle. It thus withdraws a part of the intercerebral cleft, eventually enclosing it 

 entirely as the fifth ventricle. The stages of development are shown schematically in Fig. 687 

 and its development in the human embryo is shown in Fig. 688. The corpus callosum is most 

 fully developed in man and does not appear below the marsupials. Its growth kept pace with 

 the preponderatingly greater development of the neopallium in higher forms, and it may be 

 looked upon as an index of the elaboration of at least one division of the association systems 

 those concerned with bilateral coordinations. 



