THE CORPUS CALLOSUM. 



127 



of and lateral to the hippocampus major. This eminence corresponds with the 

 collateral fissure externally, and is known as the eminentia collateralis or pes acces- 

 sorius (fig. 89, coll.} ; this term is also by some authors used to include the smooth 

 eminence at the junction of the cornua. The triangular space at this junction is 

 termed by Schwalbe trigonum ventriculi. 



The shape of the ventricles is best shown in a cast of the cavities, which can be 

 made in plaster of Paris. The appearance of such a cast, viewed respectively from 

 above and from the side, is given in the accompanying figure (fig. 90, A, B). 



The corpus callosum or great commissure (trabs cerebri) (figs. 91. 92, 102), 

 a white structure, about 10 centimeters long, which connects the two hemispheres 

 throughout nearly half their length, approaches closer to the front than the back of 



for. Man, 



cpfc. luc. \ 



r \ s 



pineal stria, 

 post. comm. 

 pineal bodu 



monticuius 



in^aaaib 

 pit. 



tub. valv. 



Fig. 9J. PORTION OF A MEDIAN SECTION OF THE BKAIN, SHOWING THE CORPUS CALLOSUM, THIRD 



VENTRICLE, AQUEDUCT AND FOURTH VENTRICLE, PONS, CEREBELLUM, ETC. (G. D. T. ) f. 



the hemispheres. It is about 20 mm. in width behind, and somewhat narrower in 

 front. Its thickness is greater at the ends than in the middle, and is greatest 

 behind (8 mm.). It is arched from before backwards, its lower surface being 

 concave and its upper surface convex. Its upper surface is distinctly marked by 

 transverse furrows, which indicate the direction of the greater number of its fibres. 

 It is also marked in the middle by a longitudinal furrow (raphe), which is bounded 

 by two white tracts, placed close to each other, named the mesial longitudinal stria 

 or nerves of Lancisi (fig. 88). On each side, near the margin, are seen other 

 longitudinal lines (grey or lateral longitudinal strife). The lateral striae lie within a 

 fissure (callosalfissure) which separates the upper surface of the commissure from 

 the gyrus fornicatus (by which convolution they are entirely covered in : hence the 

 name, ta-nm tectm, which is often applied to them). Both the mesial and lateral 

 longitudinal striae are traceable, when well developed, into the gyrus dentatus 

 posteriorly. The morphological value of these structures will be afterwards referred 

 to (p. 155). 



In front, the corpus callosum is reflected downwards and backwards, forming a 



