( 346 ) 



/'.) which, in t lie mesial wall of the pallium, is not clearly dis- 

 tinguishable from the corpus callosum. Downwards, the fornix may 

 be followed as far as the anterior commissure {C.a.). In the angle, 

 where callosum and fornix meet, lies a bundle of fibres (/'>•.) 

 ventrally from the corpus callosum and coming from behind. This 

 bundle crosses in the middle-line another bundle of the same kind 

 coming from the opposite direction. This crossing-system is the 

 fornix-commissure. More fron tally this commissure is wanting and 

 only the callosum and the fornix are present in the relation I just 

 now described. 



From the topographical relation of the corpus callosum to the 

 fornix-commissure the deduction may be made that the more posterior 

 part of the callosum is equivalent to the splenium. In the same way 

 it follows from the relation of the callosum to the fornix-bundle that 

 the more anterior part of the callosum corresponds with the genu 

 of that structure. The origin of the corpus callosum therefore 

 comprises the whole commissure, and consequently the growth of 

 the corpus 'callosum does not take place by means of the apposition 

 of new systems of fibres, but by an equable enlargement in corre- 

 spondence with the growth of the pallium. 



The most preponderant change in the structure of the mesial wall 

 of the pallium at the place of origin of the callosum consists in the 

 cortex-layer bending a little inward and ending with a sharp edge. 

 The middle-layer of the wall of the pallium gets richer in nuclei; 

 these nuclei surround the callosum and the fornix like a cap. Along 

 the lower edge of the cortex-layer they penetrate into the marginal- 

 zone of the wall of the pallium. By this process the marginal-zone 

 disappears as a separate layer. 



In the zone of union of the mesial walls of the pallium the changes 

 in the structure of this wall are more considerable; the observation, 

 that I he most mesial bundles of the fornix pass through the glia- 

 tissue of this zone of union, seems of importance here, as from this 

 fact may be derived, that the re-constructed mesial wall of the 

 pallium — the later septum luciduni — comprises more than the 

 original mesial wall. 



Fig. I. 

 Frontal section of the more posterior part of the lamina terminalis. 

 Section 20 [i stained with haematoxylin and eosin. Enl. 16.5 diam. 

 < '.a. Anterior commissure. L.t. Lamina terminalis. 



C.c. Corpus callosum. L.tr. Lamina trapezoidea. 



C.ch. Corpus chorioideum. T. Taenia. 



