242 H. F. Osborn 



The upper bundle arches sharply forwards and upwards and 

 in an obliquely transverse section (fig. 16), presents the figure of 

 a horsehoe. In sagittal sections (fig. 15), we observe that the rela- 

 tions of this bundle to the foramen of Monro are quite the opposite 

 of those presented in the Amphibia. As it ascends below and in 

 front of the foramen the bundle shows two divisions, a larger anterior 

 division of less compact fibres and a smaller posterior division of 

 more compact fibres. The former I regard as the corpus callo- 

 sum proper. It breaks up into smaller fasciculi which spread like 

 rays over the entire inner wall of the hemispheres. The fibres seem 

 to penetrate the mass of cells of the central gray, which in this 

 section extend in crescentic form from above the foramen of Monro 

 to a point opposite the centre of the olfactory lobes. The separation 

 of the hemispheres from the olfactory lobes is very distinct, so that 

 if any part of this large bundle entered the olfactory lobes it could 

 not be overlooked, but I have failed to discover any such tract. 



The posterior division of the upper bundle {cal'), follows an in- 

 dependent course, it bends upwards and backwards above the foramen 

 of Monro and then passes downwards along the posterior inner fold of 

 the mantle between the ependyma and gray substance (fig. 15, 16, IS). 

 The bundle diminishes gradually but the distribution of its fibres to 

 the adjacent layer of cells has not been actually observed. I believe 

 it probable that this tract represents the commissural portion of the 

 fornix. In position it corresponds with the crura fornicis, prin- 

 cipally in its relations to the corpus callosum, the foramen of Monro, 

 and its course along the angle of inflection of the brain wall between 

 the hemispheres and the optic thalami. But if these were the crura 

 we should expect to find the corresponding columnae, ascending 

 behind the anterior commissure (compare fig. 6 fo}\). In sagittal 

 sections there is some evidence of fibres leaving the main callosal 

 bundle upon either side and passing obliquely downwards behind 

 the anterior commissure, but this is not certain, and at present we 

 may more safely compare this tract with the commissure of the 

 cornu ammonis Forel^ 



The lower bundle. The main course of the lower bundle 

 extends widely into the cerebral stem with a slight upward conca- 



* Sie »werden zum Fornix gerechnet, sind aber wenigstens größtentbeils 

 nichts Anderes als eine Kommissur für die Ammonshürner«. I talce this quota- 

 tion from RABL-ltüCKiiAUDs paper (Zoo). Anz. 1881) as I have not seen Fouel's 

 article. 



