203 



doubtedly represents the Pars olfactoria; whether a division of this 

 bundle is present has not as yet been observed. 



2. Upper Bundle. In the Amphibia, the upper bundle is 

 sometimes widely separated from the lower [MeiiohraticJms, Proteus), 

 and passes across from one hemisphere to the other, completely sur- 

 rounded by the ventricle (thus resembling the commissura mollis). 

 In other forms it is separated from the lower bundle by the molecular 

 subtance of the lamina terminalis [Rana etc.). In all cases its fibres 

 rise compactly upon either side of the ventriculus communis and then 

 turn sharply forwards over the foramen of Monro. A part of its fibres 

 are given off directly to the inner hemisphere wall, but the greater 

 part form a longitudinal bundle in the inner half of the roof of the 

 ventricle, which rapidly gives off its fibres to the cells of the central 

 gray ; these fibres supply the entire upper medial cell area of the 

 hemispheres ; the bundle thus rapidly diminishes forwards and in the 

 region of the olfactory lobes is no longer distinguishable. In the Rep- 

 tilian I Turtle) brain is found an important step towards the mammalian 

 brain, in that the upper and lower bundles lie below but somewhat 

 in front instead of behind the foramina of Monro. It follows that the 

 upper bundle does not decribe the circuitous course to its distribution 

 which is observed in the Amphibia, but ascends obliquely forwards in 

 the inner hemisphere wall and divides into two branches, the one 

 spreading upwards and backwards above the foramen of Monro, the 

 other upwards and forwards in the inner hemisphere wall. 



3. The Fish brain has not yet been thoroughly studied. I can at 

 present only confirm the observations ofFritsch^, Miklucho- 

 M a c 1 a y 6 and others that besides the lower bundle, which undoubtedly 

 is in relation with the olfactory lobes, there is an upper bundle, the 

 direction of whose fibres is distinct from that of the lower since they 

 pass upwards and backwards. It seems highly probable that the upper 

 bundle will prove identical with the same in the Amphibia and Rep- 

 tiles and confirm Miklucho-Maclay's views of its homology. 



4. The size of the two bundles in most of the Amphibia is about 

 the same. In Menohranchus and Metiopoma, however, also in the 

 Turtle, the upper bundle is somewhat larger. Bearing upon this pro- 

 portion, is the observation of Flower^, on the brains of the Mono- 

 tremes and Marsupials, »that the want of upper (corpus callosum) fibres 

 is compensated for by the immense size of the anterior commissure(f. 

 Also of Sander^ that in the Marsupials the anterior commissure sup- 



5 Untersuchungen über den feineren Bau des Fischgehirns. Berlin, 1878. 



^ Beiträge zu vergi. Neurol. Leipzig, 1870. 



^ Philosophical Transactions of the Royal Society 1865. 



8 Arch. f. Anat., Phys. u. wiss. Med. 1868. 



