THE NERVOUS SYSTEM OF THE BIRD i8i 
The homologies of the tracts in the mesial wall of the hemisphere are of great interest. 
Phylogenetically they are the most ancient. As Elliot Smith* says : ' The cerebral cortex of 
the Reptile becomes relegated in the Mammal to the region immediately surrounding the hilum of 
the hemisphere, and constitutes the true "limbic" lobe. Similarly, in Birds, the greater part of the 
mesial wall represents the precommissural body — a small part dorsal to this, the hippocampus.' 
The greater part of the pallial tract described above as passing towards the base of the brain 
in front of the anterior commissure, is represented in the precommissural fibres of the Mammal 
and in that part of the fornix which lies in the septum lucidum (Figs. 15 and 16, Plate VI). The 
smaller part, which passes posteriorly to the anterior commissure, and which is more strongly 
developed in Reptiles, forms, as shown in Edinger's figure of the Eidechse,f a tr. cortico-harben- 
dularis, and corresponds to the anterior pillar of the fornix. 
In transverse sections, two fine bands of fibres are seen, dorsal to the anterior commissure 
(Fig. 23, Plate VII). In Reptiles they are much more strongly marked, and are figured by Edinger 
(p. 149), and described as commissura anterior and posterior pallii — a psalterium system. We 
have not been able to ascertain the exact origin of these two fine strands. They have been 
described by other writers, e.g., Meckel and Turner,J under the name of the corpus callossum, but 
if by that we understand a dorsal commissure of the pallium — the corpus callossum of Man — the 
term is inaccurate ; such a commissure is limited to the eutheria, and developes pari passu with the 
great cortical formation of higher Mammals, They can be better described as com^missural fibres 
of the precommissural area and hippocampus. In the sagittal sections (Figs. 15 and 17, Plate VI), 
pallial fibres are seen to pass ventrally between the anterior and posterior divisions of the tr. 
septo-mesencephalicus, and in transverse sections (Fig. 27, Plate VII) they appear to enter the optic 
chiasma. The large system of fibres just described above is probably both an associational and 
projectional system. It connects corresponding parts of the fore brain with the thalamencephalon 
and mesencephalon, and perhaps, as indicated above, directly with the optic tract. After removal 
of the hemisphere, the Marchi method of preparation shows that the limited degeneration takes 
place. 
We do not think that this can be doubted, though MUnzer and Wiener expressly state 
the contrary. 
(b) The Striate Tracts. 
Series of horizontal sections through the brain show that the great cerebral tracts are 
connected respectively with cells situated [a) in the anterior lobe, {b) in the middle lobe, (t) in the 
posterior lobe. The strio-mesencephalic and thalamic tracts and the occipito-mesencephalic tract 
are descending tracts with well-marked fibres, and after section in the hemispheres, degenerated 
fibres can be readily stained by the Marchi method. They connect the cell masses in the 
cerebral hemispheres with the thalamencephalon and mesencephalon. 
i. Tr actus Strio-mesencephalicus. — -The anterior lobe of the brain as seen in horizontal 
section gives rise to three groups of fibres : (i) most superficially and internally to fibres which go 
* 'Journ. of Anat. and Physiology,' vol. xxx, p. 484. t Loc. cit. p. 148. % 
