i82 THOMPSON YATES LABORATORIES REPORT 
to form the pallium, fig. 4 ; (2) externally to the fibres of the occipito-frontal tract, figs. 5 and 6 ; 
and (3) to fibres between the last two which plunge deeply to the base of the brain, figs. 5-10. 
The latter fibres appear to have an extended origin from the anterior portion of the brain ; they 
collect together as they pass downwards and backwards and join the other bundles formed by the 
strio-thalamic and occipito-mesencephalic fibres. It appears to be the most ventrally placed of the 
bundles, and with the strio-thalamic tract passes beneath the anterior commissure, fig. 40, Plate 
IX, into the thalamus, and proceeds as far as the mesencephalon. It is the longest of the three 
cerebral descending tracts. 
By the degeneration method, after removal of the anterior portion or of the whole of the 
hemisphere, it can be readily demonstrated as it passes through the thalamus to the mesencephalon. 
It terminates in cells in the substance of the optic lobes at a level a little above and more lateral 
than those of the oculomotor nucleus (figs. 29 and 32-39, Plate VIII). Closely associated with 
the tract, on its ventral aspect in the mesencephalon and thalamus, is a well-marked group of 
fibres, which are found to undergo ascending degeneration ; this tract will be described later on as 
the tractus mesencephalicus striatus (see figs. 39-42, Plates VIII and IX). 
ii. Tractus Strio-thalamicm. — This tract was first seen by Bumm and afterwards described 
by Edinger. It arises chiefly from the middle mass of the hemisphere, and is well seen in figs. 6 
and 7. It joins with the preceding tract, but remaining more dorsal, passes beneath the anterior 
commissure (fig. 40, Plate IX), and terminates in the nuclei of the thalamus (figs, 35 and 36, 
Plate VIII). It degenerates after removal of a hemisphere, and the course of its fibres can be well 
traced by using the Marchi method (figs. 32-38, Plate VIII). 
(c) Tractus Occipito-mesencephalicus. 
This tract, represented in the reptiles, is especially large in birds, aild can be traced from 
the posterior segment of the hemisphere (figs. 5-8, Plates IV and V), where it has a wide origin in 
close connection with that of the anterior commissure and occipito-frontal bundle. The tract 
passes inwards and backwards on a level with the anterior commissure, and sweeps into the 
thalamus where some of its fibres cease. 
Others may proceed as shown in figs. 17 and 19 E., Plate VI, into the mesencephalon. 
According to Edinger it ends in the roof of the middle brain, close to the termination of the optic 
nerve. It degenerates completely after removal of a hemisphere, and can be traced in transverse 
sections (figs. 33-35, Plate VIII). 
(d) Tractus Occipito-Frontalis (Bumm and Edinger). 
A large superficial associational tract, extending from the posterior to the anterior part of 
the brain, which is beautifully shown in horizontal section (figs. 5, 6, 7, Plate IV). As described 
above, it has an origin in the posterior lobe, closely associated with that of the anterior commissure 
and occipito-mesencephalic tract. The posterior lobe, which is exceedingly well developed in the 
pigeon, has thus an extensive connection with the frontal lobe, with the occipital lobe of the 
opposite side, and with the optic thalamus. 
