i88 
THOMPSON YATES LABORATORIES REPORT 
ganglion isthmi, or 'ganglion opticum dorsale' (Jelgersma), by a needle passing through the 
neighbouring part of the cerebellum. He describes the resulting degeneration as existing in the 
thalamic region in an identical position to that noticed by us, but further states that the tract 
passes into the opposite optic nerve, and can be traced to end in the retina around the cells of the 
ganglionic layer. The same author has also observed a tract of fibres which degenerate towards 
the region of the ganglion isthmi after a lesion of the outer part of the cortex of the optic vesicle. 
This tract he names the tractus isthmo-tectalis. Hence we have a reflex arc, whose afferent 
system conducts from the complicated cortex of the optic lobe, and whose efferent trunk terminates 
in the retina. Wallenberg regards this system as of great importance, whose function is a 
regulation or accommodation of the retina. 
2. Tractus Mesencephalicus Stria tus. 
Series of transverse sections extending upwards from the mesencephalon always show, 
after lesion of the optic lobe, a well-marked band of degenerated fibres. These commence as a 
scattered system about the level of the Ilird nerves, and soon collect into a distinct strand which 
passes upwards into the thalamic region, where it occupies a position immediately ventral to the 
tr. striomesencephalicus. It can be traced to end in the striate region. Thus, between the corpus 
striatum and optic lobes there exist distinct ascending and descending tracts. 
MtiNZER and Weiner have arrived at the same conclusion ; they moreover state that the 
tr. mesencephalicus striatus is partly crossed, the crossed part deriving some of its fibres at least 
from the commissura inferior of Gudden. We have seen certain degenerate fibres in the com- 
missura inferior, but in our sections of the thalamic region after lesion of the optic lobe, Marchi 
preparations show degeneration only on the side of the operation. Wallenberg* has described, 
under the title tractus isthmo-striatus or bulbo-striatus, a tract which in the thalamic region 
certainly has an identical position to the tr. mesencephalicus striatus. According to this observer, 
it takes origin about the level of the ganglion isthmi, and in part crosses to a similar position on 
the opposite side ; from these two foci the tracts can be traced up to and into the ventral part of 
the corpus striatum. 
We do not think the evidence is sufficient to separate this tract of Wallenberg's from the 
tr. mesencephalicus striatus ; a crossing, such as the author describes, is always found after lesion of 
the optic lobe, but it may be part of the 'fountain decussation' characteristic of this region, and 
not a special decussation of an ascending system, whilst its course and termination again correspond 
to that of the mesencephalicus striatus. 
VII. Tracts of the Spinal Cord 
These are : — 
Ascending in the posterior columns as far as the upper extremity of the cord. 
In the lateral region, a very well marked direct cerebellar tract passing by the corpus 
restiformis into the cerebellum. 
