SUBTHALAMIC TEGMENTAL REGION. 505 
The red nucleus is a conspicuous object in sections through the lower part of this 
region. It presents the same appearance as lower down in the mesencephalon, and, 
gradually diminishing, it disappears before the level of the corpus mammullare is 
reached. Carried up around it are the same longitudinal tracts of fibres which 
have been studied in relation to it in the tegmental part of the mesencephalon. 
The fibres of the superior cerebellar peduncle, as they proceed upwards, form a coat- 
ing for it, which is distinctly thicker on its inner than on its outer surface. The 
mesial fillet, also, which in the upper part of the mesencephalon is observed to take 
up a position on the lateral and dorsal aspect of the red nucleus, maintains a 
similar position in, the subthalamic region. When the red nucleus comes to an 
end these various fibres are continued onwards and form, in the position previously 
occupied by the nucleus, a very evident and dense mass of fibres. The fibres of 
the mesial fillet and of the superior cerebellar peduncle are prolonged upwards into 
the ventral part of the thalamus, where a certain number of them end in connexion 
with the thalamic cells. The remainder apparently proceed directly through the 
ganglionic mass into the internal capsule, and thus gain the hinder part of the 
Rolandic area of the cerebral cortex. 
The substantia nigra is likewise carried into the subthalamic region, where it 
maintains its original position on the dorsal aspect of the crusta of the crus cerebri. 
As it is traced upwards, it is seen to gradually diminish in amount. It shrinks 
from within outwards, and finally disappears when the hinder part of the corpus 
mamunillare is reached. 
In coronal sections through the subthalamic region, the most conspicuous object 
which comes into view is the corpus subthalamicum or the nucleus of Luys. It isa 
smnall mass of gray 
matter,shaped like 
a biconvex lens, 
which makes its 
appearance on the 
dorsal aspect of the 
crusta of the crus 
cerebri immedi- 
ately to the outer 
Intersection of 
the corona 
radiata and 
callosal systems} 
of fibres / 
side of the sub- 
stantia nigra. At 
first it les in an 
angle, which is 
formed by the 
meeting of the 
crusta and the in- 
ternal capsule; 
but, rapidly en- 
larging in an in- 
ward direction, it 
takes the place of 
the diminishing 
substantia nigra 
oO 
on the dorsal sur- 
face of the crusta 
at the level of the 
lower part of the 
corpus mamumil- 
lare. The corpus 
subthalamicum is 
rendered all the 
Caudate nucleus 
\ 
External capsule 
Corpus callosum |: ° 
Fornix 
Anterior nucleus S< 
of thalamus 37 
Stria medullaris me 
ite eee 
Internal capsule 
Internal nucleus pes 
of thalamus [<* 
| 
\ Putamen 
| Fronto-parietal 
SS j operculum 
———— Claustrum 
Insula 
Externalnucleus | 
of thalamus | ‘| Temporal 
operculum 
Red nucleus ; 
Globus pallidus 
Nucleus of Luys ; ) 
c . . es ey 
Substantia nigra Sed Caudate nucleus 
Crusta of crus 
cerebri ~~ —~Optie tract 
R i 
~ Hippocampus 
major 
Fic. 375.—CORONAL SECTION THROUGH THE CEREBRUM OF AN ORANG PASSING 
THROUGH THE SUBTHALAMIC TEGMENTAL REGION. 
more evident by the fact that it is sharply defined by a thin capsule of white 
fibres. 
decussation across 
On its mesial aspect these fibres proceed inwards and form a very evident 
the middle line in the floor of the third ventricle, immediately 
above the hinder ends of the corpora mammillaria. 
