INTERNAL CAPSULE. 541 
insula or island of Reil. Followed in an upward direction, it becomes gradually 
thinner and ultimately disappears. As it is traced downwards, however, it 
thickens considerably, and at the base of the brain it comes to the surface at the 
anterior perforated spot and becomes continuous with the gray matter of the cortex. 
Its extent corresponds very closely with the area occupied by the insula, and its 
surface towards this portion of the cerebral cortex shows ridges and depressions 
corresponding to the insular gyri and sulci. 
Amygdaloid Nucleus—In the forepart of the temporal lobe, in front of, and 
to some extent above the extremity of the descending horn of the lateral ventricle, 
there is a round mass of gray matter, called the amygdaloid nucleus. The tail of 
the caudate nucleus joins its lower part, whilst above it is carried up into the puta- 
men. In front it is continuous with the gray cortex of the cerebrum. 
Tenia Semicircularis.—This is a band of fibres which, for the most part, arise 
in the amygdaloid nucleus. From this it runs backwards in the roof of the de- 
scending horn of the lateral ventricle, and then arches upwards and forwards, so as 
to gain the floor of the body of the lateral ventricle. In both situations it lies 
close to the inner side of the nucleus caudatus, and finally, at the foramen of 
Monro, it bends downwards towards the anterior commissure. Some of its fibres 
pass in front and others behind the commissure, and ultimately they end in the 
locus perforatus anticus (Kolliker). 
Internal Capsule—This term is applied to the broad band of white matter 
which intervenes between the lenticular nucleus, on the outside, and the optic 
thalamus, teenia semicircularis, and caudate nucleus on the inner side. It presents 
many different appearances, according to the 
plane in which the brain is cut. In the 
region of the mesencephalon, a coronal section 
through the brain shows that in great part 
the internal capsule is directly continuous 
with the crusta of the crus cerebri (Fig. 375, 
p- 505). In horizontal section the internal 
capsule is observed to be bent upon itself 
opposite the tenia semicircularis, or the 
interval between the caudate nucleus and 
the thalamus. This bend,,which points in- 
wards, is called the genu. About one-third 
of the internal capsule les in front of the 
genu, and is termed the anterior hmb; the 
remaining two-thirds, which lhe behind the 
genu, constitute the posterior limb. 
The anterior limb of the internal capsule 
intervenes between the lenticular nucleus and 
the caudate nucleus. In its lower and fore- 
part it is much broken up by the connecting 
AUDITORY RAD" 
TEMPORO-PONTINE 
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bands of gray matter which pass between the ss Bales 
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forepart of the putamen and the lenticular Vv ¢ 
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The anterior limb of the internal capsule is, SS 
for the most part, composed of corticipetal fibres 2 
Fic. 404.—DIAGRAMMATIC REPRESENTATION OF 
THE INTERNAL CAPSULE (as seen in hori- 
zontal section). 
belonging to the thalamic radiation. It likewise 
contains a distinct corticifugal tract. The corti- 
cipetal fibres are of two kinds, viz.: thalamo- 
frontal and thalamo-striate. The former, which arise in the optic thalamus, go through 
the anterior limb of the internal capsule to reach the cortex of the frontal lobe. The 
thalamo-striate fibres likewise arise in the thalamus and enter the anterior limb, to reach 
the caudate and lenticular nuclei. 
The corticifugal fibres form a tract which has already been referred to in connexion 
with the mesencephalon. It is the fronto-pontine tract, which arises in the cortex of the 
prefrontal region, traverses the anterior limb of the internal capsule, forms the inner fifth 
of the crusta of the crus cerebri, and finally ends in the nucleus pontis. 
