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les external and internal medullary laminæ, are now evident, which traverse its substance in a vertical direction and divide it into three masses. The lateral, basal,
and larger mass is termed the putamen; the two medial portions together constitute the globus pallidus.
The putamen forms much the largest part of the lentiform nucleus. It is darker in colour than the globus pallidus, and in this respect resembles the caudate nucleus. It is traversed by fine radiating bundles of fibres, which enter it from the external medul
posterior length, Septum pellucidum
as well as the
vertical depth of
the putamen, is
than in the case
of the globus
pallidus; conse- FIG. 568.-FRONTAL SECTION THROUGH THE CEREBRAL HEMISPHERES So as to cut
quently, in both
frontal and hori
through the anterior part (putamen) of the lentiform nucleus in front of the globus pallidus. Viewed from in front; looking through the anterior horn into the central part of the ventricle.
through the cerebrum it is encountered before the plane of the globus pallidus is reached.
The external capsule is loosely connected with the lateral surface of the putamen, and it can be readily stripped off. This accounts for the tendency, exhibited in hæmorrhages in this locality, for the effused blood to spread out in the interval between these structures.
The globus pallidus is composed of the two smaller and medial masses of the lentiform nucleus. They present a faint yellowish tint, and are paler and more abundantly traversed by fibres than the putamen. The mass next the putamen. (ie. the intermediate part) is much larger than the medial subdivision. It extends forwards to a point a little in front of the plane of the anterior commissure. When the lentiform nucleus is cut in a frontal direction, and in its widest part, the medial mass shows an indication of a separation into two parts, so that here the globus pallidus appears to consist of three subdivisions.
Connexions of the Corpus Striatum.-Recent clinical investigation has demonstrated the importance of the functions of the corpus striatum, which seems to exercise a "steadying influence" (Kinnier Wilson) upon the muscles which perform voluntary movements that call for delicate co-ordination. Hence it is desirable to study the connexions of these large masses of gray matter. Fibres of the internal capsule coming from the motor cortex (as well as from all other cortical areas) end in the corpus striatum (Fig. 571), so that when a voluntary movement is initiated this structure is called into activity. Fibres coming from the nucleus caudatus break through the anterior limb of the internal capsule (Fig. 572), some of them to reach the putamen, others to pass through the external medullary lamina to the globus pallidus. Other tracts pass from
FIG. 569.-FRONTAL, SECTION THROUGH THE CEREBRUM so as to cut through the three divisions of the lentiform nucleus; posterior surface of the section shown here.
FIG. 570.-FRONTAL SECTION THROUGH THE LEFT SIDE OF THE CEREBRUM OF AN ORANG
The section passes through the middle of the lentiform nucleus.
the lentiform nucleus into the caudate nucleus (fibræ lenticulocaudata). From the globus pallidus fibres arise which proceed into the internal capsule in the region of the genu and the neighbouring part
of the posterior limb (Fig. 572). Many of these fibres become collected on the inferior aspect of the lentiform nucleus, where they form a transversely directed bundle (Fig. 570), known as the ansa lenticularis, which is distributed to the thalamus (Fig. 571, fasciculus striothalamicus) and hypothalamus, the red nucleus (fasciculus striorubricus) and substantia niger (fasciculus strionigricus). These connexions afford some explanation of the difficulties of articulation and swallowing and in the performance of delicate voluntary movements that result from damage to the corpus striatum or to this system of fibres.
This system of fibres is phylogenetically very old, being the
most primitive efferent tract from the cerebral hemisphere.
Claustrum. This is a thin plate of gray substance embedded in the white matter which intervenes between the lentiform nucleus and the gray cortex of the insula. Followed in an upward direction, it becomes gradually thinner and ultimately disappears. As it is traced downwards, however, it thickens consider
Thalamo-cerebral tract to
Thalamo-cerebral tract to suprs-
closely with the area occupied by the insula, and its surface towards this portion of the lenticularis. cerebral cortex shows ridges and depressions corresponding to the
Auditory radiation. insular gyri and sulci.
Nucleus Amygdalæ.—In the anterior
-Opric radiation. part of the temporal
FIG. 572.-DIAGRAMMATIC REPRESENTATION OF THE INTERNAL CAPSULE
(AS SEEN IN HORIZONTAL SECTION).
region, above the piriform area a fusiform mass of gray matter appears upon the surface (Fig. 558, p. 630), at the lateral extremity of the substantia perforata anterior (Fig. 584, p. 657). It is part of a large rounded mass, called the amygdaloid nucleus, which occupies a position in front of, and to some extent above the extremity of the inferior horn of the lateral ventricle. The tail of the caudate nucleus joins its inferior part (Fig. 573, p. 643), whilst above it is carried up into the putamen (Fig. 570).
Inferiorly it is continuous with the gray cortex of the piriform area, to which it is functionally related, probably in the same way that the major part of the corpus striatum is associated with the neopallium.
Stria Terminalis.—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 inferior horn of the lateral ventricle (Fig. 584, p. 657, and Fig. 573, p. 643), and then arches upwards and forwards, so as to gain the floor of the pars centralis of the lateral ventricle. In both situations it lies close to the medial side of the nucleus caudatus, and finally, at the interventricular foramen, 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 neighbourhood of the substantia perforata anterior (Kölliker).
Internal Capsule. This term is applied to the broad band of white matter which intervenes between the lentiform nucleus, on the lateral side, and the thalamus and caudate nucleus on the medial side. It presents many different appearances, according to the plane in which the brain is cut. A frontal section through the brain which passes through the cerebral peduncles shows that, in great part, the internal capsule is directly continuous with the basal part of the cerebral peduncle (Fig. 580, p. 652). Viewed from the lateral aspect after removing all else of the cerebral hemisphere excepting the corpus striatum (Fig. 573), the cut ends of the fasciculi of the internal capsule form three-fourths of an ellipse, the other fourth of which is occupied by the bridge of union between the lentiform and caudate. nuclei, the substantia perforata anterior, the amygdaloid nucleus and the anterior commissure. It may be divided into an anterior (lenticulo-caudate) part, a superior (lenticulo-thalamic) part, a retrolenticular part (not labelled in the figure), and a postero-inferior (sublenticular) part. The last three parts are usually grouped together as the posterior limb. In horizontal section the internal capsule observed to be bent upon itself opposite the stria terminalis, or the interval between the caudate nucleus and the thalamus. This bend, which points medially, is called the genu. About one-third of the internal capsule lies in front of the genu, and is termed the anterior limb; the remaining two-thirds, which lie behind. the genu, constitute the posterior limb (Fig. 572).
The anterior limb of the internal capsule intervenes between the lentiform nucleus and the caudate nucleus. In its inferior and anterior part it is much broken up by the connecting bands of gray matter which pass between the anterior part of the putamen and the caudate nucleus.
The anterior limb of the internal capsule is composed largely of corticipetal fibres belonging to the anterior thalamic radiation. It contains corticifugal fibres also. The corticipetal fibres arise in the median and anterior part of the lateral nucleus of the thalamus, and go through the anterior limb of the internal capsule to reach the cortex of the frontal lobe.
The corticifugal fibres are represented by the fronto-pontine tract.
The fronto-pontine tract arises in the cortex of the frontal region, traverses the anterior limb of the internal capsule, forms the medial fifth of the basis of the cerebral peduncle, and finally ends in the nuclei pontis.
The posterior limb of the internal capsule is placed between the thalamus and the lentiform nucleus, and it extends backwards for a short distance beyond the posterior end of the putamen on the lateral side of the posterior part of the thalamus and of the tail of the caudate nucleus. The posterior limb, therefore, is spoken of as consisting of a lenticular, a retrolenticular, and a sublenticular part.
The lenticular or more properly lenticulo-thalamic part of the posterior limb is composed of both corticipetal and corticifugal fibres. The corticipetal fibres enter the internal capsule from the lateral aspect of the thalamus, and are composed of fibres which arise within the thalamus from the ventral (ventro-lateral) nucleus, and proceed upwards to the
The corticifugal fibres consist of the cerebro-spinal tract and the cortico-thalamic fibres. The great motor or cerebro-spinal tract, descending from the cerebral cortex, occupies the anterior half of the lenticular part of the internal capsule. The fibres, that go
to the nuclei of the oculomotor, trigeminal, and facial nerves, lie close to the genu, and the behind these are the fibres which go to the hypoglossal nucleus; still further back are cerebro-spinal fibres which enter the spinal medulla and end around the motor cells of the anterior column of gray matter. This cerebro-spinal tract has been observed occupying the middle part of the pedunculus cerebri, into which it passes directly from the internal capsule.
According to Monakow the posterior limb contains also an important tract of fibres passing from the motor cortex to the red nucleus (fasciculus cerebrorubricus).
The retrolenticular part of the posterior limb contains: (1) the fibres of the optic radiation as they pass from the lateral geniculate body to establish their connexions with
the occipital cerebral cortex; (2) the fibres of the acoustic radiation, or those which connect the medial geniculate body with the acoustic cortical field in the temporal lobe (Fig. 572, p. 641, and Fig. 578, p. 650); (3) the temporo-pontine tract, which is composed of fibres which take origin in the middle and inferior gyri of the temporal lobe and pass through the sublenticular section of the internal capsule to reach the lateral part of the pedunculus cerebri. Through this they reach the basilar part of the pons, in the gray matter of which they end. This tract is accompanied by the fasciculus temporothalamicus, which has a widespread origin from the temporal and occipital regions. and passes through the sublenticular part of the internal capsule.
If the fibres of the internal capsule are traced upwards they are found to spread out widely from each other in a radiating or fan-shaped manner, as they