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presence of a subjacent longitudinal strand of fibres called the stria medullaris. When these two structures, viz., the ependymal ridge and the subjacent stria, are traced backwards, they are seen to turn medially and become continuous with the stalk or peduncle of the pineal body. Behind the portion of the tænia thalami which turns medially towards the pineal body a small depressed triangular area, the trigonum habenula, situated in front of the superior colliculus, forms a very definite medial boundary for the posterior part of the superior surface of the thalamus.

The superior surface of the thalamus is slightly bulging or convex, and is of a whitish colour, owing to the presence of a thin superficial covering of nerve-fibres, termed the stratum zonale. It is divided into two areas by a faint oblique groove, which begins in front at the medial border, a short distance behind the anterior extremity of the thalamus, and extends laterally and backwards to the lateral part of the posterior extremity. This groove corresponds to the edge of the fornix. The two areas which are thus mapped out are very differently related to the ventricles of the brain, and also to the parts which lie above the thalamus. The lateral area, which includes the anterior extremity of the thalamus, forms a part of the floor of the lateral ventricle. It is covered with ependyma, overlapped by the chorioid plexus of this ventricle, and lies immediately subjacent to the corpus callosum. Along the line of the groove the epithelial lining of the lateral ventricle is reflected over the chorioid plexus of this cavity. The medial area, which includes the posterior end of the thalamus, intervenes between the lateral and third ventricles of the brain, and takes no part in the formation of the walls of either. It is covered by a fold of pia mater, termed the tela chorioidea of the third ventricle, above which is the fornix, and these two structures intervene between the thalamus and the corpus callosum.

The anterior extremity of the thalamus, called the tuberculum anterius thalami, forms a marked bulging. It projects into the lateral ventricle, behind and to the lateral side of the free portion of the column of the fornix. The foramen interventriculare, a narrow aperture of communication between the lateral and third ventricles of the brain, is bounded in front by the column of the fornix and behind by the anterior tubercle of the thalamus.

The posterior extremity of the thalamus is very prominent and forms a cushionlike projection, which overhangs the brachia of the corpora quadrigemina. This prominence is called the pulvinar. Another oval bulging on the posterior part of the thalamus receives the name of the corpus geniculatum laterale. It is situated below, and to the lateral side of, the pulvinar, and presents a very intimate connexion with the optic tract.

The medial surfaces of the two thalami are placed close together, and are covered not only by the lining ependyma of the third ventricle, but also by a moderately thick layer of gray matter, continuous below with the central gray substance which surrounds the aquæductus cerebri in the mesencephalon. A band of gray matter, termed the massa intermedia, crosses the third ventricle and joins the medial surfaces of the two thalami together.

Intimate Structure and Connexions of the Thalamus.-The upper surface of the thalamus is covered by the stratum zonale, a thin coating of white fibres derived to some extent from the optic tract, and probably also from the optic radiation. The medial surface has a thick coating of central gray matter, whilst intervening between the internal capsule and the lateral surface is the lamina medullaris externa. The lower surface merges into the hypothalamus.

The gray matter of the thalamus is marked off into three very apparent parts -termed the anterior, the medial, and the lateral thalamic nuclei-by a thin vertical sheet of white matter, continuous with the stratum zonale, termed the lamina medullaris interna. The lateral nucleus (nucleus lateralis thalami) is by far the largest of the three. It is placed between the medial and the lateral medullary laminæ, and it stretches backwards beyond the medial nucleus, and thus includes the whole of the pulvinar (Fig. 541). The medial nucleus (nucleus medialis thalami) reaches only as far back as the habenular region. It is placed between the central gray matter of the third ventricle and the internal medullary lamina. The lateral nucleus is more extensively pervaded by fibres than the medial nucleus.

(Fig. 538). It is a large ovoid mass of gray matter, which lies obliquely across the path of the cerebral peduncle as it descends from the cerebral hemisphere. The smaller anterior end of the thalamus lies close to the median plane, and is separated from the corresponding part of the opposite side only by a very narrow interval. The enlarged posterior ends of the two thalami are placed more widely apart, and in the interval between them the corpora quadrigemina are situated.

The two thalami, in their anterior two-thirds, lie close together, one on each side of the deep median cleft which receives the name of the third ventricle of the brain. The inferior and lateral aspects are in apposition with, and, indeed, directly connected with, adjacent parts of the brain, and on this account it is customary to study them by means of sections through the brain. The superior and medial surfaces are free.

The lateral surface of the thalamus is applied to a thick layer of white matter

interposed between it and the lentiform nucleus, called the internal capsule, and composed of fibres passing both upwards towards and downwards from the cerebral cortex. A large proporAnterior commissure tion of these fibres

Fornix

Foramen inter-
ventriculare

Anterior tubercle
of thalamus
-Massa intermedia

descend to form
the basis pedun-
culi. From the
entire extent of
the lateral sur-

-Third ventricle
-Stria terminalis
-Tania thalami

Trigonum habenulæ face of the thala

Posterior
commissure

mus large num

Stalk of pineal body bers of fibres

Pulvinar

Pineal body

[graphic]

Non-ventricular part of thalamus

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presence of a subjacent longitudinal strand of fibres called the stria medullaris. When these two structures, viz., the ependymal ridge and the subjacent stria, are traced backwards, they are seen to turn medially and become continuous with the stalk or peduncle of the pineal body. Behind the portion of the tænia thalami which turns medially towards the pineal body a small depressed triangular area, the trigonum habenula, situated in front of the superior colliculus, forms a very definite medial boundary for the posterior part of the superior surface of the thalamus.

The superior surface of the thalamus is slightly bulging or convex, and is of a whitish colour, owing to the presence of a thin superficial covering of nerve-fibres, termed the stratum zonale. Ît is divided into two areas by a faint oblique groove, which begins in front at the medial border, a short distance behind the anterior extremity of the thalamus, and extends laterally and backwards to the lateral part of the posterior extremity. This groove corresponds to the edge of the fornix. The two areas which are thus mapped out are very differently related to the ventricles of the brain, and also to the parts which lie above the thalamus. The lateral area, which includes the anterior extremity of the thalamus, forms a part of the floor of the lateral ventricle. It is covered with ependyma, overlapped by the chorioid plexus of this ventricle, and lies immediately subjacent to the corpus callosum. Along the line of the groove the epithelial lining of the lateral ventricle is reflected over the chorioid plexus of this cavity. The medial area, which includes the posterior end of the thalamus, intervenes between the lateral and third ventricles of the brain, and takes no part in the formation of the walls of either. It is covered by a fold of pia mater, termed the tela chorioidea of the third ventricle, above which is the fornix, and these two structures intervene between the thalamus and the corpus callosum.

The anterior extremity of the thalamus, called the tuberculum anterius thalami, forms a marked bulging. It projects into the lateral ventricle, behind and to the lateral side of the free portion of the column of the fornix. The foramen interventriculare, a narrow aperture of communication between the lateral and third ventricles of the brain, is bounded in front by the column of the fornix and behind by the anterior tubercle of the thalamus.

The posterior extremity of the thalamus is very prominent and forms a cushionlike projection, which overhangs the brachia of the corpora quadrigemina. This prominence is called the pulvinar. Another oval bulging on the posterior part of the thalamus receives the name of the corpus geniculatum laterale. It is situated below, and to the lateral side of, the pulvinar, and presents a very intimate connexion with the optic tract.

The medial surfaces of the two thalami are placed close together, and are covered not only by the lining ependyma of the third ventricle, but also by a moderately thick layer of gray matter, continuous below with the central gray substance which surrounds the aquæductus cerebri in the mesencephalon. A band of gray matter, termed the massa intermedia, crosses the third ventricle and joins the medial surfaces of the two thalami together.

Intimate Struct and the **-lamus

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Connexions of the Thalamus.-The upper surface by the stratum zonale, a thin coating of white from the optic tract, and probably also from the surface has a thick coating of central gray matter, internal capsule and the lateral surface is the lamina face merges into the hypothalamus.

is marked off into three very apparent parts and the lateral thalamic nuclei—by a thin

ous with the stratum zonale, termed the nucleus nucleus lateralis thalami) is b ed between the medial and the vards bevond the medial nucleus. 41). The medial nucleus 'nucleu e habenular region. It is place ntricle and the internal medullar pervaded by fibres than the medial

From the lateral nucleus by far the greatest number of the fibres which form the radiatio thalami pass, and these are seen crossing it in various directions towards the lamina medullaris externa. The anterior nucleus (nucleus anterior thalami) is the smallest of the three thalamic nuclei. It forms the prominent anterior tubercle, and is prolonged in a wedge-shaped manner, for a short distance, downwards and backwards between the anterior parts of the medial and lateral nuclei. The internal medullary lamina splits into two parts and partially encloses the anterior nucleus. In connexion with its large cells a very conspicuous bundle of fibres, the fasciculus thalamomamillaris, comes to an end. [As this bundle arises in the corpus mamillare, it ought to be called "fasciculus mamillo-thalamicus."]

A diffuse gray mass, imperfectly marked off from the inferior surface of the lateral nucleus, receives the name of the ventral nucleus. Its inferior part is composed of the central nucleus of Luys and the nucleus arcuatus. In section the former appears as a circular mass of gray matter, which comes into view immediately behind the point where the internal medullary lamina disappears. It would seem to be intimately connected with fibres which reach it from the red nucleus and from the posterior commissure. These fibres pass round it so as to mark it off from the rest of the thalamus, and in front of the nucleus many of them enter the internal medullary lamina. The nucleus arcuatus is a small semilunar mass of gray matter placed below and to the lateral side of the central nucleus of Luys.

The connexions of the thalamus are of an extremely intricate kind. It would appear to be a ganglionic mass interposed between the tegmental corticipetal tracts and the cerebral cortex. In its posterior part, and through its stratum zonale, it also has important connexions with the optic tract. The corticipetal tegmental tracts, which enter it from below, will be noticed in connexion with the hypothalamic

region. Suffice it to say, for the present, that these fibres end in the midst of the thalamus in connexion with the thalamic cells. In addition to these, enormous numbers of fibres, arising within the thalamus as the axons of its cells, stream out from its lateral and inferior surfaces to form the thalamic radiation. These thalamo-cortical fibres pass to every part of the cortex; and although there is no separation of them into distinct groups as they leave the thalamus, it is customary to regard them as constituting a frontal stalk, a parietal stalk, an occipital stalk, and a ventral stalk. But fibres from the cortex, cortico-thalamic fibres, likewise stream into the thalamus in large numbers, and end in fine arborisations around its cells. A double connexion with the cerebral cortex is thus established by the thalamus.

LOBE

TEMPORAL

FRONTAL

LOBE

THALAMO-FRONTAL

FIBRES

OPTIC RADIATIONS

AUDITO

RAD

احد

OCCIPITAL

FIG. 540.-SCHEMA.

CORP. CALLOSUM

LOBE

--ANTA LIMB
INT: CAPSULE

POST LIMB
INT CAPSULE

CORP GEN: MED

SUP QUAD BODY

The frontal stalk of the thalamic radiation emerges from the anterior part of the lateral surface of the thalamus and passes through the anterior limb of the internal capsule, to reach the cortex of the frontal lobe. Many of these fibres end in the caudate and lentiform nuclei, between which they proceed. The parietal stalk issues from the lateral surface of the thalamus, and, passing through the internal capsule (and to some extent, also, through the lentiform nucleus and the external capsule), gains the cortex of the posterior part of the frontal lobe and of the parietal lobe. The occipital stalk emerges from the lateral aspect of the pulvinar and constitutes the so-called optic radiation. These fibres sweep laterally and backwards round the lateral side of the posterior horn of the lateral ventricle to gain the cortex of the occipital lobe. The

ME ONCEPHALON

TEMPORO-PONTINE
TRACT

Founded on the observations of
Flechsig, and Ferrier and Turner.

ventral stalk streams out from the under aspect of the anterior part of the thalamus, in front of the hypothalamic tegmental region and the corpus mamillare. Its fibres arise in both the medial and lateral nuclei, and sweep downwards and laterally to reach the region below the lentiform nucleus. One very distinct band which lies dorsal to the other fibres (ansa lenticularis) comes from the lentiform nucleus to the thalamus, whilst the remainder (ansa peduncularis) proceed in a lateral direction from the thalamus below the lentiform nucleus and gain the cortex of the temporal lobe and of the insula.

Flechsig divides the thalamo-cortical fibres of ordinary sensation into three sensory systems. These he has been able to distinguish by studying the order in which they assume their sheaths of myelin in the foetus and infant.

Ferrier and Turner, by the degenerative method of investigation, corroborate Flechsig's results. They confirm the observation of Flechsig that, while thalamic fibres are distributed to the several regions of the cerebral cortex to an almost equal extent, there is one district, viz., the frontal pole, to which the supply is scanty. Another very important result has been obtained by these authors. They have established the fact that many of the thalamic fibres cross the median plane in the corpus callosum, and thus gain the cortex of the opposite cerebral hemisphere. Hamilton's crossed callosal tract thus receives confirmation.

Intimate Structure of the Corpus Geniculatum Laterale.-Sections through the lateral geniculate body reveal the fact that it is composed of a series of alternately placed gray and white curved laminæ. This gives it a very characteristic appearance. The white laminæ are composed of fibres which enter the body from the optic tract. The connexions of the geniculate bodies

[graphic]

Caudate nucleus

Corpus callosum

Fornix

will be studied Anterior nucleus
of thalamus
with the optic
tract.

Stria medullaris

Internal capsule
Medial nuclens
of thalamus
Lateral nucleus
of thalamus

External capsule

Red nucleus

Nucleus hypo-
thalamicus

Substantia nigra

Hypothala-
mic Region.
The tegmental
part of the pedun-vedio

culus cerebri is
prolonged up-
wards and assumes
a position below
the posterior part Basis pedunculi

da

of the thalamus. The red nucleus is a conspicuous object in sections through the lower part of this region (Fig. 541). It presents the same appearance as lower down in the mesencephalon, and, gradually diminishing, it disappears before the level of the corpus mamillare 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. Certain of these fibres, placed in immediate relation to the red nucleus, form a coating or capsule for it. This coating is partly derived from those fibres of the brachium conjunctivum which pass directly up into the thalamus and also partly from fibres which issue from the nucleus itself. The medial lemniscus also, which in the superior 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 hypothalamic region. When the red nucleus comes to an end these various fibres are continued onwards and form, in the position previously occupied

Claustrum

Temporal operculum

Globus pallidus

Caudate nucleus

FIG. 541.-FRONTAL SECTION THROUGH THE CEREBRUM OF AN ORANG PASSING
THROUGH THE HYPOTHALAMIC TEGMENTAL REGION.

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