OPTIC THALAMUS. 503 
over the choroid plexus of this cavity. The inner area, which includes the hinder 
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 velum interpositum, 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 anterior tubercle (tuberculum 
anterius thalami), forms a marked bulging. It projects into the lateral ventricle, 
behind and to the outer side of the free portion of the anterior pillar of the fornix. 
The foramen of Monro, a narrow aperture of communication between the lateral 
and third ventricles of the brain, is bounded in front by the anterior pillar of the 
fornix and behind by the anterior tubercle of the thalamus. 
The posterior extremity of the thalamus is very prominent and forms a cushion- 
like projection, which overhangs the brachia of the corpora quadrigemina. This 
prominence is called the pulvinar. Another oval bulging on the hinder part of the 
thalamus receives the name of the corpus geniculatum externum. It is situated 
below, and to the outer side of, the pulvinar, and presents a very intimate connexion 
with the optic tract. 
The mesial surfaces of the two thalami are placed very close together, and are 
covered not only by the lining-ependyma of the third ventricle, but also by a 
tolerably thick layer of gray matter, continuous below with the central gray 
substance which surrounds the aqueduct of Sylvius in the mesencephalon. A band 
of gray matter, termed the gray or soft commissure (commissura mollis), crosses the 
third ventricle and joins the inner surfaces of the two thalami together. 
Intimate Structure and Connexions of the Optic 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 inner surface has a thick coating of central gray matter, whilst 
intervening between the internal capsule and the outer surface is the lamina 
medullaris externa. The lower surface merges into the subthalamic region. 
The gray matter of the optic thalamus is marked off into three very apparent 
parts—termed the anterior, the mesial, and the lateral thalamic nuclei—by a thin 
vertical sheet of white matter, termed the lamina medullaris interna. ‘The lateral 
nucleus (nucleus lateralis thalami) is by far the largest of the three. It is placed 
between the internal and the external medullary laminz, and it stretches back- 
wards beyond the mesial nucleus, and thus includes the whole of the pulvinar (Fig. 
375). The mesial nucleus (nucleus medialis thalami) only reaches as far back as the 
habenular region. It is placed between the central gray matter of the third ven- 
tricle and the internal medullary lamina. The lateral nucleus is more extensively 
pervaded by fibres than the mesial nucleus. 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 mesial 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 bundle of Vicq d’Azyr (fasciculus 
thalamo-mammillaris), which arises in the corpus mammillare, comes to an end. 
Two other small nuclear masses are found in the substance of the optic thalamus behind the 
mesial nucleus. These are called 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 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 hinder part, and through its stratum zonale, it also 
