PAETS DERIVED FROM THE DIENCEPHALON. 611 



. presence of a subjacent longitudinal strand of fibres called the stria medullaris. 



, When these two structures, viz., the ependynial ridge and the subjacent stria, are 



, traced backwards, they are seen to turn medially and become continuous with 



i the stalk or peduncle of the pineal body. Behind the portion of the tsenia 



thalami which turns medially towards the pineal body a small depressed triangular 



area, the trigonum habenulae, situated in front of the superior colliculus, forms a very 



definite medial boundary for the posterior part of the superior surface of the thalaruus. 



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 cushion - 

 like 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 aquseductus 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 laminse, 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. 



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