THE DIENCEPHALON. 



II 19 



ventricle, the inner border is sharply defined from the mesial surface by a delicate but 

 well defined ridge, taenia thalami, produced by the thickening of the ependyma 

 of the third ventricle, along its line of reflection onto the membranous roof, and 

 the underlying strand of nerve-fibres, the stria meduUaris. Traced backward, the 

 taenia thalami becomes continuous with the stalk of the pineal body. Between 

 this ridge and the diverging mesial border of the upper surface of the thalamus, is 

 included a narrow depressed triangular area, known as the trigonum habenulae. 

 It lies on a distinctly lower level than the adjoining convex upper surface of the 

 thalamus. Since it contains a special nucleus and belongs to the epithalamus, its 

 description will be deferred until that region is considered (page 1123). The upper 

 surface is not quite even, but subdivided by a shallow oblique furrow, which runs 

 from before backward and outward and marks the position of the overlying lateral 

 border of the fornix. External to this furrow lies a free marginal zone that forms a 

 part of the floor of the lateral ventricle ; internal to it is an attached inner zone over 

 which the velum interpositum is united to the thalamus. By the attachment of this 



Fig. 966. 



Septum lucidum 



Tsenia semicircularis and 

 vena terniinalis 



Tfenia chorioidea 



Furrow for fornix 



Tcenia thalami 



rrigonum habenulae 



Pulvinar 



Corpora quadrigeniina 



Corpus callosura 



Caudate nucleus 



Anterior pillars of fornix 

 Foramen of Monro 



Anterior commissure 



Middle commissure in III 

 tfentricle 



Thalamus 



Posterior commissure 

 Pineal body 



l.ingula 



Thalami, caudate nuclei and ventricles viewed from above after removal of corpus callosum, fornix and 

 velum interpositum ; third ventricle shows as narrow cleft between mesial surfaces of thalami. 



sheet to the fornix above and to the thalamus below, direct communication between 

 the third and lateral ventricles is shut off save through the foramen of Monro. In 

 front, the superior surface ends on the rounded elevation (tubercuhim anterius thalami) 

 which marks the anterior pole of the ganglion, while behind it goes over onto 

 the prominent posterior projection, the pulvinar, which overhangs the superior 

 brachium and the corpora geniculata. The mesial surface forms the greater part 

 of the lateral wall of the third ventricle. It is covered by a layer of gray matter 

 prolonged from the central gray of the Sylvian aqueduct, over which stretches the 

 immediate lining of the ventricle, the ependyma. The upper boundary of the mesial 

 surface is sharply defined by the taenia thalami, which behind is continuous with the 

 stalk of the pineal body (Fig. 966). Its lower limit is indicated by an oblique 

 furrow, the sulcus hypothalamicus, which separates the thalamic from the 

 hypothalamic regions. Somewhat in advance of their middle, the mesial surfaces 

 of the two thalami are connected by a bridge of gray matter, known as the 

 middle commissure (massa intermedia), usually about 7-8 mm. in diameter and 

 oval in section, but very variable in thickness and form. From the meagre number 

 of meduUated nerve-fibres that it contains, its importance, at least in man, seems 

 to be small. The lateral surface of the thalamus is inseparably blended with the 

 adjacent thick and conspicuous stratum of white matter, the internal capsule, 

 ■which intervenes between the thalamus and the more laterally placed lenticular 



