THE CEREBRUM 447 



venes between the thalamus and the caudate nucleus and 

 contains the vena terminalis and the stria terminalis. On 

 the medial side, the superior surface of the thalamus is 

 separated from the medial surface, in its anterior half, by a 

 sharp edge, or prominent ledge, of the ependyma of the 

 third ventricle. The ledge is called the tcenia thalami. It 

 is produced by a longitudinal strand of fibres, called the stria 

 medullaris, which lies beneath the ependyma. A short distance 

 anterior to the pineal body the taenia lies upon the upper 

 border of a raised white band, called the habenula, which 

 is directly continuous with the stria medullaris. 



The habenula divides posteriorly into two parts, one of which becomes 

 associated with the cells of the grey matter of the trigonum habenulse of 

 the same side (see below] ; the fibres of the other part pass through the roof 

 of the third ventricle immediately in front .of the upper part of the stalk 

 of the pineal body. They go to the trigonum habenulse of the opposite 

 side and, together with their fellows of the opposite side, they form the 

 habenular commissure (Fig. 177). 



Between the habenula medially and the upper quadrigeminal body 

 posteriorly, lies a small triangular depressed area, the trigonum habenula. 



The superior surface of the thalamus is slightly convex, 

 and is of a whitish colour owing to the presence of a thin 

 superficial coating of nerve fibres (stratum zonale). It is 

 divided into two areas by a faint oblique groove which 

 begins near the anterior extremity of the thalamus, and ex- 

 tends obliquely, laterally and backwards. The sulcus corre- 

 sponds to the free edge of the fornix. The two areas thus 

 mapped out are very differently related to the ventricles of 

 the brain. The lateral area includes the anterior extremity 

 of the thalamus, and forms a part of the floor of the lateral 

 ventricle ; it is covered with ependyma, and overlapped by 

 the chorioid plexus. The medial area 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 

 with the tela chorioidea, above which is the fornix. It 

 includes the posterior extremity of the thalamus. 



The anterior extremity of the thalamus, called the anterior 

 tubercle, is rounded and prominent. It projects into the 

 lateral ventricle, lies postero - lateral to the corresponding 

 column of the fornix, and forms the posterior boundary of the 

 interventricular foramen. 



The posterior extremity of the thalamus is very prominent, 

 and it projects backwards over the mesencephalon (Fig. 176). 



