THE CEREBRUM 503 



end of the thalamus shows another prominence. This is 

 situated below and to the lateral side of the pulvinar. It is 

 oval in form, and receives the name of the corpus geniculatum 

 laterale. 



The medial surfaces of the two thalami are placed very 

 close together, and are covered not only with the lining 

 ependyma of the third ventricle, but also with a moderately 

 thick layer of grey matter continuous with the grey matter 

 which surrounds the aquaeductus cerebri (Sylvius). A band 

 of grey matter, termed the massa intermedia, crosses the third 

 ventricle and joins the two thalami together. 



Corpus Pineale. This is a small body of a darkish colour, 

 and about the size of a cherry-stone, which is placed between 

 the posterior extremities of the two thalami on the dorsal 

 aspect of the mesencephalon (Fig. 206). It occupies the 

 depression between the two superior quadrigeminal bodies, 

 and is shaped like a fir-cone. Its base, which is directed 

 anteriorly, is attached by means of a hollow stalk or peduncle. 

 This stalk is separated into a dorsal and a ventral part by a 

 continuation into it of a pointed recess of the cavity of the 

 third ventricle. The dorsal part of the stalk becomes con- 

 tinuous on each thalamus with the taenia thalami ; the ventral 

 part is folded round a narrow but conspicuous cord-like band 

 of white fibres (posterior commissure} which crosses the median 

 plane immediately below the base of the pineal body. 



Commissura Anterior Cerebri. In the anterior part 

 of the cleft between the two thalami, and immediately 

 anterior to the columns of the fornix, a round bundle of 

 white fibres will be seen crossing the median plane. This 

 is the anterior commissure. It is very much larger than the 

 posterior commissure, and will be afterwards followed towards 

 the temporal lobe, in which the greater part of it ends. 



Ventriculus Tertius. This name is given to the deep 

 narrow cleft between the two thalami. It is deeper anteriorly 

 than posteriorly, and extends from the pineal body posteriorly 

 to the anterior commissure and lamina terminalis anteriorly. 

 Its floor is formed by the parts already studied within the 

 interpeduncular fossa on the base of the brain, viz., the tuber 

 cinereum, the corpora mamillaria, and the grey matter of the 

 substantia perforata posterior, and also, more posteriorly, by the 

 tegmenta of the cerebral peduncles. Anteriorly, it is bounded 

 by the lamina terminalis and the anterior commissure ; whilst 



