THE THIRD VENTRICLE AND INTER-BRAIN. 139 



uncus and amygdala. The pars occipito-temporalis connects 

 the tentorial areas of the two hemispheres together, regions which 

 are not connected by the corpus callosum. In man it is larger 

 than the pars olfactoria. A thin transverse sheet of gray matter, 

 called the lamina terminalis, extends downward and forward 

 from the anterior commissure to the optic chiasma and completes 

 the anterior wall of the ventricle (Figs. 17, 28 and 73). Between 

 the chiasma and the lamina terminalis is a sharp angle which 

 terminates on either side in a small pit, called the optic recess. 



The floor of the third ventricle is very narrow (Figs. 21 and 

 27). It is formed by the interpeduncular structures plus the 

 tegmenta, namely: optic chiasma, tuber cinereum and infundibu- 

 lum, corpora mammillaria, posterior perforated substance and 

 the tegmenta. The last two are portions of the mid-brain; the 

 others belong to the fore-brain with the surface of which we have 

 already studied them, and all extend laterally beneath the thalami. 



The third ventricle has its lateral wall formed chiefly by the 

 thalamus and the columna of the fornix (Figs. 27 and 28). Below 

 a slight longitudinal groove, extending from the optic recess 

 to the cerebral aqueduct and called the sulcus hypothalamicus, 

 the thalamus is covered by upturned hypothalamic gray matter 

 and by the upper part of the central gray substance of the mid- 

 brain. The thalamus forms the immediate lateral wall above 

 this hypothalamic groove. The columna of the fornix, diverging 

 from its fellow, proceeds downward and backward to the corpus 

 mammillare through the medial part of the thalamus. In the 

 ventricle, the pars libera of the columna fornicis is covered by 

 the ependymal epithelium. It bounds the interventricular fora- 

 men in front. 



Thalamus. (Thalamus a bed, Figs. 40, 42, 43 and 44). It is 

 the great ganglion of the inter-brain. The thalamus is an impor- 

 tant sensory relay station. In it or in the hypothalamic nuclei 

 almost every impulse of general sensation, in its journey to the 

 cerebral cortex, is transferred to a higher neurone. The third 

 ventricle separates the thalami from each other, except at the 

 mid-point where they are joined by the massa intermedia. The 

 thalamus is situated behind and medial to the corpus striatum, 



