THE BRAIN. 



281 



stem and beneath the cerebellum, to the central canal of the spinal cord 

 below. Both the roof and the floor of the irregular third ventricle are 

 thin, while its lateral walls are formed by two robust masses, the optic thai 'ami, 

 the mesial surface of one of which forms the background of the space when 

 viewed in sagittal section. The roof of the ventricle is very thin and consists 

 of the delicate layer of ependyma, as the immediate lining of the ventricular 

 spaces is designated, supported by the closely adherent fold of pia mater 

 which in this situation pushes before it the neural wall and contains within 

 its lateral border a thickened fringe of blood-vessels, the choroid plexus. 

 The two structures, the ependyma and the pia mater, together constitute 

 the membranous velum interpositum that forms the roof of the ventricle. 

 Behind, just over the upper end of the Sylvian aqueduct, lies the cone- 



Corpus callosum 

 Septum lucidui 



Optic thalamus, 

 dorsal surface 



Lateral wall of third 

 ventricle (optic 

 thalamus) 



Anterior commissure 



Foramen of Monro 



Lamina cinerea 



Optic commissure 



Cerebral peduncle 



Roof of Sylvian 

 aqueduct 



Floor of third ventricle 



Mammillary body 



Aqueduct of Sylvius 



Pons 



Fourth ventricle 



Occipital lobe 

 Superior medullary velum 



.White core of cerebellum 

 Inferior medullary velum 



^pinai cora i 



FIG. 328. Human brain seen in mesial sagittal section, showing relations of brain-stem, cerebrum and 



cerebellum and the ventricles. 



shaped pineal body. The floor of the third ventricle is also, for the most 

 part, thin and irregular. It corresponds to the median part of the lozenge- 

 shaped area, the interpeduncular space, bounded behind by the diverging 

 cerebral peduncles and in front by the optic chiasm and optic tracts. Passing 

 forwards from the deep recess between the cerebral peduncles, the paired 

 corpora mamillaria, the tuber cinereum and the stalk of the pituitary body 

 occupy the interpeduncular space. 



The Sylvian aqueduct, the narrow canal connecting the third and fourth 

 ventricles, is surrounded ventrally and laterally by the dorsal part or te^ men- 

 turn, of the cerebral peduncles. Above it lies a plate of some thickness, 

 whose free dorsal surface is modelled by two pairs of rounded elevations, the 

 corpora quadrigemina. The fourth ventricle in sagittal section appears as a 

 triangular space, the anterior or basal wall being the dorsal surface of the 

 pons and medulla and the posteriorly directed apex lying beneath the cere- 

 bellum. When viewed from behind, the ventricle is rhomboidal in outline, 

 the lateral boundaries above being the superior cerebellar peduncles, that 



