356 DK. G. ELLIOT SMITH ON 



In a mesial sagittal section of the brain the ventricles are opened up, but ^s their 

 features agree with those of the Eodentia, Carnivora, TJngulatn, and the main body of 

 Mammals, we merely need review them very briefly. 



The third ventricle is a vertical slit-like cavity between the optic thalami. These 

 bodies, however, meet in the mesial plane and fuse so as to form a large elliptical bridge 

 of grey substance — the commissura mollis— wh.\c\\ therefore causes tlie disappearance of 

 a large part of tlie third ventricle. In fact tlie third ventricle becomes reduced to a 

 narrow channel which surrounds the soft commissure (figs. 4. 17, and 18). The roof of 

 this ventricle is formed by a thin epithelial layer which extends from the upper part of 

 the psalterivm ventrale to the upper part of the pineal body. At the situation of its 

 attachment to the pineal body a little transverse strand of nerve-fibres is found crossing 

 the middle Hue in the roof of the pineal recess. This consists of fibres proceeding from 

 the gaiiglioH Jiahenulcr and tcenia thalami of each side to the ganglion Jiabenulce. of the 

 other side, and is hence known as the coinriiissura habenularum.. It is sometimes called 

 the commissura superior, but Ziehen has recently caused some confusion by calling the 

 dorsal commissure in Monotremes and Marsupials by this name*. 



Between the roof of the third ventricle and the posterior superior part of the soft 

 commissure the ganglion habenulae may lie seen in the lateral wall of the ventricle as a 

 peai'-shaped projection (figs. 1, 17, and IS). The anterior wall of the third ventricle is 

 formed by the lamina terminalis, which extends from the epithelial roof above to the 

 optic chiasma below, and contains the psalterium ventrale and the anterior commissure. 



Between the soft commissure and the intercommissural copula (/. e. the part of the 

 lamina tei*minalis [fig. 23, l.t'.'] which links the anterior commissure to the psalterium) 

 a small elliptical aperture will be found in the lateral wall of the ventricle. This is the 

 foramen of Monro, leading into the lateral ventricle. The floor of the ventricle is formed 

 by a thin lamina extending from the optic chiasma to the corpora mammMlaria. This 

 floor is drawn downward into a funnel-shape— the ivfimdibuJiim — and to its apex the 

 pituitary body is attached (figs. 4 and 18). 



The posterior walls of the third ventricle are formed by a very solid mass composed of 

 the anterior end of the mesencephalic tegmentum and the corpora iimmmillaria. The 

 anterior surface of this mass presents a regular curve which leads back to the floor of 

 a narrow channel opening out of the third ventricle. This is tlie aqueduct of Sylvius. 

 The roof of the anterior aperture of this channel is formed by a large mass of nerve- 

 fibres presenting a sickle-shape in section. This is the posterior commissure. This broad 

 funnel-shaped anterior opening rapidly contracts to a narrow canal, which extends back- 

 ward, with a slight obliquity upward, and tunnels the mesencephalon between the anterior 

 quadrigemiral bodies and the tegmentum, which forms its floor. As the aqueduct 

 approaches the posterior extremity of the anterior quadrigeminal bodies it gradually 

 expands not only in vertical extent, but even more markedly in the lateral direction, so 

 that an elongated funnel is produced, the wide mouth of which looks in the caudal 



* Ziehen, op cit. 



