THE STRUCTURE OF THE BRAIN STEM 



part of the optic thalamus, in which they probably end. It is possible that a 

 certain proportion pass through the optic thalamus and run straight to the 

 cerebral cortex of the Rolandic area. The lateral fillet has disappeared fin.,, 

 the region of the tegmentum and passed into the inferior corpora quadri- 

 gemina. The mesial fillet forms a flat band lying to the outer side of the red 

 nucleus and comes into close relation with a ganglion of the fore-brain, known 

 as the internal geniculate body. The roof of the mid-brain is formed by the 

 corpora quadrigemina. The inferior corpora quadrigemina are composed of 

 central grey matter encapsulated by white matter, derived chiefly from 

 the lateral fillet. The superior corpora quadrigemina are composed of several 

 layers of grey matter traversed by nerve fibres, derived partly from the 

 fillet, partly from the optic tract, and partly from the occipital lobe of the 

 cerebral hemisphere. 



THE FORE-BRAIN 



In the fore-brain the most important feature is the optic thalami, the 

 two head ganglionic masses of the brain stem (Fig. 189). In this region 

 the central neural canal, which .in the mid-brain forms the Sylvian iter, 

 widens out to the third ventricle, in the lateral walls of which are developed 

 the two optic thalami. It is a narrow cleft, rapidly deepening in depth from 

 behind forwards. As we trace sections forwards we see that the two crura 

 cerebri diverge from one another. The floor of the third ventricle is thus left 

 thin. It is formed from behind forwards by a thin layer of grey matter with 

 numerous vessels, the locus perfomtus posticus, two small eminences, the 

 corpora mammillaria, and in front of these another lamina of grey matter 

 known as the tuber cinereum. In front of the tuber cinereum is the infun- 

 dibulum, which leads to the posterior lobe of the pituitary body. In front 

 of the infundibulum the optic chiasma is closely attached to the lowest part 

 of the anterior wall of the ventricle. The front wall is formed by a thin layer 

 of nervous matter, the lamina cinerea, at the upper border of which, project- 

 ing slightly into the ventricle, is a strand of white fibres connecting the an- 

 terior parts of the two optic thalami and known as the anterior commissure. 

 The roof of the third ventricle is formed entirely of epithelium, the ependyma, 

 along the upper surface of which is the layer of pia mater, the velum inter- 

 positum. The roof is invaginated into the cavity by two delicate vascular 

 fringes, the choroid plexuses. At the back part of the roof is attached the 

 stalk of the pineal body, and behind this stalk, between the anterior parts 

 of the anterior corpora quadrigemina, is a small space known as the tngonum 

 Jiabenulce, which contains a well-marked collection of nerve-cells known as the 

 ganglion habenulce. The lateral walls are formed entirely by the optic 

 thalami. The upper surface of the optic thalamus looks into the lateral 

 ventricle of the cerebral hemispheres, from which it is 'separated by the 

 velum interpositum and by the ependyma, the epithelium completing the* 

 inferior wall of the lateral ventricle in this region. It consists of thre 

 masses of grey matter the anterior nucleus, the lateral nucleus (the largest 

 of the three), and the mesial nucleus. Its outer surface is in contact with tl 

 layer of nerve fibres formed by the crusta of each crus cerebri as it diverge 



