98 MESENCEPHALON. 



All along the upper curved margin of the lateral wall, from the pillar of the fornix 

 to the pineal gland, runs' a white stria, known as the stria pinealis, stria meduUaris, 

 or tmnia fornicis (fig. 73, Tfo). The floor of the ventricle is formed posteriorly by 

 the iegmenia of the crura cerebri, and where the crura diverge from one another by 

 the following parts, which have been already mentioned as seen at the base of the 

 cerebrum ; viz., commencing from behind, the grey matter of the posterior perforated 

 space, the corpora albicantia, sen mamillaria, the tuber cinereum and infundibnlum ; 

 the lamina cinerea serves to close the ventricle in front. The roof of the cavity is 

 limited before and behind by two commissures, named from their position, anterior 

 and posterior. Of these the anterior will be described with the cerebral hemispheres. 



The third ventricle is lined, like the other cavities already described, by ciliated 

 epithelium, which is thin and flattened over the roof, i.e., lining the velum and 

 choroid plexuses, but longer and more columnar at the bottom and sides. The 

 floor, which is narrow, is formed, underneath the epithelium, of grey matter con- 

 tinuous with that of the Sylviau aqueduct, and this central grey matter extends a 

 short distance upwards on the wall of the thalamus. The central grey substance 

 rests behind upon the still conjoined part of the tegmenta ; but anteriorly, after 

 these have diverged, it comes to the surface at the base of the brain as the posterior 

 perforated lamina and the tuber cinereum. The lateral walls of the ventricle have 

 but a thin covering of neuroglia (ependyma) underneath the lining epithelium ; so 

 that the white covering (stratum zonale) of the thalami comes to view through it. 



The epithelial covering of the roof of the ventricle is not free but covers the 

 under surface of the median portion of an expansion of pia mater named the velum 

 interposilum, which overlies the third ventricle as well as the larger part of the optic 

 thalami. The epithelium follows all the inequalities of two fringed vascular tracts 

 (choroid plwnises of the third ventricle) which project downward from the mem- 

 brane, and it becomes torn away when the pia mater is removed. At the pineal stria 

 (fig. 73, Tfo) on either side it is continuous with the epithelium covering the lateral 

 wall. This stria therefore represents the limit of the third ventricle so far as the 

 lateral boundaries of the roof are concerned : the upper surface of each optic thala- 

 mus is excluded from this cavity. 



The central grey matter of the aqueduct (fig. 76, c. gr.) is a layer 2 to 3 milli- 

 meters thick which surrounds the aqueduct, and is prolonged from the grey matter 

 of the fourth ventricle. It contains, scattered through its substance, nerve-cells of 

 varying size, the largest being prolonged upwards from the locus cceruleus of the 

 fourth ventricle ; the cells are very numerous and small at the dorsal side of the 

 aqueduct. In addition to these scattered cells the grey matter of the aqueduct 

 contains certain more defined groups or columns of cells which are connected with 

 the roots of the third and fourth, and of the fifth cranial nerves. Amongst the 

 cells there is a network of fine medullated nerve fibres, whilst near the aqueduct 

 and immediately under the ependymal layer many fine longitudinal fibres are seen. 



The nuclei of the third and fourth nerves (fig. 76, n.IIL, IV. extend on 

 either side along almost the whole length of the ventral part of the aqueduct, 

 close to the middle line, the nuclei of the two sides being only separated from one 

 another by the raphe ; at one part they even meet across this (fig. 78, n.III}. 

 The cells of these nuclei are large and irregular in shape, and of a yellowish 

 colour. The nucleus from which the root-bundles of the fourth nerve spring 

 does not begin to show itself until the level of the upper part of the inferior 

 corpora quadrigemina, and it here lies just below and rather to the side of that 

 from which the bundles of the third originate. From here the bundles of the fourth 

 pass obliquely downwards towards the pons, and just before reaching this the nerve 

 turns sharply dorsal-wards, and passes into the superior medullary velum, in which 

 it crosses horizontally, decussating with that of the opposite side (fig. 72). 



