416 Proceedings of Boy al Society of EdinbiirgJi. [sess. 
happens in the normal brain, this growth was too feeble to reach 
the anterior wall of the ventricle and to reduce the brain substance 
covering it to a single layer of cubical epithelium. 
Below the cavity v. the representatives of the two optic thalami 
were fused together over the mesial plane, except where a small slit 
remained to represent the infundibuliform portion of the third 
ventricle. The pituitary body (p.b.) was found surrounded with 
thickened membranes (fig. 6). 
Fig. 7 was drawn from a section made through the optic thalami 
immediately behind the infundibulum about the anterior part of 
the large globular mass described above, and shows the fusion of 
these structures with a narrow slit between them. A series of 
sections made backwards showed that this narrow slit-like ventricle 
terminated below the commencement of, and was not in communica- 
tion with, the aqueduct of Sylvius. Above, it joined the laige 
ventricular cavity of the hemisphere. It was, therefore, evidently, 
a much narrowed third ventricle. The manner in which the 
cerebral mantle folds over the optic thalami is well seen in this 
diagram. In order to ascertain the condition of the thickened 
membranes round the fused optic thalami, etc., and the condition of 
the optic nerve, which had not as yet been detected with the naked 
eye, a series of microscopic sections was made through the globular 
body in front of the pons varolii. Fig. 8 is drawn from a section 
through the hinder portion of the optic thalamus, and shows the 
great thickness and vascularity of the membranes. The two 
thalami are completely fused together, with the exception of two 
small openings in the mesial plane, of which the anterior is the 
extreme lower extremity of the infundibulum, and the posterior 
the upper portion of the aqueduct of Sylvius. Beneath the 
infundibulum are seen nerve fibres decussating with each other — 
few in number, but which, from their position, could only represent 
imperfectly-developed optic tracts. 
Staining by Weigert’s haematoxylin method demonstrated the 
existence of medullation of these fibres. 
Sections made immediately below this level showed extremely 
well the remarkable thickening of the membranes (fig. 9). 
Sections through the nuclei and roots of origin of the third nerve 
demonstrated that, to all appearance, these were normal. The fillet 
