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D. DAVIDSON BLACK 



find the hippocampal formation developed in the edges of the 

 pallial cup posteriorly, and above the fimbria we find choroidal 

 invaginations of the ependyma. 



Turning to the thalamus, it will be seen that the absence of 

 the pars optica and pars mammillaris hypothalami would mate- 

 rially reduce both the volume of the thalamus and the size of its 

 ventricular cavity. 



The use of such a model is justified only in so far as it helps 

 to demonstrate the mechanical tendencies that would arise in a 

 brain in which these areas are missing from the start. The 

 brain at this stage of development was taken because, while 

 the relations are simple, it is yet possible to outline the areas 

 occupied by the corpus striatum, and so forth, fairly accurately. 



Fig. 2.3 A B and C. Diagrams of medial sagittal sections through the brain 

 illustrating the hypothetical closure of the connection between the cavity of 

 the cerebral vesicle and the third ventricle. Cor.c, cortex cerebri; Cur. pin., 

 pineal body; Ep.R., ependymal roof; Th., thalamus; Vent. I., cavity of cerebral 

 vesicle; Vent.IIL, third ventricle; A", in figure 23 C. indicates the final point of 

 attachment of the much expanded thin roof to the front of the thalamus. 



Sometime during the development of this case the condition 

 of hydrocephalus set in and, as a result, the roof of the cerebral 

 vesicle bulged upward. If one refer now to the diagrams (fig. 

 23), it can be readily seen how the discontinuity in the cavity 

 of the already reduced third ventricle might be brought about 

 through pressure of the expanding roof of the cerebral vesicle. 

 This thin cerebral roof being confined above by the skull, ex- 

 tended backward as a pocket over the thalamus. Pressure of 

 the fluid contents being transmitted equally in all directions 

 would then tend to close the fore part of the third ventricle. 

 This closure might subsequently be followed by adhesion be- 



