630 Prof. Dendy. Structure, etc., of the Pineal Organs [June 22, 



delicate organs of the pineal complex in situ by the injection into the cranial 

 cavity of acetic bichromate of potash. After fixation the pineal eye itself, 

 with the parietal plug, can be dissected clean out of the parietal foramen, 

 and the necessity of decalcification before section cutting thus avoided. 



The " pineal complex " is formed chiefly by the dorsal sac, the paraphysis 

 and the pineal sac (" epiphysis " or right pineal organ), united in a common 

 pial investment and forming a bilaterally flattened, funnel-shaped structure 

 attached above to the cranial roof by the dura mater and below to the 

 optic thalami and habenular ganglia. 



Across the subdural space numerous fine threads of connective tissue 

 extend from the surface of the brain to the dura mater, and these are con- 

 centrated to form imperfectly developed vertical supporting membranes for 

 the pineal complex, one placed transversely on either side of the dorsal sac 

 and one lying in the sagittal plane behind the pineal sac. The very large 

 subdural space arises late in development, which perhaps indicates tliat the 

 relatively small size of the brain is a Cfenogenetic character due to arrested 

 growth. 



The fundamental relations of the different parts of the fore-brain and its 

 derivates in Sphenodon are already to a large extent familiar to us, but tlie 

 following points may be noticed : — 



Gisi has described the lateral choroid plexuses (jdcxus hnnisjjhasrviwi) as 

 arising from a transverse fold behind the paraphysial opening, while Elliot 

 Smith has described them as arising from a transverse fold in front of tlie 

 paraphysial opening. The explanation of this discrepancy is that wliile 

 they really arise one on either side of the paraphysial opening their roots of 

 attachment may extend a little in front of and behind the latter. There is 

 no unpaired plexus medianus as described by Gisi. 



The paraphysis, as I have previously shown, is part of the same system of 

 folds of the epithelial lam,ina supraneuroporica which gives rise to the plexus 

 he7nisphcerium, but growing outwards instead of inwards. It originally opens 

 into the prosencephalon immediately in front of the commismira aherraris. 

 In an advanced stage of development, however, a longitudinal supra-com- 

 missural canal is formed above the covmissura ahcrrans, and this leads to the 

 formation of a new opening for the paraphysis in the adult, directly into 

 tlie dorsal sac at some distance above the commissure. The original opening 

 of the i)ara])]iyHiH, in front of the covimissiira aherraiis, is blocked u]) by tlie 

 growth of tlie anterior choroidal veins and arteries. 



There is no true commissura mollis as described by (iisi, though the lateral 

 walls of the t/hird ventricle come into contact with one another over a 

 considerable area. 



