NERVE SUPPLY OF PINEAL SYSTEM OF SPHENODON 265 



within the stalk, it is probable that the difference in the route which 

 is taken by the nerve-fibres in the two cases is not of morphological 

 importance. Finally, it is probable that if Dendy had been in possession 

 of the large amount of negative evidence with regard to the supposed 

 function of the pineal gland which we possess at the present time, he 

 would not have used the word " evident " with reference to the presumed 

 change of function which at that time the pineal organ was thought to 

 have undergone. 



The Connections of the Pineal Organs with the Central 

 Nervous System 



The nerves of the pineal system in Sphenodon appear to end (Fig. 185) 

 in the habenular and posterior commissures and in the habenular ganglia, 

 and to be continued from the latter by means of the bundles of Meynert to 

 the base of the brain (fasciculi retroflexi, v. tractus habenulo-peduncularis). 

 Each of the fasciculi passes obliquely downwards and backwards in front 

 of the recessus geniculi to the interpeduncular ganglion. The habenular 

 and posterior commissures are recognized at a very early stage of develop- 

 ment. The former, being much the smaller of the two, connects the right 

 and left habenular ganglia and lies beneath the median habenular ganglion ; 

 where it forms with the latter a projection into the third ventricle just 

 above the pineal recess. The posterior commissure is closely connected 

 from the first with the tractus pinealis, as well as with other structures 

 outside the pineal complex. It grows rapidly in size and soon becomes 

 folded transversely ; at the same time, the epithelium beneath it becomes 

 converted into the subcommissural organ, which in its turn is connected 

 with the anterior end of Reissner's fibre, that remarkable elastic filament 

 which extends in Sphenodon, as in Geotria (Fig. 134, Chap. 17, p. 188), 

 along the whole length of the central canal of the spinal cord. 



The right and left habenular ganglia are situated above the postero- 

 median and dorsal part of the optic thalami and are prolonged upward on 

 each side into the wall of the dorsal sac ; the prolongation is greater on the 

 left than on the right side, the difference being associated with the termi- 

 nation on this side of the left pineal nerve (n. parietalis) in the apex of the 

 habenular ganglion. Beyond this extension, however, there is no obvious 

 difference in the size of the right and left habenular ganglia, as is the case 

 in Geotria (p. 188), and there is no appreciable difference in the size of the 

 bundles of Meynert as in Geotria. The lack of any marked difference in 

 the size of a right and left habenular ganglia in Sphenodon, notwithstand- 

 ing the absence of a right parietal nerve, is probably largely due to the 

 presence of other connections of the habenular ganglia besides those with 

 the pineal system, and the very small size of the pineal nerve. 



