NERVE SUPPLY OF PINEAL SYSTEM OF SPHENODON 263 



In one case he was able to recognize nerve-fibres even before the two 

 pineal vesicles had separated from one another. Both the habenular 

 and the posterior commissures were recognizable. The nuclei in the 

 ventral or future retinal wall of the pineal eye were just beginning to 

 show an arrangement into principal layers separated by a nerve-fibre 

 layer. These fibres were limited to the retinal part of the eye-vesicle, and 

 he concluded that as, now, fibres were visible in the adjacent wall of the 

 brain, the fibres appear first in the retina and from thence grow into 

 the brain wall, as in the case of the ordinary paired eyes. As the pineal 

 eye becomes separated from the roof of the brain by the development 

 of intervening mesoderm, the left pineal nerve, which has now grown out 

 from the retina, grows longer and longer, being attached at its ventral 

 extremity to the brain roof. It is absolutely independent ot the pineal sac 

 and its stalk. 



In two series of transverse sections at a later stage of development, 

 Dendy was able to trace the nerve continuously from the eye to the left 

 side of the habenular commissure, and he also traced the nerve into the 

 left habenular ganglion in a longitudinal series of sections, belonging to 

 the same developmental stage. He never found it joining the brain roof 

 in the middle or on the right side (see p. 261). 



Development of the Right Pineal Nerve. — When the first nerve-fibres 

 have become recognizable in the retinal wall of the pineal eye, a similar 

 band of fibres is visible in the posterior wall of the pineal sac, close to 

 where it joins the roof of the diencephalon in the region of the posterior 

 commissure (Fig. 186). These fibres appear in the outermost zone of 

 the wall, external to all the nuclei. From this point the nerve-fibres 

 extend at a later stage of development upwards into the wall of the pineal 

 sac and downwards into the posterior commissure. When later the nuclei 

 in the wall of the pineal sac assume their characteristic arrangement in 

 two principal layers, the nerve-fibres are found between the two layers 

 of nuclei, as in the retina of the pineal eye. Also when the cavity of the 

 proximal portion of the pineal sac becomes obliterated the nerve-fibres 

 form the most important constituent of the stalk or " pineal tract." The 

 nerve-fibres are associated here with the remains of epithelial cells, 

 connective tissue, and blood-vessels, as in the adult. 



Dendy believed that there was no doubt about the connection of the 

 right pineal nerve with the posterior commissure, but that the right 

 pineal organ (pineal sac) was also primarily associated with the right 

 habenular ganglion ; and he considered also that the connection of the 

 pineal sac with the posterior commissure was secondary and " possibly 

 correlated with change of function which the pineal sac has evidently 

 undergone." Several points of interest arise from this description. 



