262 



THE PINEAL ORGAN 



traced to the median habenular ganglion and the right habenular ganglion, 

 but the bulk of the fibres joined the left ganglion, and although some non- 

 nucleated fibres from the nerve-fibre layer of the pineal sac joined the 

 series of fibres coming from the left pineal nerve, Dendy considered that 

 the greater number of fibres which entered the left habenular ganglion 

 came from the left pineal nerve. 



(2) The right pineal nerve (tractus pinealis) (Fig. 185). Dendy, 

 using the term " tractus pinealis " in the wide sense in which this name 

 was employed at the time, namely, as including the " entire band or cord of 

 tissue which unites the lower extremity of the pineal sac to the roof of the 

 brain," describes it as commencing in the nerve-fibre layer of the wall 

 of the pineal sac, the greater number of its fibres leaving at its lower 

 RE. C. P.S. Epd. 



P.Co. 



S.C. 



Fig. 



186. — Semi-diagrammatic Representation of a Medial Longitudinal 

 Section of an Embryo Sphenodon, showing a Constriction marking off 

 the Anterior (left) Lobe, or Parietal Organ, from the Posterior (right) 

 Lobe, or Pineal Sac The Anterior End of the Organ is Directed 

 Forward and is to the left in the Figure. (After A. Dendy.) 



C. : Constriction. L.N.F.R.: Layer of nerve-fibres in retina. 



Epd. : Epidermis. P. Co. : Posterior commissure. 



Le. : Lens. P.E. : Pineal eye. 



L.N. F. P.S. : Layer of nerve-fibres in P.S. : Pineal sac. 

 pineal sac. P. St. : Pineal stalk. 



S.C. : Superior or habenular commissure. 



pole and passing downwards in the stalk to the roof of the brain ; here, 

 between the habenular commissure in front and the posterior commissure 

 behind, some fibres pass to the right and others to the left of the infra- 

 pineal recess and, passing towards the posterior extremities of the habenular 

 ganglia, some fibres appear to terminate in these ganglia, while others 

 appear to end in the median habenular ganglion. Dendy also stated 

 that some fibres passed backwards towards the posterior commissure and 

 subcommissural organ and that possibly some fibres were continued 

 directly into the bundles of Meynert. 



The Early Development of the Pineal Nerves in Sphenodon 



Dendy also studied the early stages of development in the parietal 

 organ of Sphenodon. The earliest indication of the development of 

 nerve-fibres was in embryos, of approximately Stage O when the visceral 

 clefts have begun to close. 1 (Fig. 186). 



1 Dendy, A., 1899. Q. J. Micr. Sc, 42, N.S. 



