246 THE PINEAL ORGAN 



evagination, as indicated in the drawings of Strahl and Martin (1888), 

 Anguis fragilis, Lacerta vivipara ; Beraneck (1892) in Anguis and Lacerta, 

 and Nowikoff (1910), L. agilis, L. vivipara, and A. fragilis. The earlier 

 observations made on the embryos of Lacerta and Anguis were later con- 

 firmed by Studnicka, and additional observations were made by Klincko- 

 stroem, Spencer, Dendy, and others on different species of reptiles, e.g. 

 Iguana, Varanus, and Sphenodon. The general result of these researches 

 may be briefly stated as follows. The parietal nerve originates from 

 ganglion cells in the retina of the parietal sense organ ; it courses back- 

 wards over the dorsal sac and then turns downwards towards the brain in 

 front of the epiphysis and between this and the posterior wall of the dorsal 

 sac, and finally terminates in the roof of the diencephalon, where it has 

 been traced into : the right habenular ganglion — Iguana tuberculata 

 (Strahl and Martin) ; the left habenular ganglion — Sphenodon (Dendy) ; 

 two parietal nerves, right and left, each of which ended in the habenular 

 ganglion of the corresponding side — Iguana (Klinckostroem) ; a single 

 nerve terminating in a nucleus in the roof of the diencephalon (formed 

 according to Studnicka by the approximation of the right and left habenular 

 ganglia) — Anguis fragilis . This was named by Beraneck le noyau parietal . 

 Spencer described a case of longitudinal splitting of the parietal nerve in 

 Varanus giganteus (Fig. 176), and he also described nerve-fibres passing 

 in the remnants of the original stalk of the pineal eye to the epiphysis. 

 Ritter in Phrynosoma coronatum describes the parietal nerve as extending 

 between the posterior commissure and the parietal eye ; and cases have 

 been described in which a nerve is absent. The failure to find a nerve in 

 adult or later larval stages is explained by the degeneration of a nerve 

 which, although developed as a transitory structure in embryonic life, has 

 either disappeared altogether or is represented only by its fibrous sheath. 

 The nerve-fibres when the whole nerve is at the height of its development 

 are provided with medullary sheaths and there is an outer covering of 

 neurilemma, the trunk being invested by a fibrous sheath which is con- 

 tinuous with the capsule of the parietal eye and with the pia mater. 



The pineal nerve, 1 or tractus pinealis, or nerve of the epiphysis differs 

 from the parietal nerve, or nerve of the parietal sense organ, in its position 

 relative to the epiphysis and its termination ; arising from cells in the wall of 

 the pineal sac or end vesicle, it passes towards the brain on the posterior 

 aspect of the proximal part of the organ and ends in the posterior commis- 

 sure (Figs. 166, 187, pp. 232, 264). The earlier description of nerve-fibres 



1 The name " pineal nerve " is still frequently used to denote the nerve of the 

 " parietal sense organ," or " pineal eye." The distal connection of the nerve with 

 the parietal organ should, however, prevent confusion when the term " pineal " is 

 used in this sense. 



