THE SENSE ORGANS 



591 



have in time shifted their position to the present antero-posterior arrange- 

 ment, remains a disputed question. That they were formerly paired 



PARAPHYSIS SUPERIOR 



A CYCLOSTOME HABENULAR 

 /\. «_T(_L.<JilUME. COMMISSURE. 



PINEAL TRACT 



B. ANURAN 



PARIETAL EYE 



PINEAL ORGAN 



CORIUM^EPII^ERMIS 



M I DBRA I N 

 iPOSTERIOR COMMISSURE 



DORSAL SAC— &^ 



SUPERIOR HABENULAR/ 



COMMISSURE 



POSTER I C 

 COMMISSURE 



D. MAMMAL 



MIDBRAIN 



C REPTILE 



Fig. 488. — Parietal and pineal organs of vertebrates as seen in median longitudinal 

 section. A, Cyclostome; B, Anuran; C, Reptile; D, Mammal. Both parietal and pineal 

 organs have served as median eyes. In amphibia it is the pineal organ, in reptiles the 

 parietal organ. In man and mammals the pineal organ becomes a gland of doubtful 

 function. (Redrawn from Oppel, after Studnicka.) 



organs is suggested by the fact that in geotria, a cyclostome, each has a 

 lateral position in relation to the other ; and the parietal organ has nervous 



Fig. 489. — Diagrammatic longitudinal section of brain, ac, anterior commissure 

 in lamina terminalis; aq, aqueduct; c, cerebral hemisphere; cb, cerebellum; cp, chorioid 

 plexus; cs, corpus striatum; cv, cerebral ventricle; h, hypophysis; he, habenular com- 

 missure; i, inf undibulum ; ip, inferior chorioid plexus; m, mesencephalon; ml, myelen- 

 cephalon; p, pinealis; pa, paraphysis; pc, posterior commissure; pe, parietal eye; v, 

 anterior medullary velum; vl, velum trans versum with aberrant commissure; III and 

 IV, third and fourth ventricles. (From Kingsley's "Comparative Anatomy of 

 Vertebrates.") 



connexion with the left habenular ganglion, while the pineal organ is con- 

 nected with the right habenular ganglion; and also by the fact that the 

 anlagen of the two in the embryos of fishes, amphibia, and reptiles lie in 



