CHAPTER XIX 

 MEDIAN EYES 



We have already seen ^ that it is not unusual for the region of the 

 mid-brain (diencephalon) of certain Vertebrates — and particularly the 

 ependymal cells lining the posterior portion of the first embryonic vesicle 

 which persists as the third ventricle of the brain — to show evidences of an 

 optical as well as a glandular function. From this region the optic vesicles 

 which form the lateral eyes emerge as out-pouchings ; from the floor is 

 derived the neural portion of the pituitary gland ; in the ventral area are 

 nuclei of internal secretion ^ ; and from the thin roof is given off the pineal 



h 



Fig. 852. — Descartes's Orkjinal Diagram Illustrating the Effect of Light 

 UPON THE Rational Soul Lying in the Pineal Gland (from a iDhotograph 

 by Prof. J. F. Fulton). 



apparatus (or epipliysis), which, although usually glandular in function, 

 becomes differentiated into a median eye in some species. This dorsal 

 up-growth of the roof of the diencephalon is represented in varying degrees 

 in all Vertebrates with the exception that the pineal process is absent in 

 the dugong {Halicore), a decadent and sluggish sea-cow, and in whales 

 (Cetaceans), while the pineal body is absent in the armadillo {Dasypus) and 

 in the dolj>hin {Dclphhius). 



The significance of the pineal body has always been an enigma. The ancient 

 Romans described it as the glandula pinealis and by snch anatomists as William 

 Cooper (1666-1709) and Jacob Henle (1809-1885) it was considered as a lymphatic 

 " gland." In the more speculative philosophy of Rene Descartes the body was a 

 machine directed by a " rational soul " which dwelt in the pineal gland. This con- 

 ception, sarcastically derided by Voltaire, is illustrated in Fig. 852, taken from 

 Descartes's work De homine figuris et latinitate donatus a Florentio Schuyl (Leyden, 

 1 p. 537. 2 p_ 557^ 



711 



