THE PINEAL BODY 



541 



The pineal body.— The dorsal upgrowth from the roof of the 

 thalaniencephaluu is represented, though to a varying extent, in all 

 Vertebrates. It consists of two parts, a pineal body or epiphysis 

 proper, and a parietal organ, which arises as a rule from the epiphysis 

 but may have an independent origin in front of it. It is probable that 

 they were originally right and left members of a pair. The parietal 

 organ may become atrophied, but in some cases, especially in Reptiles, 

 it is terminally differentiated into a httle body known as the pineal 

 eye. This was entirely an enigma until De Graaf discovered its eye- 

 like structure in Anguis, and Baldwin Spencer securely confirmed this 

 in the New Zealand " lizard " [Sphenodon), where the pineal body 

 shows distinct traces of a retina. In Petromyzon both the epiph3^sis 

 and the parietal organ show an eye-like structure, most marked in the 

 case of the epiphysis. 



In Elasmobranchs the pineal process (epiphysis) is very long, and, 

 perforating the skull, ter- 

 minates below the skin in a 

 closed vesicle. In the young 

 frog it also comes to the sur- 

 face above the skull, but 

 degenerates in adolescence. 

 In Sphenodon the stalk passes 

 through the skull by the 

 " parietal foramen," so that 

 the " eye " itself, developed 

 from the parietal organ, lies 

 close beneath the skin, the 

 scales of which in this region 

 are speciaUsed and trans- 

 parent. In Iguana, Anguis, 

 Lacerta, etc., the epiphysis 

 loses connection with the 

 " eye " portion ; and it is 

 also to be noticed that in 

 Anguis and Iguana the pineal 

 body (on the end of the 



parietal organ) receives a nerve from a " parietal centre " near the 

 base, but independent of the epiphysis ; this nerve is transitory in 

 Anguis, more or less persistent in Iguana. Above Reptiles the pineal 

 stalk is relatively short, and its terminal portion is glandular. Among 

 mammals the epiphysis is absent in the dugong and some Cetaceans ; 

 the pineal body is absent in Dasypus and the dolphin. 



The significance of the pineal body is uncertain. According to 

 some, its primitive function is that of an unpaired, median, upward- 

 looking eye— a function retained only in the Reptiles mentioned 

 above, the organ having elsewhere undergone (independent) degenera- 

 tion. It may be, however, that the optic fimction is not primitive, 

 but the result of a secondary transformation. 



The second primary vesicle of the brain forms the third 

 region, that of the optic lobes (mesencephalon or mid- 



'S^-® ,^>' 



Fig. 306. — Vertical section of the pineal 

 eye in an embryo of Spkenodon. — 

 After Dendy. 



£., Epidermis ; D., dermis ; L., lens; I.W., 

 inner wall of the eye ; O.W., outer wall 

 of the eye ; PA .N., parietal nerve ; 

 PA.S., parietal stalk; C, cartilage. 



