486 



STRUCTURE OF VERTEBRATA. 



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, terminates below the 

 skin in a closed vesicle. In 

 the young frog it also comes 

 to the surface above the skull, 

 but degenerates in adoles- 

 cence. 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 specialised 

 and transparent. 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 



FIG. 259. Vertical section of the 

 pineal eye in an embryo of Spheno- 

 don. After Dendy. 



E., Epidermis; >., dermis; L., lens; I.W., 

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

 the eye; PA.N., parietal nerve; PA.S. t 

 parietal stalk ; C. t cartilage. 



of thft 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 function is not primitive, but 

 the result of a secondary transforma- 

 tion. 



FlG. 260. Diagram of the parts 

 of the brain in Vertebrates. 

 After Gaskell. 



c.h.. Cerebral hemispheres; c.pl., 

 choroid plexus; o.th., optic thai- 

 ami; o.l., optic lobes; cl>., cere- 

 bellum; c.pl., choroid plexus; 

 M.O.y medulla oblongata ; S.C. ( 

 spinal cord. 



