THE PINEAL BODY. 



441 



In Elasmobranchs the pineal process 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 

 adolescence. In Sphenodon the stalk passes through the skull by the 

 "parietal foramen," so that the "eye" itself 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 receives a nerve from a " parietal centre " near 

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

 in Anguis, more or less persistent in 

 Iguana. Above Reptiles the pineal 

 stalk is always relatively short, and 

 its terminal portion forms a glandu- 

 lar structure. In fact, the develop- 

 ment of the pineal body is much 

 more complicated than at first ap- 

 peared ; thus, according to Locy's 

 researches on Acanthias embryos, it 

 represents the fusion of an extra fair 

 of eyes. 



The full significance of the pineal 

 body is thus uncertain. According 

 to one view, 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) degeneration. It has 

 also been interpreted as an "organ 

 for the perception of warmth." It 

 may be, however, that the optic 

 function is not primitive, but the 

 result of a secondary modification. 

 Thus one of the first interpreta- 

 tions (Dohrn's) connected the pineal 

 and the pituitary outgrowths with 

 a supposed passage of the original hypothetical mouth through the 

 nerve-cord. 



The second primary vesicle of the brain forms the 

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

 brain), in the adult brain. The floor and lateral walls form 

 the thickened crura cerebri ; the roof becomes the two optic 

 lobes, which are hollow in almost all Vertebrates. In 

 Mammals a transverse furrow divides each optic lobe into two 

 (corpora quadrigemina). The cavity of the vesicle becomes 

 much contracted, and forms the narrow iter or aqueduct of 

 Sylvius, a canal connecting the third ventricle with the fourth. 



Fig. 189. — Diagram of the parts 

 of the brain in Vertebrates. — 

 After Gaskell. 



c.k., Cerebral hemispheres; c.pi., 

 choroid plexus; o.th., optic thai- 

 ami; o.l., optic lobes; cb. , cere- 

 bellum; c.pl., choroid plexus; 

 M.O., medulla oblongata; S.C.. 

 spinal cord. 



