THE PINEAL BODY. 



469 



more complicated than at first appeared ; thus, according to Locy's 

 researches on Acanthias embryos, it represents the fusion of an extra 

 pair of eyes. 



The full significance of the pineal body is thus uncertain. Accord- 

 ing to one view, its primitive function is that of an unpaired, median, 

 upward-looking eye a function retained only in the Reptiles men- 

 tioned above, the organ having elsewhere undergone (independent) 

 degeneration. It may be, however, that the optic function is not 

 primitive, but the result of a secondary modification. 



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 cere- 

 bri ; 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 be- 

 comes much contracted, and 

 forms the narrow iter or aque- 

 duct of Sylvius, a canal con- 

 necting the third ventricle with 



the fourth. 



T-U 4-u- j -i FIG. 228. Diagram of the parts 



The third primary vesicle O f the brain in Vertebrates. - 



gives rise to the metence- After Gaskell. 



phalon, Or hind-brain, Or region c./i., Cerebral hemispheres; <:.//., 



of the cerebellum, and to the 

 myelencephalon, or after-brain, 



or region of the medulla ob- 



choroid plexus; o.t/i., optic thai- 

 ami; o.l., optic lobes; c/>., cere- 

 bellum; c.pl., choroid plexus; 

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

 spinal cord. 



longata. 



In the metencephalon the roof develops greatly, and 

 gives rise' to the cerebellum, which often has lateral lobes, 

 and overlaps the next region. In the higher forms the 

 floor forms a strong band of transverse fibres --the pons 

 Varolii. 



From the region of the medulla oblongata most of the 

 cranial nerves are given off. Here the roof, partly over- 

 lapped by the cerebellum, degenerates, becoming thin and 



