The Nervous System 



1 1 1 



medulla oblongata is more or less modified or specialized. The 

 brain of the shark is relatively more highly developed than that 

 of the bony fishes, although in most other regards the latter 

 are more distinctly specialized. 



The Pineal Organ. Besides the structures noted in other 

 fishes the epiphysis, or pineal organ, is largely developed in sharks, 

 and traces of it are found in most or all of the higher vertebrates. 

 In some of the lizards this epiphysis is largely developed, bear- 



R 



P.. 



FIG. 82. 

 (After Dean.) 

 II. Second cranial nerve. 



IV. Fourth cranial nerve. 



V. Fifth cranial nerve. 



VII. Seventh cranial nerve. 



VIII. Eighth cranial nerve. 

 IX. Ninth cranial nerve. 

 X. Tenth cranial nerve. 



FIG. 81. 

 FIG. 81. Brain of a Perch, Perca flavescens. 



R. Olfactory lobe. 



P. Cerebrum (prosencephalon). 



E. Epiphysis. 



M. Optic lobes (mesencephalon). 



EP. Cerebellum (epencephalon). 



ML. Medulla oblongata (metencephalon). 



I. First cranial nerve. 

 FIG. 82. Petromyzon marinus unicolor (Dekay). Head of Lake Lamprey, showing 

 pineal body. (After Gage.) 



ing at its tip a rudimentary eye. This leaves no doubt that in 

 these forms it has an optic function. For this reason the struc- 

 ture wherever found has been regarded as a rudimentary eye, 

 and the "pineal eye" has been called the "unpaired median 

 eye of chordate" animals. 



It has been supposed that this eye, once possessed by all 

 vertebrate forms, has been gradually lost with the better de- 



