BRAIN. 



101 



hemispheres, coalescent, and provided with an upper median 

 groove like the prosencephalon. At their base a pair of lobi 

 inferiores are constant, with the hypophysis and saccus vascu- 

 losus (a conglomeration of vascnlar loops without medullary 

 substance) between them. 



The cerebellum is very large, overlying a portion of the 

 optic lobes and of the sinus rhomboidalis, and is frequently 

 transversely grooved. The side-walls of the fourth ventricle, 

 which are formed by the corpora restiformia, are singularly 

 folded, and appear as two pads, one on each side of the cere- 

 bellum (lobi posteriores s. lohi nervi trigemini). 



The brain of the Cyclostomes (Figs. 44, 45) represents a type 

 different from that of other fishes, showing at its upper surface 



Fig. 44. — Brain of Bdellostoma. 

 I., Upper; II., Lower aspect. 



(Enlarged, after Miiller.) 

 Letters as in Fig. 45. 



three pairs of protuberances in front of the cerebellum ; they 

 are all solid. Their homologies are not yet satisfactorily deter- 

 mined, parts of the Myxinoid brain having received by the 

 same observers determinations very different from those given 



