402 Central Nervous System in Vertebrate Animals. [Mar. 9.. 



The large tuberosity situated behind the cerebellum is termed by 

 the author the tuberculum impar, and consists of six layers, counting- 

 from the outside. 



L The first layer has small cells which become deeply coloured by 

 the staining fluid. 



2. The second contains sections of obliquely directed nerve-fibres. 



3. The third is smoothly granular and does not become so highly 

 coloured as the outside layer, but shows faint indications of radial 

 striations. 



4. The fourth is a narrow stratum of moderately sized cells of 

 varying dimensions, which become intensely coloured by the staining 

 fluid. 



5. The fifth consists of a complex of nerve-fibres. 



6. The sixth layer only found at the anterior end of the tubercle is 

 composed of finely granular material ; the corresponding portion of 

 the posterior end is occupied by a circular space. 



In addition to these six layers there is intercalated between the 

 first and second layers a body of granular neuroglia, in which 

 extremely large cells are collected ; at the anterior end of the tuber- 

 culum impar, this structure takes the place of the first layer and 

 becomes interposed between the tubercle in question and the cere- 

 bellum . 



The author then discusses the modes of origin of the nerves which 

 are present in these fishes. The trochleares and the abducentes appear 

 to be absent. The trifacial and the vagus, in addition to their ordinary 

 origins, derive the greater number of their fibres from the tuberculum 

 impar, the former from the anterior end, the latter from quite the 

 posterior edge. The facial and the glossopharyngeal are parts of 

 these two nerves respectively. 



The author finds, contrary to the opinion of Bellonci,* that the 

 optic nerve has an origin from the hypoarium as well as from the 

 tecta lobi optici. The other nerves arise as in Teleostei. 



The author then proceeds to indicate the probable homologies of 

 these two extraordinarily developed organs. Taking the brain of a 

 Ballan wrasse (Labrus maculatus) and examining the part which is 

 generally termed the valvula cerebelli, he finds that it has a central 

 part resembling the cerebellum in structure, together with a wing on 

 each side, occupying much more of the ventricle of the optic lobe 

 than in many other Teleostei. 



These wings ai*e formed by an extension of the molecular layer of 

 the central part, which even here shows two or three folds based on 

 an extension of the granular layer. He therefore puts forward the 

 idea that if these wings of the valvula cerebelli of the L. maculatvs 



# " Ueber d. Ursprung des Nervus Opticus und den feineren Bau des Tectum Op- 

 tician." " Zeitsch. f. Wiss. Zool.," Bd. sxxt, 1880. 



