Cha7'acteristics of the Vertebrata. Ixxvii 



ance relatively to the other divisions of the brain, which it main- 

 tains in the higher Vertebrata, and developes lateral ventricles in 

 its two halves, which communicate with similar cavities in the 

 rhinencephalon. These two divisions of the brain are solid in 

 most Fishes, the rhinencephalic lobes appear to be attached laterally 

 and by pedimcles to the prosencephalic in the Elasmobranchii ; 

 they are pedunculate in many Teleosfei, but sessile in the Ganoidei. 

 Besides developing- in Silurus and Cyprinoids a supero-median 

 ' lobus impar,' or ' nodulus,' the medulla oblongata presents in 

 various families certain lateral ganglionic enlai'gements, which^ 

 from their connection with peripheral nerves, are known in Cypri- 

 noids as ' vagal/ as ' lobi nervi trigemini ' in the Sharks^ and as 

 ' electric lobes ' in the Torpedo. 



In Dipnoi, and to some extent in Marsipohranchii, the muscles of 

 the eye are supplied by the fifth nerve ; and in most Fishes the 

 nervous supply of the superficial structures in the maxillary, hyo- 

 mandibular, and palatine regions supplied in higher Vertebrata, with 

 the exception of the Anurous Amphibia, by the jwrtio dura of the 

 seventh pair, is dependent partly upon factors from the fifth nerve, 

 as well as upon an independent stem. The nervus lateralis which 

 supplies the sensory organs of the lateral line, as well as the medio- 

 dorsal region of the trunk, anastomosing in its course with the 

 sj)inal nerves externally, much as the sympathetic chain anasto- 

 moses with them within the thoracico-abdominal cavity, is in 

 Elasmohranchii, Ganoidei, and many Physostomous Teleostei, con- 

 stituted by the vagus. In some osseous Fish [Gadiis), it is formed 

 chiefly by the fifth nerve. The glossophaiyngeal is not always 

 differentiated from the vagus, and in some Rays it interchanges 

 fibres with the auditory nerves. The spinal accessory nerve does 

 not exist in Fish, and the place of the hypoglossus is taken by the 

 first spinal nerve. The existence of the sympathetic has not been 

 demonstrated in Marsipohranchii ; in Teleostei the bilateral gang- 

 liated chain does, in Elasmohranchii it does not, extend into the 

 caudal region. 



Organs of tactile sensibility are constituted in some Fishes by the 

 development of flexible rays upon their fins ; in some others, and 

 more commonly, by that of similar structures, in the shape of 

 barbules upon the snout ; but in most cases by the muco-nervous 

 organs developed upon the head, where they are supplied by the 



