PHYSIOLOGICAL SERIES. 



actions than in higher vertebrates in which tracts from the 

 cortex, cerebellum, and other parts of the brain form the irreater 

 part of its white mailer. 



In most bony fishes, Ganoids. I.Mpnoi, and larval Amphibian-, 

 the ventral (motor) tracts contain a pair of giant nerve-iibres 

 (Mauthner's fibres) which arise from a pair of giant cells in the 

 neighbourhood of the acoustic nucleus. They cross in the floor 

 of the medulla and at the hinder end of the cord break up and 

 enter the motor nerves of the tail-fin. A number of somewhat 

 similar fibres (Miiller's fibres) occur in the ventral tracts of the 



Fig. 254. 



MAUTH. FIB. 

 PERIPH.V.COMM. 



cord in Cyclostomi. In many Fishes a number of remarkable 

 giant ganglion-cells are found in the dorsal fissure, usually at 

 the anterior end of the cord ; their axis cylinders apparently run 

 in some cases forwards, in others backwards, and form connec- 

 tions either with the dorsal roots of the spinal nerves or with 

 the sensory branches of the trigeminal. The development of 

 these element- has been noticed to be particularly strong in 

 tish with >pecial epidermal sensory processes ( Aoy /////., tiatra- 

 clms tau). Whether these cells are hoinologou> wherever they 

 occur, and \vh:>t their .-X.-K-I relation^ may he to similar cell- 

 that form a tran.-icnt nervou- Y8teiH in Kla-mohranch emhrvo-, 

 is -till a matter of doubt. 



