Royal Physical Society. 391 



there is a small bone unconnected with any vertebra, viz., the 

 incus, joining the stapes and malleus (Weber), but there is 

 no vestige of it in Malapterurus. But from the under aspect 

 of the transverse process of the third vertebra a long and 

 slender spiculum passes inwards and forwards between the 

 malleus and the body of the vertebra, and expands into a 

 little plate the size of a pin-head, which lies over the lateral 

 foramen. When the malleus is pressed upon, as it would be 

 by the distended swimming-bladder, its curved extremity 

 presses this little plate down upon the foramen, and its an- 

 terior extremity comes in contact with the stapes and presses 

 it inwards. This curious little process, if it exists in the 

 Nilotic Malapterurus (which one would be disposed to think it 

 did, from the close similarity of the neighbouring parts), has 

 escaped the eye of Dr Bilharz ; neither does it appear from 

 Weber's account that we have anything of the sort in Silurus 

 glanis. As to the function of the little bones in Silurus glanis, 

 of which two, the malleus and stapes, are very similar to those 

 in this fish, Weber writes as follows : — " When the superior 

 part of the swimming-bladder is expanded, the malleus is 

 pressed upon the incus, the incus on the stapes, and the stapes 

 on the atrium of the sinus impar, whose water, being pro- 

 pelled into the sinus impar and vestibule, expands and dis- 

 tends the membranous labyrinth. The upper part of the 

 bladder being relaxed, the malleus, by the elasticity of the 

 processus folianus, returns to its old position, and the whole 

 labyrinth is relaxed. Moreover, tremors of the swimming- 

 bladder itself are no doubt transferred to the labyrinth by the 

 ossicula.*" 



On the same principle is the arrangement in the fish that 

 we are considering. When the fish descends in the water 

 the ossicula are pressed on, and the labyrinth distended ; and 

 thus a pressure from within is furnished to counterbalance 

 the increased pressure from without. At the same time, the 

 little processes of the third vertebra will be pressed upon the 

 branchial veins, and must more or less impede the circula- 

 tion in the head and gills ; so that they would appear to mo- 



* Weber, op. cit., p. 57. 

 VOL. I. 2 M 



