334 PROCEEDINGS OF THE CANADIAN INSTITUTE. 
Whether the sac, receptaculum dorsale (rsi), acts as a reservoir for - 
this fluid or serves to receive any excess driven out of the atria, lam 
unable to say, but its distension is not likely to produce any imme- 
diate effect on the spinal cord, separated as it is from it by the thick ~ 
cushion of loose adipose tissue which would entirely redistribute any 
pressure. That the forward movement of the fluid in the cavwm 
sinus imparis should have any direct effect on the base of brain, as. 
suggested also by Hasse, is, I conceive, improbable, owing to the 
thick cushion of adipose tissue which separates the brain from the 
floor of the skull. (Fig. 9, Pl. VI.) I am inclined to believe, 
then, that it is solely through the auditory nerve, and specially through 
its saccular branches, that the central nervous system is informed of 
the movements of mallews and stapes, and consequently of the state 
of distension of the air-bladder. 
It is probable that the currents in the endolymph produced in this. 
way are different in character from those brought about by ordinary 
sound waves, but on the other hand the difference is not likely to be 
of such moment as to remove the phenomena in question entirely 
from the domain of sound. 
Whether the air-bladder and apparatus in connection with it are 
a]so sensitive to the alternations of pressure incident to sound waves, 
and whether this be not one of the principal channels through which 
the endolymph of the partes inferiores of the labyrinth is set in 
motion, must be a matter for further investigation. No very free 
interchange of endolymph can take place between the superior and 
inferior parts of the labyrinth, for the ductus sacculo-utricularis is 
thick walled and its narrow lumen is blocked up by a valve project- 
ing obliquely across it. Although the endolymph, then, in the 
superior part may be very readily set in motion by the vibrations 
transmitted through the thin wall of the skull opposite the recessus 
utriculi, yet the inferior part must be in a great measure protected 
from such by its concealed position. 
Hasse (/.c. 599) while not entirely excluding the possibility of 
alterations in volume of the air-bladder exerting an influence on the 
production of auditory sensation, adduces several arguments for 
believing that such must be of very subordinate nature in the 
Cyprinide. The first of these is that the direction of the stroke 
of the stapes not being coincident with the plane of the apertura 
posterior of the cavum, the fluid contents of the aériwm will not be: 
j 
