1896-97.] Dr Albert A. Gray on Direction of Sound. 
Whatever view be taken as to the causation of the facts stated 
above, it is evident, from them and from the experiments of Lord 
Kelvin and Prof. Thompson, that our lower centres, at least, 
are cognisant of the relative positions of the two tympanic 
membranes, and it has occurred to the writer that this may be 
one of the means by which we are enabled to estimate the 
direction of a sound. Thus it is evident that supposing a sound 
to affect two ears in different phases, the two membranes will 
not be in exactly the same condition at any given moment ; the 
one may be moving in while the other is moving out, or one 
may be just beginning its inward movement when the other is 
completing it, and so on. It would not be safe, however, to 
assume that such a supposition is correct, unless we find facts 
that bear evidence in favour of its truth. Have we any such 
facts? I think the following may be considered as tending to 
corroborate the theory advanced : — 
(1) If we listen to a sound conducted by water, and with our 
ears under the water, we lose all sense of the localities of the 
sound ; it appears to arise in the ear. In this case, although 
the membranes are vibrating in and out, yet, as a whole, they 
have taken up a position very much further inwards than under 
normal conditions, because of the pressure of the water. Hence, 
in these circumstances, there is a state of matters in both ears 
similar to that which is produced in one ear by pressing in the 
chain of ossicles with a probe, and we are conscious of a similar 
intra-tympanic muscles causes diminution of the hearing power. These 
physiologists would therefore not be satisfied with that explanation, even 
admitting that a reflex action of the muscles mentioned does occur ; they 
would look for some other cause. I might suggest the following explana- 
tion, though, of course, it is purely speculative : When the constant inward 
pressure of the ossicles produces increased labyrinthine pressure, a constant 
stimulus passes up the auditory nerve to the nuclei in the medulla. But 
this stimulus is not periodic, and therefore no sound is heard. But sup- 
posing a sound to affect the opposite ear at the same time, then the two 
stimuli will mix in the nuclei of the medulla ; and we may suppose that the 
constant stimulus will add to the strength of the periodic one before the 
latter passes on to the higher centres ; that is, that a stimulus to one 
auditory nerve, even if it is not of such a nature as to produce a sensation 
of sound by itself, may yet increase the subjective intensity of a sound heard 
by the other ear. Of course, as above stated, such an explanation is purely 
speculative, and must remain so, more or less, until the nature of the nerve- 
current is better known than at present. 
