448 Proceedings of Royal Society of Edinburgh. [sess. 
anatomical relationships of the auditory nerve. As regards the 
first of these, we have absolute evidence of a similar reflex 
occurring between the two ears as before stated, viz., stimulation 
of one ear by a sound causes contraction of the tensor tympani 
of both ears. It is therefore probable that this occurs in the 
experiments described. But we are still left to account for the 
fact that pressure of the chain of ossicles inwards causes increased 
perception for all notes, low as well as high ; now contraction of 
the tensor tympani alone will not increase the acuteness of hear- 
ing for low notes; if anything, it will diminish this power. We 
must therefore seek for some other reflex besides this. I think 
such a reflex will be found to account for this fact, viz., that of 
the stapedius. We know that contraction of this muscle increases 
the acuteness of the hearing power, and if such a reflex could 
be demonstrated when the chain of ossicles is pressed inwards 
we would then be able to account for the whole phenomenon. 
Unfortunately I can find no description of such a reflex in 
physiological literature, and only experiment upon the living 
animal could give absolute proof. From the anatomical relation- 
ships of the accessory nucleus of the cochlea and the nucleus of 
the facial nerves before they leave the medulla, and the way in 
which the cochlear nerve decussates, there is certainly, anatomi- 
cally speaking, a path for such a reflex. In the lower animals 
a reflex between the cochlea and the facial nerve does take place 
by this path, viz., the pricking up of the ears when the animal 
hears a sound; and I think it is highly probable that at the 
same moment a reflex action of the stapedius occurs, could we 
but see it. In the meantime, however, we must wait for absolute 
evidence, and be content at present with saying that in all 
probability it does occur, while the reflex action between the 
cochlea and the tensor tympani of both ears certainly does. 
The path of the reflex to the tensor tympani is as follows : — 
Afferent impulse from the cochlea to tuberculum acousticum, up 
through the lateral lemniscus to the motor nucleus of the fifth 
nerve ; from there the efferent impulse passes to the otic ganglion, 
and thence to the tensor tympani.* 
* To some physiologists, however, the above explanation of the facts will 
not appear satisfactory, because it is held by some that contraction of the 
