250 The Structure of the Cochlea and the Perception of Tone 
hair cells prevents the impulses in the endolymph from coming in direct 
contact with the hair cells and, therefore, invalidates the hypothesis that 
these cells may act as their own agent in selecting their stimuli directly 
from the impulses passing through the endolymph. ‘The conclusion to 
which these facts logically lead is that the stimulation of the hair cells 
is accomplished only by the vibration in the membrana tectoria trans- 
mitted to it by the impulses passing through the endolymph. The 
membrana tectoria must, therefore, be the mechanism which mediates 
impulses passing through the endolymph to the hair cells. 
A consideration of the following facts renders this theory much more 
plausible than the hypothesis that this function is accomplished by the 
vibration of the membrana basilaris. The sound waves transmitted 
through the mebrana tympani and the chain of ossicles produce impulses 
first in the labyrinthine fluid contained in the vestibule. It seems much 
more probable that these impulses, passing into the scala vestibuli, im- 
pinge directly upon the membrana tectoria, producing vibrations in it, 
since the delicate membrana vestibularis in no way hinders the passage 
of these impulses to the endolymph, than to assume that they must pass 
through the entire length of the scala vestibuli, then through the helico- 
trema into the scala tympani and then back again through the entire 
length of the cochlea in order to carry the basilar membrane upwards 
and push the hair cells against the membrana tectoria. Physically, the 
delicate semi-fluid structure of the membrana tectoria renders it ad- 
mirably suited for responding to the most delicate impulses passing 
through the fluid in which it is suspended. Quite the opposite is true 
of the membrana basilaris, the fibers of which are so short and rigid that 
even in its most favorable part this has always been recognized as an 
objection to the idea that it could be a vibrating structure,—an objection 
first raised by Helmholtz himself. 
A striking characteristic of the membrana tectoria when examined 
throughout the several coils of the cochlea is its great variation in size 
from one end of the cochlea to the other. This is well illustrated in Figs. 
6 to 10 which have been drawn accurately with the aid of the camera 
lucida from sections of different parts of the cochlear tube of a single 
labyrinth. It is seen at a glance that the size of the membrana tectoria 
near the apex of the cochlea is many hundred times its size near the be- 
ginning of the basal coil. Beginning with the lower end of the basal 
coil there is a gradual increase in the size of the membrana tectoria until 
the apex of the cochlea is reached. Another characteristic which this 
