THE EAR AND HEARING. 
201 
the effect being intensified by tlie bombardment of tlie 
otoconia, found chiefly in the vestibule, and by the peculiar 
arrangement found at the orifices of tlie semicircular canals, 
which have a diameter of about jjfc inch. Where these join 
the vestibule they dilate into three ampullae. On the inner 
walls of these ampullae are found a number of minute hair-like 
filaments, which, being connected at their bases with auditory 
nerve filaments, are very sensitive to impressions of 
sonorous undulations. The function of the semicircular 
canals has been thought to be the collection in their fluid 
contents of the sonorous undulations communicated through 
the cranial bones, and the magnification of the vibrations 
excited, in the ampullae and utriculus, in which they are assisted 
by the crystalline pulverulent ear stones, which, as aibove noted, 
tend to reinforce sonorous vibrations both by their resonance 
and by their bombardment of the epithelial cells of the 
vestibule. 
The Cochlea is situated in front of the vestibule on the 
inner side of the internal meatus of the ear. It is a very 
complicated structure, somewhat resembling a snail shell, 
having two turns and a half in its spiral. We may, perhaps, 
best represent it by a tube about half an inch long, having a 
diameter of one-tenth of an inch at its base and one-twentietli 
of an inch at its termination, divided longitudinally into 
three unequal compartments, and coiled two and a-lialf times 
round a central conical pillar (the modiolus). The base of 
the modiolus is pierced by canals for branches of the auditory 
nerve (entering through the internal meatus), and blood¬ 
vessels. The largest of these canals is called the central 
canal of the modiolus. The middle and smallest of the 
chambers of the cochlea is called the Scala Media (or canal is 
cochleans). It is a continuation of the membranous laby¬ 
rinth, and completes the division between the other two 
chambers. It is closed towards the top of the cochlea and 
opens below by a small neck into the Sacculus of the vestibule. 
The other two chambers are named from their connections. 
The Scala Vestibuli opens into the bony vestibule, and is thus 
open only to the action of the perilymph; the Scala Tympani 
communicates with the tympanum and the fenestra rotunda. 
At the top of the cochlea the scalae tympani and vestibuli 
communicate by a small aperture called the Helicotrema, left 
between the top of the modiolus and the Hamulus, or small 
hooked termination of the Lamina Spiralis which is the 
partition, partly bony and partly membranous, separating 
the two larger scal?e. 
(To be continued.) 
