372 On the Morphology of the Semicircular Canals. [June, 
of a wall about it, is at first of a protective character, and the final 
formation of a cavity is the further attainment of the same end. 
This, however, would entail a greater or less modification of func- 
tion. The exposed sensory macula would probably perceive 
vibrations clearly and distinctly. As soon, however, as enclosure 
in a cavity takes place, interference of the vibrations by their con- 
tinued reflection from the walls of the cavity must necessarily 
occur. Such a condition of affairs is probably present in sensory 
structures like the “mucous” ampullz of sharks and rays and 
the Savian vesicles. This difficulty is overcome if, instead of 
there being a cavity closed on all sides, the macula is situated in 
a tube. Then interference would either not result at all, or be - 
greatly diminished, for the vibrations would be dispersed in one 
or two definite directions. The formation of such a structure 
may have been the first step in the evolution of the ear. Indeed, 
its parallel is presented to us in the lateral lines. The very same 
conditions obtain. 
The question now arises, when the area containing’ the struc- 
tures about to form the ear became involuted, in what way was 
the function of the tubes converted into the semicircular canals 
modified? In other words, what is the function of the semicir- 
cular canals? The fact that both ends of each canal communicate 
with the utriculus, probably lessens their use as dispersing vibra- 
tions, but at the same time another and very important condition — 
obtains. The utriculus, a comparatively small cavity, has the 
area of its enclosed surface materially diminished by the presence 
of five openings, three of which are comparatively large. In this 
way the capacity of its walls for reflecting vibrations must be con+ 
siderably lessened, and interference of vibration consequently 
much diminished. Thus one function of the semicircular canals 
is probably to assist clearness of perception,’ and in this respect 
their function would be intrinsically the same as that of the canals 
of the side organs. It has at various times been argued that the 
semicircular canals prevent interference of vibrations by the sound 
waves entering the same canal at opposite ends, and then meeting 
in opposite phases, thus mutually destroying each other. How 
this could occur I cannot imagine; there is no special contrivance 
at either end of a canal by which the vibrations would be retarded 
just half of a wave-length, and there is no reason why such an 
assumption should be made. On the contrary, I cannot see any- 
1 This was the idea of Prof, Jackson, formerly of the University of Pennsylvania. 
