of Edinburgh, Session 1873-74. 
257 
one direction about one axis. But we have six semicircular canals, 
three in one ear and three in the other, and these are arranged 
in pairs — the two exterior being nearly in the same plane, and 
the superior in one ear being nearly parallel to the posterior in 
the other. We have thus a system of three rectangular axes, 
each axis having two semicircular canals at right angles to it, — 
one influenced by rotation in one direction about the axis, the 
other by rotation in the opposite direction. Any rotation what- 
ever of the head can be resolved into three rotations, one about 
each of the said three rectangular axes, and will thus in general 
affect three ampullae. If the ampullae affected are known, and 
the amount of pull at each is known, the axis about which rotation 
takes place and the rate of the rotation can be deduced.* 
I am at present engaged in making measurements and experi- 
ments in reference to this inquiry, and hope before long to lay 
a more complete account of the various phenomena before the 
Society. 
* When rotation has continued for some time, friction of the periosteum 
of the bony canals against the perilymph, and fluid friction in the perilymph, 
gives to the perilymph, and, of course, also to the membranous canal, the same 
rotation as the bony canal has ; the perception of rotation will thus cease. 
If we now stop the rotation of the head the bony canal stops, but the peri- 
lymph and the membranous canal move on, and a pull takes place at the 
opposite ends of the semicircular canals, causing a perception of rotation 
round the same axis in the opposite direction. 
The members of the three pairs of semicircular canals are not always 
accurateiy parallel to each other, and in some animals the three axes are not 
accurately at right angles, so that in the most general case we have two 
systems of co-ordinates, not necessarily rectangular, which we may call z, y, z, 
and £, 77, C — © ac h °f these six axes having an organ capable of being influenced 
by rotation about the axis in one direction. But in all cases, as far as I know, 
these six axes and the corresponding organs are so placed that a different 
set of impressions will be produced by each form of rotation, that is, by each 
combination of axis, direction, and rate. 
