A. bony labyrinth of human ear (after Sömmering). c, c., the cochlea; s.c., superior semicircular canal; p.c., posterior semicircular canal; h.c., horizontal semicircular canal; a, a, a, their swellings, or ampullæ; f.o., f.r., fenestra ovalis and rotunda (oval and round windows) in the vestibule. B. Diagram of semicircular canal to illustrate effect of rotation. The large arrows indicate the direction of the rotation. The small arrow to the left indicates the resulting flow of the inner fluid into the ampulla; that to the right, the flow of the outer fluid into the vestibule.] From Professor Crum Brown's paper in_ Nature_ I transcribe, with some verbal modifications, his account of how the semicircular canals enable us to feel these changes of motion. Let us consider the action of one canal. If the head be rotated about a line at right angles to the plane of the canal, with the ampulla leading, there will be a tendency for the fluid within the sac to flow into the ampulla, and for the fluid around the semicircular canal to flow into the cavity in which the sac lies. These movements will conspire to stretch the membranous ampulla, and thus to stimulate the hair-cells. This stretching will not take place in that canal if the rotation be in the reverse direction. But on the opposite side of the head is another canal in the same plane, but turned the other way. In the reversed rotation the ampulla in this canal will lead, and its hair-cells will be stimulated. Thus by means of the two canals on either side of the head in the same plane, rotation in either direction can be appreciated. And since there are two other pairs of semicircular canals in two other planes, rotation in any direction will be recognized by means of one or more of the six canals. It is thus by means of the semicircular canals that we can appreciate acceleration of rotatory motion.[FB] But we can also appreciate acceleration of movements of translation--forwards or backwards, up or down. And Professor Mach has suggested that it is through the stimulation of the hair-cells in the patch in the sac itself (the so-called _macula acustica_) that we are able to appreciate these changes. The otoliths, held loosely and lightly in position by the gelatinous substance in which they are embedded, may, through their inertia, aid in the stimulation of the sense-hairs.