848 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



of the other side. Any possible movement of the head would thus produce 

 an increase of endolymph-pressure upon the hair-cells in one ampulla and a 

 decrease of pressure in the ampulla of the parallel canal, and every change of 

 position would be accompanied by the irritation of definite ampullae with defi- 

 nite degrees of excitement (Fig. 289). Experiments on man afford considerable 



support to this theory of the function of the 

 semicircular canals. A person with eyes closed 

 and with muscular and tactile sensations elimi- 

 nated, supported on a table which can be rotated 

 in all directions, can determine with consider- 

 able accuracy not only that he is moved, but 

 in what direction and, to some extent, through 

 how great an angle. Further, when brought 



FIG 289. Diagrammatic horizontal . . t . , 



section through the head to illustrate to rest after a series of rotations the person 

 the planes occupied by the semicircu- un( j er observation feels a sensation of motion 



lar canals (after Waller) : s, superior . 



canal ;P, posterior canal ;H, horizontal m the Opposite direction. JLach ot these T6- 



canal - suits should be expected to follow were the 



theory in question correct. The observations of James have shown that 

 with deaf mutes in whom the internal ear was at fault rapid rotation in 

 an ordinary " swing " failed to produce the dizziness which is the common 

 effect in ordinary individuals. On the other hand, diseases which may be sup- 

 posed to alter the intra-labyrinthine pressure are characterized by the symp- 

 toms of vertigo and inco-ordination of movement. The presumable effect of 

 cutting the semicircular canals is that the escape of endolymph changes the 

 pressure upon the sensory hair-cells and gives the animal the sensation of 

 falling in one direction or another, so that he is impelled to make compensa- 

 tory or fwced movements to counteract this imaginary change of position. In 

 birds and in fishes, whose life is passed more or less exclusively in a medium 

 in which tactile and muscular sensation can contribute little to the sense of 

 equilibrium, the semicircular canals are especially well developed. 1 In fishes, 

 though section of the canals themselves produces no disturbance, division of 

 the nerves supplying the ampullae usually gives rise to marked forced move- 

 ments, as shown in somersaults, spiral swimming, etc. when set free in the 

 water. When, however the nerves are cut with great care, with sharp scis- 

 sors, so as to avoid traction on or crushing of the nerves, such forced move- 

 ments do not follow. The movements in this case, then, as in that of the 

 pigeon, are the outcome of direct irritation of the equilibrium mechanism, 

 and, according to our present conception of the function of the semicircular 

 canals in its relation to equilibrium, we must regard it as a terminal organ 

 which is exceedingly sensitive to such mechanical irritations as may arise from 

 variations of endolymph-pressure upon the ampullary hair-cells, but which 

 may be destroyed without causing inco-ordination of movement, and which 

 may therefore more or less completely be substituted in function by other 

 sense organs. 



1 Sewall : Journal of Physiology, 1884, iv. p. 339. 



