406 AN AMERICAN TEXT-BOOK OF I'll VSIOLOGY. 



of a chicken whose head lias been cut off. Essentially the same results have 

 been determined to follow injury of the semicircular canals of widely different 

 groups of animals. 



These results have l>c<n explained by the assumption that the hair-cells on 

 the crista <icnxtic<r. of the ampulla of the semicircular canals are irritated by 

 increase or decrease of pressure of the endolymph upon them, and thus give 

 rise to sensory impressions from which ideas of change of position are derived. 

 Section of the canal, by draining off the endolymph, would cause abnormal 

 pressure-irritation. The anatomical relations of the semicircular canals afford 

 an obvious basis for this view, for the canals of each ear are almost exactly at 

 right angles to one another, occupying the three planes of space ; considering 

 the two ears, the horizontal canals are nearly in the same plane, and the ante- 

 rior vertical canal of one side i- nearly parallel with the posterior vertical canal 

 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 ainpullse with defi- 

 nite degrees of excitement (Fig. 202). 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 

 1 how great an angle. Further, when brought 



Fig. 202. — Diagrammatic horizontal . . 



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

 the planes occupied ».>• the semicircu- un( ] er observation feels a sensation of motion 



lar canals (after Waller): s, superior p 



canal ; p, posterior canal ; h, horizontal in the opposite direction. Each of tliese re- 

 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 forced 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 

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



