THE SENSE OF EQUILIBRIUM. 847 



organs ; the result is disturbance of equilibrium. A very common outcome 

 of this conflict of sensations is dizziness or nausea. The distress arising from 

 wearing ill-fitting glasses and the sensations experienced when one looks down 

 from a high eminence are examples in point. Internal disorders exciting nerves 

 of common sensation have the same effect, though the relation borne by visceral 

 sensations to equilibrium is very ill known. A false idea of position of the 

 body, a sense of falling in one direction or another, may lead to sudden effort 

 of recovery by which the person is precipitated to the opposite side. Thus, 

 when looking at rapidly-moving water erroneous ideas of equilibrium are 

 gained through the visual sense, and there is a strong tendency for the body 

 to precipitate itself in one direction or another. When, in going up a stair- 

 case, one miscalculates the number of steps, a peculiar sensation of want of 

 equilibrium is aroused through the muscular sense. It is clear, then, that 

 the sense of equilibrium is served by various sense organs, and a complete 

 discussion of this function would entail a consideration of the whole field of 

 nerve-muscle physiology. There is, however, good reason for believing that 

 there is a special sense organ for determining the position and direction of 

 movement of the head and, by inference, of the whole body. The terminal 

 organ of this sense apparatus of equilibrium is found in the system of semi- 

 circular canals of the internal ear. 



Experiments on the lower animals, chiefly performed on birds, show a con- 

 stant motor disturbance to follow division of any or all of the semicircular 

 canals. These disturbances are of two kinds. When the animal is at rest it 

 does not stand in a natural fashion, but sprawls in a more or less exaggerated 

 degree. It holds its head in an unnatural position, as with the vertex touch- 

 ing the back, or with the beak turned down toward the legs or bent over to 

 one side. Immediately after the operation, and whenever it is disturbed, the 

 animal goes through peculiar forced movements, together with rolling or 

 twitching of the eyes, of various kinds and degrees of violence, depending on 

 the position and number of canals severed. The disturbance varies from 

 simple unsteadiness in gait, with swaying motions of the head, to complete 

 lack of co-ordination and a violence of movement almost comparable to that 

 of a chicken whose head has 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 been explained by the assumption that the hair-cells on 

 the cristce acusticce 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 is nearly parallel with the posterior vertical canal 



