THE SENSE OF EQUILIBRIUM. 405 



sibility, as in locomotor ataxy, cannot stand with eyes closed, and his move- 

 ments, even when sight is employed, are exaggerated and unnatural. Attention 

 has previously been called to the fact that air-waves, irrespective of those 

 producing sound-sensations, exert an influence upon the tympanic membrane 

 by which we are capable of appreciating the presence and, to some extent, the 

 physical character of objects. Whether this sensation involves the nerves of 

 touch, those of common sensibility, or those distributed to the internal ear, is 

 uncertain. 



In the absence of any of these sensations the loss may be made up by more 

 perfect development of others. Ordinarily, the sensory information from all 

 these sources, when compared in consciousness, harmonizes and gives rise to 

 a concrete idea of position. Frequently, however, one of the sources of sense- 

 impression suddenly fails us or its testimony conflicts with that of other sense 

 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 benl 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 eves, 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 



