THE MACHINERY OF COORDINATED MOVEMENTS. 645 



Injury to the bony canals alone is insufficient to produce the symptoms ; 

 the membranous canals themselves must be divided or injured. The cha- 

 racteristic movements of the head may, however, be brought about in a bird 

 without opening the bony canal, by suddenly heating or cooling a canal, 

 especially its ampullar terminations, or by the making or breaking of a con- 

 stant current directed through the canal. 



There can be no doubt that these characteristic movements of the head 

 are the result of afferent impulses started in the nervous endings of the 

 auditory nerve over the ampulla of the canal and conveyed to the brain 

 along that nerve. And that injury to or other stimulation of each of the 

 three canals should produce in each case a different movement of the head, 

 the direction of the movement being different according to the plane in 

 which the canal lies, shows that these impulses are of a peculiar nature. 

 This is further illustrated by the following experiment : If the horizontal 

 canal be carefully laid bare, and the membranous canal opened so as to ex- 

 pose the endolymph, blowing gently over the opened canal with a fine glass 

 canula will produce a definite movement of the head, which is turned to the 

 one side or to the other, according as the current of air drives the endolymph 

 toward or away from the ampulla. From this it is inferred that a movement 

 of the endolymph over, or an increased pressure of the endolymph on, the 

 nervous endings in the ampulla gives rise to afferent impulses which in 

 some way determine the issue of efferent impulses leading to the movement 

 of the head. It is further suggested that since the planes of the three canals 

 lie in the three axes of space, any change in the position of the head must 

 lead to changes in the pressure of the endolymph on the walls of the ampul- 

 lae or to movements of endolymph over those walls, and so must give rise 

 to impulses passing up the auditory nerve ; and that since every change of 

 position will affect the three canals differently (whereas, the changes of pres- 

 sure of the endolymph involved in a " wave of sound " will affect all three 

 ampullae equally), those impulses will differ according to the direction of 

 the change. A still further extension of this view supposes that since in 

 any one position of the head the pressure of the endolymph will differ in 

 the three ampullae, mere position of the head, as distinguished from change 

 of position, is adequate to generate afferent impulses differing in the differ- 

 ent positions. 



Let us now for a while turn aside to ourselves and examine the coordina- 

 tion of the movements of our own bodies. When we appeal to our own con- 

 sciousness we find that our movements are governed and guided by what 

 we may call a sense of equilibrium, by an appreciation of the position of our 

 body and its relations to space. When this sense of equilibrium is dis- 

 turbed we say we are dizzy, and we then stagger and reel, being no longer 

 able to coordinate the movements of our bodies or to adapt them to the 

 position of things around us. What is the origin of this sense of equilib- 

 rium ? By what means are we able to appreciate the position of our 

 body ? There can be no doubt that this appreciation is in large measure 

 the product of visual and tactile sensations ; we recognize the relations of 

 our body to the things around us in great measure by sight and touch ; we 

 also learn much by our muscular sense. But there is something besides 

 these. Neither sight nor touch nor muscular sense can help us when, placed 

 perfectly flat and at rest on a horizontal rotating table, with the eyes shut 

 and not a muscle stirring, we attempt to determine whether or not the table 

 and we with it are being moved, or to ascertain how much it and we are 

 turned to the right or to the left. Yet under such circumstances we are 

 conscious of a change in our position, and some observers have been even 

 able to pass a tolerably successful judgment as to the angle through which 



