SENSIBILITY OF THE INTERNAL OEGANS 119 



or left, so long as the angular velocity is increasing. When the 

 velocity becomes uniform this sensation gradually disappears. 

 When the speed diminishes the subject has an illusory sensation of 

 rotating in the opposite direction. This illusory sensation reaches 

 its maximum intensity when the rotation ceases, lasts for a few 

 seconds after it has stopped, and disappears directly the movement 

 recommences. If the bandage is taken off the eyes during the 

 sensations of vertigo, visual vertigo will be substituted for the 

 subjective sensation of rotation, so that all the objects in the 

 environment ' seem to turn in the same direction. This post- 

 motor visual vertigo is due to nystagmus of the eyes, as Purkinje 

 recognised. 



If the rotatory movements are made with the head inclined 

 sideways, upward, or downwards, there will be a sensation of 

 turning, not on a horizontal plane, but on a plane vertical to the 

 axis of the head. If, during rotation of uniform velocity, when all 

 sense of movement is lacking, the head moves from the vertical 

 to an inclined position, the sense of rotation reappears, but in the 

 inclined plane that corresponds to the degree and direction of the 

 inclination of the head. These phenomena prove that the organs 

 by means of which the rotatory movements are perceived lie in 

 the head. 



In addition to these phenomena, there is seen during the 

 acceleration of the rotatory movements not only in man, but also 

 in mammals and birds blind or blindfolded nystagmus of the 

 head and eyes in the direction opposite to the movement, which 

 becomes less and ceases as the angular velocity becomes uniform. 

 When the rotatory movement is slowed down and stops, the 

 nystagmus of the head and eyes reappears, but in the direction of 

 the plane of rotation, lasting for some seconds and accompanied 

 by oppression in the head, vertigo, tendency to vomiting, etc. 

 These effects, as Breuer showed, are very similar to those above 

 described after section or removal of the semicircular canals. 



To understand this fact it is necessary with Breuer, Mach, and 

 Crum Brown to admit that there is in the head a sense-organ 

 that is excited by its rotatory movements in various planes, and 

 the only organs that can reasonably be credited with this office 

 are the semicircular canals, owing to their arrangement in three 

 planes vertical to each other. 



This theory finds confirmation in the observations .of James, 

 Kreidl, and Bruck on deaf-mutes. According to these observa- 

 tions, in deaf-mutes (in a proportion corresponding to that in 

 which the semicircular canals are so much altered that it is 

 impossible that they should function) arrest of passive rotation 

 with the eyes bound does not produce rotatory vertigo, and 

 nystagmus of the eyes and head is absent at the commencement of 

 rotation. 



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