NEURO-MUSCULAR MECHANISM 147 



the pitch of a note and the existence of over-tones. The fibres 

 of the basilar membrane may be compared to the strings of a 

 piano, each one of which, or each set of which, will be made to 

 vibrate by a particular note. 



D. PROPRIO-CEPTIVE MECHANISM OF THE HEAD 

 Labyrinthine Sense 



Just as the reflex response of the limbs to external stimuli 

 leads to a stimulation of the proprio-ceptive mechanism in the 

 muscles and joints which plays an important part in guiding the 

 subsequent movement (see p. 107), so the reflex response to visual 

 stimuli in the muscles of the head and neck brings into play a 

 delicate proprio-ceptive mechanism developed from the internal 

 ear which has an important action in guiding the movements 

 of the body as a whole, and very specially in guiding the co- 

 ordinated movements of the eye muscles. 



This is the labyrinthine mechanism consisting of the utricles 

 and the semi-circular canals. 



The structure of this mechanism has been described on p. 142. 



The mode of action may be analysed by a study of the 

 sensations which accompany its activity. 



That there is no special mechanism making us aware of 

 uniform movement is proved by the fact that we are not 

 conscious of whirling through space with the earth's surface, 

 and that in a smoothly running train we lose all sense of 

 forward movement. It is only as the train starts or stops 

 that we have a sensation of movement or retardation. The 

 same thing has been demonstrated by strapping a man to a 

 table rotating smoothly round a vertical axis and setting the 

 table spinning. A sense of rotation is experienced as the 

 table starts, but is lost when the movement becomes uniform. 

 Stopping the table gives rise to a sensation of being rotated 

 in the opposite direction. 



The semicircular canals are the mechanism which act in 

 this way. They are arranged in pairs in the two ears. The two 

 horizontal canals are in a horizontal plane, the superior canal 

 of one side and the posterior canal of the other are in parallel 

 planes oblique to the mesial plane of the body (fig. 74, a). 



The horizontal canals may be considered as forming the arc 



