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PHYSIOLOGY FOR DENTAL STUDENTS. 



however, evident that there must be some definite arrangement 

 of the tubes. This is provided for in the disposition of the semi 

 circular canals in three planes, namely, a horizontal and two 

 vertical (Fig. 51). Taken together the three canals form a struc- 

 ture which looks somewhat like a chair, the horizontal canals 

 being the seat of the chair and the two vertical canals joining 

 together to form its back and arms. The back of each chair is 

 directed inwards so that they are back to back. At one end of 

 each canal is a swelling, the ampulla, in which the sensory nerve 



Fig. 51. The semicircular canals of the ear, showing their arrangement 

 in the three planes of space. (From Howell's Physiology.) 



apparatus above described is .located. It is evident that when 

 the head is moved in any direction the fluid in some of these 

 canals will be set in motion. It is this movement of the fluid 

 which stimulates the hair cells. That this is really the function 

 of the semicircular canals is proven by the fact that if they ;uv 

 irritated or destroyed, grave disturbances occur in the bodily 

 movements. This is what occurs in Meniere's disease, in which 

 attacks of giddiness, often severe enough to cause the patient 

 to fall, and accompanied by extreme nausea, are the chief symp- 

 toms, the lesion being a chronic inflammation involving the 



