THE NERVOUS SYSTEM 165 



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 demon- 

 strated 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, while stopping the 

 table gives rise to a sensation of being rotated in the opposite 

 direction. 



The semicircular canals form a mechanism which is 

 capable of acting in this way. They are arranged in pairs 

 in the two ears thus 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. 89, a). 



FIG. 89. (a) Arrangement of the Semicircular Canals on the two sides ; (6) Bony 

 and Membranous Canal and Ampulla to illustrate their mode of action. 



The horizontal canals may be considered as forming the 

 arc of a circle with an ampulla at each end. The superior 

 canal of one side has its ampulla in front, while its twin the 

 posterior of the opposite side has its ampulla behind, and 

 they together form the arc of a circle with an ampulla at 

 each end (Fig. 89, a). 



The membranous canals are very narrow, and occupy but 

 a small part of the osseous canals. The membranous 

 ampullae are large and almost fill the osseous ampullae 

 (Fig. 89, 6). 



If the head is moved in any plane, certain changes will be 

 set up in the arnpullse towards which the head is moving, 

 and converse changes in the ampullae at the other end of the 

 arc of the circle. 



