148 VETERINARY PHYSIOLOGY 



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. 74, 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. 74, b). 



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

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

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

 arc of the circle. 



If, for example, the head is suddenly turned to the right, 

 the inertia of the endolymph and perilymph tend to make 



FIG. 74. (a) Arrangement of the semicircular canals on the two sides ; (b) bony 

 and membranous canal and ampulla to illustrate their mode of action. 



them lag behind. Thus the endolymph in the ampulla of the 

 left horizontal canal will tend to flow into the canal, but the 

 canal is so small that it will merely accumulate in the ampulla, 

 and thus a high pressure will be produced (fig. 74, b + +). 

 The perilymph will tend to lag behind, and a low pressure will 

 result outside (fig 74, 6 ). The converse will take place in 

 the opposite horizontal canal. 



When the movement is continued the pressures will be 

 readjusted, and, on stopping the movement, the opposite con- 

 ditions will be induced, and a sensation of moving in an 

 opposite direction will be experienced. 



In forward movement, the two superior canals have the 

 pressure of endolymph increased in their ampullae in backward 

 movement this occurs in the two posterior canals. In nodding 

 to the right the superior and posterior canals of the right 

 ear undergo this change. In all probability the utricle with 

 its otoliths mounted on the top of the hair cells also plays a 



