366 



ADVANCED PHYSIOLOGY 



not a simple one, but from its main central portion (Fig. 181) 

 three tubes lead out into the surrounding bone in different 

 directions, make half circle turns and come around into the 

 central cavity again. On the posterior side of this cavity a 

 longer canal, twisted into a spiral form, goes out into the bone. 

 In this curiously shaped cavity and floating in its fluid are 

 two thin-walled sacs; the larger is called the utricle and is 

 connected with the smaller, the saccule, by a slender canal. 

 From the utricle membranous tubes, called the semicircular 

 canals, run through the three half circular canals noted above. 

 Extending from the other sac, i.e. the saccule, and following 

 the course of the spiral tube in the bone, is another mem- 

 branous canal. This spiral bony canal and its contained 

 membranous canal are together called the cochlea. The 

 utricle, the saccule and all the tubes in connection with them 

 are filled with a clear, watery fluid, the endolymph. 



FUNCTION OF THE SEMICIRCULAR CANALS 



.-Posterior Semicircular Canal 

 { ^External 



\ \ Superior^ 



'ochlea 



Saccule 

 Utricle 



Ampulla 

 FIG. 182 A. DIAGRAM 



OF EAR PARTS 



The ear is usually thought of 

 as an organ of hearing rather 

 than one of balancing, but the 

 semicircular canals are con- 

 cerned with the latter function. 



Figure 182A shows that each 

 canal has a swelling, an am- 

 pulla, at one end near where it 

 leaves the utricle. The inner 

 structure of an ampulla is 

 shown in Figure 183. 



On one side of the ampulla is a ridge, and on top of the 

 ridge are a number of hair-like projections, among which are 

 lime granules. The direction in which the canals run should 

 also be noted. No two pass around through the bone in the 

 same plane; one lies approximately in a horizontal plane, 

 another in a vertical plane, in a right-to-left direction, and the 



