SEMICIRCULAR CANALS 200 



The maculae are so placed that when the head (of the rabbit) is 

 held in its normal position they lie with their hairs vertical, 

 bearing the otoliths above them. Now. if the rabbit is held so 

 that the head has turned through 180 degrees and the macular 

 hairs are again vertical, but the otoliths arc now hanging from them, 

 it has been found that the limbs are extended maximally. That is, 

 when the otolith in each macula presses against the hairs, one has 

 minimal limb tonus, and, conversely, when the otolith in each 

 macula pulls on the hairs, one has maximal tonus. Degrees of 

 tonus may be obtained by degrees of rotation. 



Saccules. The saccule of each ear has its macula on the inner 

 lateral wall, i.e. in the normal position of the head (of the rabbit) 

 the hairs will be horizontal and pointing in opposite directions in 

 each ear. The macula of the saccule is least stimulated when the 

 otolith presses against the hairs, and undergoes maximal stimulation 

 when the otolith hangs vertically from the hairs (cf, utricle). 

 That is, e.g. when the head is hanging over towards the left side of 

 the animal, the right saccule will receive minimal and the left 

 saccule maximal stimulation, producing an asymmetric alteration 

 of muscle tonus, the extensor muscles of the limbs on the left side 

 having their tone increased and those on the right side undergoing 

 diminution of extensor tone, whereby a " righting " action is 

 developed (Fig. 68). 



Semicircular Canals. These structures, of which there are six, 

 are arranged so that the three on each side are in three planes at 

 right angles to one another. The two canals, which lie externally 

 in each ear, are situated in a plane which is almost horizontal in 

 the erect position of the human head. The two other pairs of 

 canals are, therefore, vertical, as they are at right angles to the 

 external canals. The two groups of three canals each are set, as 

 it were, back to back, i.e. mirror images of one another, so that 

 any rotatory movement of the head will tend to produce equal and 

 opposite movements of the fluid pressure in the canals and their 

 membranous ducts. The ducts running within the osseous canals 

 occupy about one-fourth of the volume of the canals except where 

 they are widened to fill the ainpullae, which are bulbous cavities 

 at one end of each canal (anterior end of external canals and 

 external ends of anterior and posterior canals). The bony ampullae 

 are about twice the diameter of the canals, and, therefore, each of 

 the membranous ducts undergoes an 800 per cent, enlargement. 

 On the concave side of each membranous ampulla is a thickened 

 ridge rising almost to the axis of the duet and covered with 

 columnar cells forming the crista ampidlaris. From the surface of 

 the crista project long flexible hairs, thicker and more bristle-like 



