The Senses of Animals. 269 



In briefly describing the auditory apparatus in man, mention 

 was made of three curved membranous loops, the so-called 

 semicircular canals. A few more words must now be said 

 about them and the membranous sac with which they are 

 connected. 



The sac lies in a somewhat irregular cavity in a bone 

 at the side of the head, in the walls of which are five 

 openings leading into curved tunnels in the bone in which 

 lie the membranous loops. The planes in which the three 

 semicircular canals lie are nearly at right angles to each 

 other, and they are called respectively the horizontal, 

 the superior, and the posterior. The two latter unite at 

 one end before they reach the sac ; hence there are five, and 

 not six, openings into the cavity. At one end of each semi- 

 circular canal is a swelling, or ampulla, in each of which is 

 a ridge, or crest, abundantly supplied with hair-cells. And 

 in a little recess in the sac there is, occupying its floor, its 

 front wall, and part of its outer wall, a patch of hair-cells 

 covered by a gelatinous material with numerous small 

 crystalline otoliths. The only other point that calls for 

 notice is that the membranous sac does not fit closely in 

 the bony cavity in which it lies, while the diameter of the 

 membranous semicircular canals is considerably less than 

 that of their bony tunnels, except at the ampullae, or swellings, 

 where they fit pretty closely. Both the bony cavity and the 

 membranous labyrinth (as it is called) are filled with fluid. 

 From its close connection with the organ of hearing, 

 this apparatus was for long regarded as in some way 

 auditory in its function, and it was surmised that it enabled 

 us to perceive the direction from which the sound came. 

 But how it could do so was not clear. In 1820 M. 

 Flourens made the observation that the injury or division 

 of a membranous canal gave rise in the patient to rotatory 

 movements of the animal round an axis at right angles to 

 the plane of the divided canal ; and he, therefore, suggested 

 that the canals might be concerned in the co-ordination of 

 movement. They are now regarded as the organs of a 

 sense of rotation or acceleration. 



