SECTION XIII 

 THE LABYRINTHINE SENSATIONS 



THROUGHOUT almost the whole of the animal kingdom, and in 

 practically all freely moving metazoa, we find a sense-organ 

 which has often been designated as an auditory organ. This 

 organ, which is situated in the integument, is in the form of 

 a small sac generally open to the exterior, and lined by cells pro- 

 vided with hairs and richly supplied with nerves. Resting among 

 the hairs is a small concretion, generally of carbonate of lime, which 

 is known as an otolith. These sacs have generally been regarded 

 as auditory in function, hence the term otolith applied to the con- 

 cretion. The evidence for audition, i.e. the power of appreciating 

 vibrations in the elastic medium surrounding them, is scanty. Thus 

 in fishes this power has been stated to be absent unless the vibrations 

 are of sufficient amplitude to affect the sense-organs of the skin.* 

 On the other hand, there is evidence that these otolith organs are 

 connected with equilibration. Section of the nerves going to them 

 in the crayfish causes disturbance of locomotion. Steinach has 

 succeeded in the crayfish in replacing the concretion by a small particle 

 of iron. The animal's behaviour and movements were perfectly 

 normal until it was brought within a powerful magnetic field. Under 

 the influence of this field the effect of gravity on the iron particle 

 was annulled and replaced by a force of attraction in another direction, 

 and the effect was at once seen as pronounced disorders of locomotion, 

 the animal swimming in an abnormal position. 



From a sac, such as that present throughout the lower animals, 

 the organ of hearing in the higher vertebrata is developed. Arising 

 as a pit in the epiblast in the neighbourhood of the hind-brain, the 

 auditory sac becomes shut off from the exterior, and then, by an 

 outgrowth in various directions, forms the complex membranous 

 labyrinth of the internal ear. This membranous labyrinth, as we 

 have seen, can be divided into two parts, viz. the canalis media 

 of the cochlea in front, and the saccule, utricle, and semicircular 

 canals behind. The canalis media of the cochlea is concerned with 

 the reception and analysis of sound waves. In the lower vertebrates 



* On the other hand, Piper has succeeded in detecting an electrical variation 

 in the eighth nerve of fishes in response to a sound stimulus. 



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