PART VIII 

 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 integu- 

 ment, is in the form of a small sac generally open to the exterior, and lined 

 by cells provided 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 concretion. 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 mem- 

 branous 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 semi- 

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

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

 which auditory sensations are wanting the cochlea is absent, and in fishes 

 is represented merely by a small diverticulum known as the lagena. With 

 the development of air-breathing vertebrates we see the first signs of a special 

 organ of hearing. Thus a primitive cochlea is present in the amphibia, and 

 especially in the anura, and in some of the reptiles as well as in birds it 

 acquires a bend and shows the beginning of a spiral arrangement. Only in 

 the mammals does it attain a degree of development at all comparable with 



* Piper, however, has detected an electrical variation in the eighth nerve of 

 fishes in response to a sound stimulus. 



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