SENSATION 



All these geotropic sensations belong to the same group 

 of barotactile phenomena, as the fall of a stone or any 

 other effect of gravitation that depends on an inorganic 

 feeling of attraction. 



As a result of these adaptations, we find a distinct 

 sense of space developed in the higher, free-moving 

 animals. The feeling of the three dimensions of space 

 becomes an important means of orientation, and in the 

 vertebrates, from the fishes up to man, the three spiral 

 canals in the inner ear are developed as special organs 

 of this. These three semicircular canals, which lie 

 vertically to each other in the three dimensions of space, 

 are the organs of the sensation that guides the move- 

 ments of the head, and, in relation to this, for the 

 normal posture of the body and the feeling of equilibrium. 

 If the three spiral canals are destroyed, the equilibrium 

 is lost; the body totters and falls. Hence, these organs 

 are not of an acoustic, but a static or geotactic charac- 

 ter; and the same may be said of the so-called "auditory 

 vesicles" of many of the lower animals — round vesicles 

 which contain a liquid and a solid body, the otolith. 

 When this body changes its position with the change 

 of posture of the whole frame, it presses on the fine 

 auditory hairs, or delicate terminations of the auscultory 

 nerve, which enters the vesicle. In fact, the sense of 

 equilibrium is often combined with the sense of hearing. 



The perception of noises and tones, which we call 

 hearing, is restricted to a section of the higher, free- 

 moving animals ; if, that is to say, the above-mentioned 

 "auditory vesicles" in the lower animals do not have 

 acoustic as well as static sensations. The specific sensa- 

 tion of hearing is due to vibration of the medium in 

 which the animal lives (air or water) , or to vibrations of 

 solid bodies (such as tuning-forks) which are brought 

 into touch with them. If the vibrations are irregular, 

 they are felt as "noises"; if regular, they are heard as 



3" 



