SENSATION AND SENSE-ORGANS. 493 



mouth and nose), and others (in the skin) to variations in 

 pressure or temperature. 



All our sensations are thus modifications of one common 

 primary sensibility, represented by that of the skin, or rather 

 by the primitive representative of the skin in such an animal 

 as the Hydra (see Zoology). The cutaneous sensations, being 

 less differentiated, shade off more readily into the common 

 sensibility of the other living tissues than do the activities of 

 the highly differentiated cells in the eye and ear. We find, 

 accordingly, that while a powerful pressure or a high tem- 

 perature acting on the skin readily arouses a sensation of 

 pain, that this is not the case with the more specialized visual 

 and auditory organs. Their super-excitement may be dis- 

 agreeable, but never passes into pain, in the ordinary sense 

 of the word. Similarly the special skin sensations, touch 

 and temperature, may sometimes be confounded, while a 

 sound and a sight cannot be : the modality of the less modi- 

 fied skin-senses is less complete. 



The study of comparative anatomy and development 

 shows that the irritable parts of our sense-organs are but 

 special differentiations of the primary external layer of cells 

 which covered the Body when it was very young. Some of 

 these cells become nerve end-organs in the eye, others end- 

 organs in the ear, and so on; while others, less changed, re- 

 main in the skin as organs of touch and temperature; and 

 so, from a general exterior surface responding equally readily 

 to many external natural forces, we get a surface modified so 

 that its various parts respond with different degrees of read- 

 iness to different external forces; and these modified parts 

 constitute the essential portions of our organs of special sense, 

 ^very sense organ thus comes to have a special relationship to 

 some one natural force or form of energy is a specially 

 irritable mechanism by which such a force is enabled to excite 

 sensory nerves; and is, moreover, commonly supplemented by 

 arrangements which, in the ordinary circumstances of life, 

 prevent other forces from stimulating the nerves connected 

 with it. Not all natural forces have sense-organs with ref- 

 erence to them developed in the Human Body; for example, 

 we have no organ standing to electrical changes in the same 

 relation that the eye does to light or the ear to sound. 



The Essential Structure of a Sense-organ. In every 

 sense-organ the fundamental part is one or more end-organs, 



