466 THE HUMAN BODY. 



we gather by it how much our various muscles are con- 

 tracted: and so learn the position of various parts of the- 

 Body, on the one hand, and the resistance opposed to bodily 

 movement by external objects, on the other. In fact, we 

 cannot draw a sharp line between the special senses and 

 common sensations: all the Body, we conclude from ob- 

 servations on the lower animals, is, at an early stage of its 

 development, sensitive; very soon its cells separate them- 

 selves into an outer layer exposed to the action of external 

 forces and an inner layer protected from them : and some 

 of the former cells become especially sensitive. From them, 

 as development proceeds, some are separated and buried be- 

 neath the surface to become the brain and spinal cord; of 

 those which remain superficial, some are modified so that 

 they (in the eye) become especially excited by ethereal vi- 

 brations; others (in the ear) become especially responsive 

 to sound vibrations; others to slight chemical changes (in 

 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 sen- 

 sations, 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 pres- 

 sure or a high temperature 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-ex- 

 citement maybe disagreeable, but never passes into pain, in 

 the ordinary sense of the word. Similarly the special skin 

 sensations, touch and temperature, may sometimes be con- 

 founded, while a sound and a sight cannot be: the mo- 

 dality of the less modified skin-senses is less complete. 



The study of comparative anatomy and development shows 

 that the irritable parts of our sense-organs are but special dif- 

 ferentiations of the primary external layer of cells covering 

 the Body when it is very young. Some of these become nerve 



