TOUCH 



2 5 



sensations of which we can form little if any idea. And even 

 when with reasonable certainty we can correlate sense-organs 

 possessed by such animals with some of our own, it by no means 

 follows that the range of a given sensation is the same for one 

 of them as for ourselves. As regards hearing, for example, 

 there is reason to think that some animals can hear sounds which 

 are pitched much higher than any by which we are affected; nor 

 is this very surprising when we reflect that the range of hearing 

 is not the same in all human beings. Many persons, for example, 

 cannot hear the high and piercing sounds made by Bats. These 

 remarks are made as a warning against applying the results of 

 human physiology to lower animals with too great assurance. 



TOUCH 



Undoubtedly the most primitive of all the senses is that of 

 Touch, and we may broadly state that the skin is the Tactile 

 Organ, remembering that its outer layer, commonly known as 

 the epidermis, is no other than the ectoderm or outer cell-layer 

 of the embryo. We must also include here the cellular lining 

 of the mouth-cavity and, when such exist, the nasal cavities, 



St.- 



-r. 



-Gl* 



Hrain 



Fig. 1029. Tactile Organs. A, Cells from the ectoderm of a Sea-Anemone (Actinia,}; T., a touch-cell, with outer 

 end produced into a stiff process; St., stinging cell, with sensitive trigger-hair (Tr,}; Gl. t glandular cell. B, Head of 

 a Freshwater Annelid (Bohemilla comata], seen from above, and showing epidermis in optical section, enlarged; 

 T., tactile processes of some of the epidermic cells, which are continuous internally with nerve-fibres. 



since these have been developed as in-pushings of the ectoderm. 

 The external agents of stimuli which by their action upon the 

 skin evoke sensations of touch are of two sorts. There are, in 

 the first place, mechanical agents, such as contact or pressure, 

 and, in the second place, heat-rays. The sensations which result 

 are respectively known as haptic and thermal. 



