DISTRIBUTION OF CUTANEOUS SENSE AREAS 439 



quality depends not upon the nature of the stimukis but 

 upon the particular fiber which is stimulated. In other 

 words, the fibers and end organs which 

 transmit impulses of warmth are as distinct 

 from those which transmit impulses of 

 pressure as they are from those of sight, 

 hearing, and smell. For this reason the 

 old term, touch sensation, has been re- 

 placed by the word cutaneous (skin) sen- j,,<,205._Atouch 

 sations, and the variety of end organs in- corpuscle in a 

 eluded under this head are to be considered corpuscle; jvi 

 as four distinct kinds of sense organs. ^^'^''^ ^^''^^' 



Cutaneous end organs. — The sensory fibers which end in 

 the skin show several varieties of end organs. Some form 

 networks in the outer dermis where the axis cylinders lose 

 their sheaths, and from this network extend out into the 

 epidermis as tiny naked filaments. Others show special- 

 ized structures which may be classified under three heads, 

 as tactile cells, tactile bulbs, or tactile corpuscles. At pres- 

 ent it is impossible to tell by the look of a cutaneous end 

 organ what kind of impulse it will transmit. In other 

 words, unlike the fibers which end in the eyes and ears, we 

 cannot be sure from their structure whether a given form of 

 end organ produces pressure impulses or heat impulses, 

 pain or cold. 



Distribution of cutaneous sense areas. — The most inter- 

 esting fact concerning cutaneous senses is that all parts 

 of the skin are not equally sensitive to all kinds of stimuli. 

 For example, in a given area of skin there will be spots which 

 respond only to pressure, and not to cold or heat. Other 

 spots in the same area will respond to heat and not to pres- 

 sure, cold, or pain. In short it is possible to map any given 



