6 THEORETICAL BIOLOGY 



quality of diKsrtion. WTien. as we havne seen, stimulation 

 is applied at different points on the skin, it is easy to show 

 that there are two ditterent kinds of relationship ; tlie quality 

 of direction, however, does not appear. We have only to 

 think of the " pins-and-needles " feeling that comes when 

 a limb " goes to sleep " : we then localise very exactly the 

 indi\'idual pricklings in our skin by means of their different 

 qualities, but we ha\^ absolutely no sensation of direction. 



The sensation of direction comes in onlv when one local 

 sign strikes up while the others most nearly related to it have 

 not yet quite died a\^*ay ; with " pins-and-needles " this does 

 not happen. In " pins-and-needles " we feel tlie change in 

 the quaUty ven* precisely ; but what we call '* motion " is 

 only that change in the quaUty which is connected \^'ith 

 the quality of direction. 



DIRECTION-SIGNS 



To simplify the description, I wiW call that which connects 

 the local signs a direction-sign. It would seem that all the 

 local signs of the skin, wheresoever situated, are invariably 

 connected with their neighbours by two direction-signs only ; 

 we had best call these " transverse-signs *' and '* longitudinal- 

 signs," 



On closer investigation, we observe that the direction- 

 signs have 4 very important property which distinguishes 

 them from all local signs, i.e. they are reversible. With- 

 out losing its character as transveise-sign or longitudinal- 

 sign, every direction-sign wiU sene for " hence " as well as 

 for " hither." 



No quahtj' known to us has this power ; the order in 

 which the qualities appear can be reversed, but the individual 

 quahty itself never can. On that account, it is open to ques- 



