542 .MANUAL OF PHYSIOLOGY. 



tion. Thus, when an edge is placed on our skin, we do not feel a 

 series of points corresponding to the individual fields with which 

 it comes in contact, but the confusion of the stimuli gives rise to 

 an uninterrupted sensation, and we have a right perception of the 

 object touched. 



THE SENSE OF PRESSURE. 



There seems to be a reason for separating the perception of dif- 

 ferences in the degree of pressure exercised by a body from the 

 simple tactile or local impression. If we support a part of the 

 body so that no muscular effort be called into play in the support 

 of an increasing series of weights placed upon the same area of 

 skin, we can distinguish tolerably accurately between the different 

 weights. It has been found that if a weight of about 30 grammes 

 be placed on the skin a difference of about 1 gramme can be recog- 

 nized that is, we can distinguish between 29 and 30 grammes, 

 if they are applied soon after one another. If the weights em- 

 ployed are smaller, a less difference can be detected ; if larger 

 weights are used the difference must be greater, and it appears 

 that the weight-difference always bears the same proportion to the 

 absolute weight used. We can perceive a difference between 7i 

 and 7J, 14^ and 15, 29 and 30, 58 and 60, etc., the discriminating 

 power decreasing in proportion as the absolute degree of stimula* 

 tion increases. 



One of the reasons why the sense of locality is regarded as dis- 

 tinct from that of pressure is that the latter is found not to be 

 most keenly developed in the same parts where the impressions 

 of locality are most acute. Thus judgment of pressure can be 

 more accurately made with the skin of the forearm than the 

 finger-tip, which is nine times more sensitive than the former to 

 ordinary tactile impressions, and the skin of the abdomen has 

 an accurate sense of pressure though deficient in ordinary tactile 

 sensation. 



It has been said above that the weights by which pressure- 

 sense is to be tested should be applied rapidly one after the other. 

 This facts depends upon the share taken in the mental judgment 

 by the function we call memory. In a short time the recollection 



