There seems to be a reason for separating the perception of differences 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 recognized - that is, we can distinguish between 29 and 30 grammes, if they are applied soon after one another. If the weights employed 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 7 1/4 and 7 1/2, 14 1/2 and 15, 29 and 30, 58 and 60, etc., the discriminating power decreasing in proportion as the absolute degree of stimulation increases.
One of the reasons why the sense of pressure is regarded as distinct from that of locality is that the former is found not to be most keenly developed in the 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, while the skin of the abdomen has an accurate sense of pressure, though dull to 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 fact depends upon the share taken in the mental judgment by the function we call memory. In a short time the recollection of the impression passes away, and there no longer exists any sensation with which the new stimulation can be compared.
At best we can form but imperfect judgments of pressure by the skin impressions alone. When we want to judge the weight of a body we poise it in the free hand, which is moved up and down so as to bring the muscles which elevate it into repeated action. Hereby we call into action a totally different evidence, namely, the amount of muscle power required to raise the weight in question, and we find we can arrive at much more accurate conclusions by this means. The peculiar recognition of how much muscular effort is expended is commonly spoken of as muscle sense, which may arise from a knowledge of how much voluntary impulse is expended in exciting the muscles to action, but more probably it depends upon afferent impulses arriving at the sensorium from the muscles. By its means we aid the pressure sense in arriving at accurate conclusions of the weight of bodies, so that in the free hand we can distinguish between 39 grm. and 40 grm.