432 Dynamic Theory. 



pressure and the muscular sense almost always accompany each other, 

 because they are almost always exercised by the same act. The abso- 

 lute sense of pressure is very vague, because it depends so much on the 

 surface covered by the weight. The body lifted always seems heavier 

 if the part by which we hold it presses upon a smaller surface of skin. 

 E. H. Weber's experiments showed that when both muscle sense and 

 pressure sense were combined in lifting a weight, a difference could be 

 felt between 19|- and 20 oz. , or one fortieth, or the addition of -^-. But 

 where a weight was laid upon the skin so as not to call upon muscular 

 exertion, only the difference between 14^- and 15 ounces could be per- 

 ceived, or the addition of ^ ; the same proportion held for other 

 weights. The perception of differences depended upon the rapid succes- 

 sion of their application, one after the other. The memory could not 

 hold the first weight more than 10 seconds. After 30 seconds between 

 the application of the weights the sense could not discriminate if the 

 difference were less than i of the first weight. The sense or memory of 

 the impression vanishes thus rapidly from the brain. This could, no 

 doubt, be improved by practice. The sense of pressure does not accom- 

 pany the delicacy of the sense of touch. The skin of the forehead and 

 stomach have a delicate sense of pressure, but only a dull sense of localit}'. 



Our sense of temperature is entirely relative ; we call a thing warm 

 which transfers heat to us, and cold which conducts it away. Of two 

 objects of the same temperature, we call one warm and the other cold. 



Weber found that with the finger he could perceive a difference when 

 it amounted to half a degree Fahrenheit, and this proportion of rela- 

 tive difference could be discovered up to blood heat. The sensitiveness 

 of different parts depends largely upon the thickness of the skin, the 

 thin skin being the most sensitive, for the obvious reason above adverted 

 to, that the extra thickness of skin is in the horny, nerveless scales of 

 the epidermis, built in front of the nerve-bearing papillae by excessive 

 stimuli for their own use the use, that is, of the excessive stimuli. So 

 that the cuticle in such parts is no longer so easily affected by moder- 

 ate stimuli. The back of the hand, elbow, eyelids, lips and tongue, are 

 the most sensitive. Weber found that warm bodies appear lighter than 

 cold ones. A cold coin placed upon the forehead appeared as heavy as 

 two warm ones afterward placed on the same spot ; as if the contraction 

 or pressing together of the epidermal cells, in consequence of the cold- 

 ness communicated from one coin, was equal to the pressing together of 

 the same cells b} r their weight. 



There is probably no animal, however small or simple, that is desti- 

 tute of the sense of touch. The jelly-fishes, Medusae, have long tenta- 

 cles, very sensitive, which contract from a slight touch. Some species 

 have tactile hairs on the margin of their umbrella-like bodies. These 



