166 HAND-BOOK OF PHYSIOLOGY. 



The estimate of weight derived from a combination of these two sensa- 

 tions (as in lifting a weight) is more accurate than that derived from the 

 former alone (as when a weight is laid on the hand) ; thus Weber found 

 that by the former method he could generally distinguish 19J- oz. from 

 20 oz., but not 19f oz. from 20 oz., while by the latter he could at most 

 only distinguish 14 oz. from 15 oz. 



It is not the absolute, but the relative, amount of the difference of 

 weight which we have thus the faculty of perceiving. 



It is not, however, certain, that our idea of amount of muscular force 

 used is derived solely from sensation in the muscles. We have the 

 power of estimating very accurately beforehand, and of regulating, the 

 amount of nervous influence necessary for the production of a certain de- 

 gree of movement. When we raise a vessel, with the contents of which 

 we are not acquainted, the force we employ is determined by the idea we 

 have conceived of its weight. If it should happen to contain some very 

 heavy substance, as quicksilver, we shall probably let it fall; the amount 

 of muscular action, or of nervous energy, which we had exerted being in- 

 sufficient. The same thing occurs sometimes to a person descending stairs 

 in the dark; he makes the movement for the descent of a step which does 

 not exist. It is possible that in the same way the idea of weight and 

 pressure in raising bodies, or in resisting forces, may in part arise from 

 a consciousness of the amount of nervous energy transmitted from the 

 brain rather than from a sensation in the muscles themselves. The men- 

 tal conviction of the inability longer to support a weight must also be 

 distinguished from the actual sensation of fatigue in the muscles. 



So, with regard to the ideas derived from sensations of touch combined 

 with movements, it is doubtful how far the consciousness of the extent 

 of muscular movement is obtained from sensations in the muscles them- 

 selves. The sensation of movement attending the motions of the hand is 

 very slight; and persons who do not know that the action of particular 

 muscles is necessary for the production of given movements, do not sus- 

 pect that the movement of the fingers, for example, depends on an action 

 in the forearm. The mind has, nevertheless, a very definite knowledge 

 of the changes of position produced by movements; and it is on this that 

 the ideas which it conceives of the extension and form of a body are in 

 great measure founded. 



(c) Temperature. The whole surface of the body is more or less 

 sensitive to differences of temperature. The sensation of heat is distinct 

 from that of touch; and it would seem reasonable to suppose that there 

 are special nerves and nerve-endings for temperature. At any rate the 

 power of discriminating temperature may remain unimpaired when the 

 sense of touch is temporarily in abeyance. Thus if the ulnar nerve be 

 compressed at the elbow till the sense of touch is very much dulled in 

 the fingers which it supplies, the sense of temperature remains quite 

 unaffected (Nothnagel). 



The sensations of heat and cold are often exceedingly fallacious, and 

 in many cases are no guide at all to the absolute temperature as indicated 



