554 HANDBOOK OF PHYSIOLOGY. 



It is not. however, certain that our idea of the 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 

 degree 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 or muscular action, or of nervous energy, which we had ex- 

 erted being insufficient. 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 mental 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 particu- 

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

 suspect 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 un- 

 affected. 



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 

 by a thermometer. All that we can with safety infer from our sensations 

 of temperature, is that a given object is warmer or cooler than the skin. 

 Thus the temperature of our skin is the standard; and as this varies from 

 hour to hour according to the activity of the cutaneous circulation, our 

 estimate of the absolute temperature of any body must necessarily vary 

 too. If we put the left hand into water at 40 F. and the right into 

 water at 110 F., and then immerse both in water at 80 F., it will feel 

 warm to the left hand but cool to the right. Again, a piece of metal 



