INTRODUCTION. 3 



energy which ultimately leaves the body as heat exists for a while 

 within the living body in other forms than heat, though eventually 

 transformed into heat. 



The changes in the surroundings affect the dead body at a 

 slow rate and in a general way only, simply lessening or increasing 

 the amount or rate of chemical change and the quantity of heat 

 thereby set free, but never diverting the energy into some other 

 form such as that of movement ; whereas changes in the surround- 

 ings may in the case of the living body rapidly, profoundly and in 

 special ways affect not only the amount but also the kind of energy 

 set free. The dead body left to itself slowly falls to pieces, slowly 

 dissipates its store of energy, and slowly gives out heat ; a higher 

 or lower temperature, more or Jess moisture, a free or scanty supply 

 of oxygen, the advent of many or few putrefactive organisms, these 

 may quicken or slacken the rate at which energy is being dis- 

 sipated but do not divert that energy from heat into motion ; 

 whereas in the living body so slight a change of surroundings as 

 the mere touch by a hair of some particular surface,, may so 

 affect the setting free of energy as to lead to such a discharge 

 of energy in the form of movement that the previously apparently 

 quiescent body may be suddenly thrown into the most violent 

 convulsions. 



The differences therefore between living substance and dead 

 substance though recondite are very great, and the ultimate object 

 of Physiology is to ascertain how it is that living substance can do 

 what dead substance cannot, can renew its substance, and replenish 

 the energy which it is continually losing, and can according to the 

 nature of its surroundings vary not only the amount but also the 

 kind of energy which it sets free. Thus there are two great 

 divisions of Physiology : one having to do with the renewal of 

 substance and the replenishment of energy, the other having to do 

 with the setting free of energy. 



4. Now the body of man (or one of the higher animals) is a 

 very complicated structure consisting of different kinds of material 

 which we call tissues, such as muscular, nervous, connective, and 

 the like, variously arranged in organs, such as heart, lungs, muscles, 

 skin, &c., all built up to form the body according to certain 

 morphological laws. But all this complication, though advan- 

 tageous and indeed necessary for the fuller life of man, is not 

 essential to the existence of life. The amoeba is a living being ; 

 it renews its substance, replenishes its store of energy, and sets 

 free energy now in one form, now in another ; and yet the amoeba 

 may be said to have no tissues and no organs ; at all events this is 

 true of closely allied but not so well-known simple beings. Using 

 the more familiar amoeba as a type, and therefore leaving on 

 one side the nucleus, and any distinction between endosarc and 

 ectosarc, we may say that its body is homogeneous in the sense 

 that if we divided it into small pieces, each piece would be like all 



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