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 sur- 

 roundings 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 less moisture, a free or scanty 

 supply of oxygen, the advent of many or few putrefactive organ- 

 isms, — these may quicken or slacken the rate at which energy is 

 being dissipated but do not divert that energy from heat into 

 motion ; whereas in the living body so slight a change of surround- 

 ings 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 replen- 

 ish 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 mate- 

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

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

 muscles, skin, etc., all built up to form the body according to 

 certain morphological laws. But all this complication, though 

 advantageous 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 



