INTRODUCTION. 



19 



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 com- 

 plicated 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, etc., all built up to 

 form the body according to certain morphological laws. But all this com- 

 plication, though advantageous and indeed necessary for the fuller life of 

 man, is not essential to the existence of life. The amoeba [Fig. 1] is a 



[FIG. 1. 



Amoeba princeps, sho\Vn in different forms (A, B, C) assumed by the same animal.] 



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 the others. In another sense it is not homogeneous. For 

 we know that the amoeba receives into its substance material as food, and 

 that this food or part of it remains lodged in the body, until it is made use 

 of and built up into the living substance of the body, and each piece of the 

 living substance of the body must have in or near it some of the material 

 which it is about to build up into itself. Further, we know that the amoeba 

 gives out waste matters such as carbonic acid and other substances, and each 

 piece of the amoeba must contain some of these waste matters about to be, 

 but not yet, discharged from the piece. Each piece of the amoeba will, 

 therefore, contain these three things, the actual living substance, the food 

 about to become living substance, and the waste matters which have ceased 

 to be living substance. 



Moreover, we have reasons to think that the living substance does not 

 break down into the waste matters which leave the body at a single bound, 

 but there are stages in the downward progress between the one and the 

 other. Similarly, though our knowledge on this point is less sure, we have 

 reason to think that the food is not incorporated into the living substance at 

 a single step, but that there are stages in the upper progress from the 

 dead food to the living substance. Each piece of the body of the amoeba 



