SOURCES OF DEMAND FOR ALIMENT. 479 



sufficient amount, the evolution of the organism is either retarded or checked 

 altogether.* But there is one tissue, namely, Fat, the peculiar characters of 

 which are derived from the presence of a non-azotized substance in its cells ; 

 and this cannot be developed, unless there be in the food either oily, saccha- 

 rine, or amylaceous matters, from any of which the fatty compounds may be 

 generated. 



632. The full development of the Animal fabric, however, does not by any 

 means involve the cessation of the demand for food ; in fact, during the whole 

 period of that development, it may be observed that the amount of nutriment 

 ingested is far greater than that which is applied to the simple extension of 

 the structure ( 269). One source of this constant demand is to be found in 

 the continual ivaste or disintegration of the fabric, which goes on to a certain 

 extent under all circumstances, but which varies in degree according to cer- 

 tain conditions not difficult to be understood. All organized substances are 

 liable, from the peculiarity of their chemical composition, to interstitial de- 

 cay ; and this operates in the living organism, as much as in the dead body 

 ( 268). The difference is, that, in the living fabric, there is a provision for 

 at once removing the products of decay, so that they may be cast out of the 

 system as soon as possible ; whilst in the dead body they remain, and act as 

 ferments, accelerating the decomposition of other parts. Now the amount of 

 this interstitial decay varies with the temperature ; being increased by warmth, 

 and retarded by cold. It is consequently greatest in warm-blooded animals, 

 the temperature of whose bodies is constantly sustained at a high standard ; 

 it is reduced to its minimum in the torpid condition of cold-blooded animals, 

 which is brought on by the agency of cold; and will be lowered to nearly 

 the same degree in the hybernating state of certain Mammalia. There is 

 another source of waste and decay, which is common to Animals, and all but 

 the simplest Plants ; this results from the limited duration of life in the indi- 

 vidual parts, which are most actively concerned in the Vegetative Functions. 

 We have seen that the essential instruments in the various functions of 

 Absorption, Assimilation, Respiration, Secretion, and Reproduction, are cells; 

 each of which goes through a certain series of processes and then dies and 

 decays, just as do the isolated cells, which compose the entire fabric of the 

 simplest Cryptogamic Plants. This is evidenced to us in the Vegetable king- 

 dom by the " fall of the leaf;" which is nothing else than the result of the 

 death and decay of the component cells of that organ, after having fulfilled 

 their peculiar functions ; these consisting in the preparation or elaboration of 

 the nutritious sap, from which the various tissues and secretions of the plant 

 are subsequently generated. The same process is continually taking place, 

 though in a less obvious manner, in the Animal body ; the rate of death and 

 renewal of each group of cells being greater, as the functions to which it 

 ministers are energetically performed ; whilst the energy of these operations 

 is mainly dependent upon the demand set up by the exercise of the Minimal 

 functions, for the reparation of the Nervous and Muscular tissues. 



633. The great source of waste and decay in the Animal body, and con- 

 sequently the chief source of the demand for food, is the disintegration of the 

 Nervous and Muscular tissues, which has been shown to be a necessary con- 

 dition of their functional activity. Every manifestation of Nervous power, 

 of whatever kind, seems to require the combination of Oxygen with the ele- 

 ments of Nervous matter ; the normal composition of which is thus destroyed, 



The very curious discovery has lately been made, in regard to the integuments of the 

 Tunicated or Ascidian Mollusca (the lowest class of that sub-kingdom), that they contain a 

 considerable quantity of Cellulose a substance which had not been previously supposed to 

 be a normal constituent of the Animal Fabric. See Annales des Sciences Naturelles ; 3me 

 Serie, Zool., torn. v. p. 193 et sea. 



