238 OF FOOD AND THE DIGESTIVE PROCESS. 



of inert habits, is suitable and beneficial to one who is leading a life of 

 continual exertion ; and this difference manifests itself in the require- 

 ments of the same individual who makes a change in his habits the 

 indolent man acquiring an appetite by vigorous exertion, and the active 

 man losing his disposition to hearty feeding by any cause that keeps 

 him from his accustomed exercise. We see precisely the same contrast 

 between Animals of different tribes, whose natural instincts lead them 

 to different modes of life. The Birds of most active flight, and the 

 Mammals which are required to put forth the greatest efforts to obtain 

 their food, need the largest and most constant supplies of nutriment ; 

 but even the least active of these classes stand in remarkable contrast 

 with the inert Reptiles, whose slow and feeble movements are attended 

 with so little waste, that they can sustain life for weeks and even 

 months, with little or no diminution of their usual activity, without a 

 fresh supply of food. 



410. The waste and decay just adverted to, however, do not affect 

 the muscular and nervous tissues alone; for all the operations of nutri- 

 tion involve it to a certain extent. It has been already shown that the 

 acts of absorption, assimilation, respiration, secretion, and reproduction ; 

 all those, in fact, by which the material for the nutrition of the 

 nervous and muscular tissues is first prepared, and subsequently main- 

 tained in the requisite purity, are effected in the Animal, as in the 

 Plant, by the agency of cells, which are continually dying and requiring 

 renewal. In most Vegetables, the death of the parts concerned in 

 these functions takes place simultaneously, as soon as they have per- 

 formed them ; the whole crop of leaves ceasing at once to perform its 

 proper actions, and dropping off; to be replaced by another, at an in- 

 terval that solely depends upon the temperature under which the tree 

 is living ( 99). In the evergreen, however, the process bears a close 

 resemblance to that which we observe in the Animal ; for the leaves die 

 one by one, and not simultaneously ; and are constantly undergoing re- 

 placement, so that the vigour of the system and the activity of its 

 nutritive processes never suffer a complete suspension. 



411. In the Animal body, the different classes of cells, to which 

 allusion has been made, are in like manner constantly undergoing death 

 and renewal ; and this with a rapidity proportioned to the energy of 

 their functions. Hence a supply of food is as requisite, to furnish the 

 materials of their growth, as it is in Plants to furnish the materials of 

 the growth of the leaves. A large part of these materials are subse- 

 quently used for other purposes in the economy ; thus, as the leaves 

 prepare the sap which is to nourish the woody stem, and to form new 

 shoots, so do the absorbing and assimilating cells prepare and furnish 

 the fluid elements of the blood, which are to repair the waste of nerve 

 and muscle, bone and cartilage, &c. But still a considerable amount is 

 expended^ in the simple nutrition of these organs themselves, whose 

 duration is transient, and whose solid parts are cast off as of no further 

 use. Thus the skin and all the mucous surfaces are continually form- 

 ing and throwing off epidermic and epithelial cells, whose formation re- 

 quires a regular supply of nutriment; and only a part of this nutriment 

 (that which occupies the cavity of the cells) consists of matter, that is 





