610 OF NUTRITION. 



still firming part of the stem, without interfering with the nutritive operations 

 of the tree ; and if we could possibly remove it entirely, without doing injury 

 by the operation to the rest of the structure, its absence would be productive 

 of no other evil consequences than those which would necessarily result 

 from the withdrawal of the mechanical support afforded by it. The same 

 may be said of the Epidermic Appendages of Animals, and of the External 

 Skeletons of many Invertebrata ; which remain equally unchanged from the 

 time of their first formation. Now as long as these structures hold together, 

 it is evident that the organized part of them must have undergone little change 

 from the condition in which it existed in the living fabric ; and that their 

 death takes place, in reality, only when the structures decay, this decay 

 being, in fact, the consequence of it. The law of existence of such cells, 

 therefore, is that of indefinitely-prolonged duration; this law must have been 

 impressed upon them from their origin; and the power by which their walls 

 secrete and deposit the consolidating materials, appears to be the chief means 

 of keeping it in operation. 



in. In all the higher forms of Animal structure, the Cells originally com- 

 posing it are only the means of generating tissues of other kinds, in which 

 the Cellular character is less obvious. Thus the Muscular and Nervous tis- 

 sues have their origin in cells, which at first appear in no respect different 

 from others, but which subsequently undergo a peculiar metamorphosis, and 

 themselves no longer exist as such. Upon all these primordial cells, there- 

 fore, a law of transformation is impressed, from the time of their first pro- 

 duction. In their original aspect, -they cannot be distinguished from the cells 

 which are not destined to undergo any such metamorphosis; but, just as the 

 first cell of the embryo, from which Man is produced, must have some real 

 though not apparent difference from that in which the Polype originates, so 

 must the cell which is afterwards developed into Muscular Fibre, be inherently 

 different from that which is subsequently converted into Nervous tissue. 



iv. The tissues, thus formed by the transforming processes to which certain 

 Cells are subject, are evidently governed by the same laws of Nutrition as 

 those which regulate ordinary Cell-growth; these are modified in their action, 

 however, by the conditions in which they are placed, in regard to the activity 

 of the function which the Tissue is called upon to perform. In all instances, 

 however, these Tissues have a limited period of existence. They are gene- 

 rated, they grow from the alimentary materials with which they are supplied, 

 they arrive at maturity, they 'decline, they die, and they decay ; just as do the 

 isolated vesicles constituting the humblest forms of vegetation. For all of 

 them there is an appointed duration of life, just as there is for the entire Man. 

 Now on this view we can explain many physiological phenomena, which 

 cannot otherwise be very satisfactorily accounted for. It is owing to the con- 

 tinual death and decay of its component cells, that the process of decomposition 

 goes on with such constancy and uniformity in the living body; whilst, on the 

 other hand, it is by the continual reproduction of new cells, in the place of 

 those which have disappeared, that the normal organization is maintained. 

 The limited duration of the life of the cells composing the various tissues is 

 further made evident, by the rapid disappearance of the normal organization, 

 and by the loss of the functional power of those tissues, when the cessation 

 of their activity prevents the development of the new cells, by which alone 

 their character can be maintained. Of the change of structure and loss of 

 power which result from disuse and consequent want of nutrition in Muscular 

 and Nervous tissues, instances have already been given ( 588 and 790). 

 The ordinary processes of Decomposition and Interstitial Absorption are pro- 

 bably less rapid than usual under such circumstances ; so that the length of 

 time required for the disappearance of the characteristic structure, and the 



