FORMATION OF THE TISSUES GENERAL CONCLUSIONS. 505 



its walls, and cuts it off, more or less completely, from the general course of 

 Vital Action. When this is the case, and the hard matter is not itself liable to 

 decomposition, the duration of the cell-walls, which are protected by their pe- 

 culiar aggregation from exposure to decomposing agents, may undergo little or 

 no change for an almost indefinite period. Thus the heart-wood of Plants, the 

 Bones of Animals, and still more their Hair, Hoofs, Horns, &c., may remain 

 unaltered through a long series of years. Of some of these parts it can scarcely 

 be said that they are less alive, when removed from the organism to which 

 they belonged, than when included in it. In the heart-wood of a Plant, for 

 example, no vital change takes place from the time that the woody tubes and 

 cells are once consolidated by internal deposition ; it may decay whilst still 

 forming 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 de- 

 posit the consolidating materials, appears to be the chief means of keeping it 

 in operation. 



III. 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 tissues 

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

 others, but which subsequently undergo a peculiar metamorphosis, and them- 

 selves no longer exist as such. Upon all these primordial cells, therefore, a 

 law of transformation is impressed, from the time of their first production. 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 appa- 

 rent 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 definite 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 continual 

 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 



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