VARYING DURATION OF CELL-LIFE. 609 



4. Varying duration of different parts of the Organism. 



810. From the foregoing details the obvious inference results, that each 

 part of the organism has an individual Life of its own, whilst contributing to 

 uphold the general Life of the entire being. This Life, or state of Vital 

 Action, depends upon the due performance of the functions of all the subordi- 

 nate parts, which are closely connected together. The lowest classes of or- 

 ganized beings are made up of repetitions of the same elements ; and each 

 part, therefore, can perform its functions in great degree independently of the 

 rest. But, in ascending the scale, we find that the lives of the individual parts 

 become gradually merged (so to speak) in the general life of the structure ; 

 for these parts gradually become more and more different in function, and 

 therefore more and more dependent on each other for their means of support ; 

 so that the activity of all is necessary for the maintenance of any one. 



811. The doctrine of Development from Cells gives us a clearer idea of the 

 nature of the continual process of decay and renewal, which take place in the 

 Animal body. Every Cell has, to a certain degree, an individual life of its 

 own. This individuality is much more decided in the lower forms of organ- 

 ized being, where each cell can maintain an independent existence, than it is 

 in the higher, in whose fabric a- large number having different functions are 

 united into one structure, the combined activity of the whole of which is 

 necessary to the life of any one. But, even in the highest, it is evident that 

 each cell will possess a certain duration of its own ; and that, from its first 

 period of development, all the changes which it undergoes are governed by 

 laws peculiar to it. In the various parts of the Vegetable, as in those of the 

 Animal, we find a great difference in the duration of the existence of the cells 

 composing them. These differences may be reduced to five heads. 



i. Cells may be generated, Avhich have a very transient existence, and 

 which disappear again, without undergoing any transformation. This may be 

 seen in the Vegetable ovule, and in the Germinal Vesicle of the Animal Ovum ; 

 as well as in many other parts. Thus we have Absorbent Cells ( 181), 

 Secreting Cells ( 179), and probably Assimilating or Fibrine-elaborating 

 Cells ( 154) ; all of which originate in pre-existing germs, attain their full 

 development (in the course of which they perform their allotted function), 

 and then disappear by rupture or liquefaction. In such instances it is obvious 

 that, from their first origin, the cells are subject to a law of limited duration, 

 and that their death and decay are as much the result of their inherent consti- 

 tution, as are those of each entire Animal or Vegetable organism. 



n. The contrary extreme to this may be found in those Cells, of which the 

 function, instead of being transient, is to be indefinitely prolonged; such are 

 those of which the organs of mechanical support are usually formed. Here 

 the cell, instead of changing its form, or of giving origin to new cells within 

 itself, becomes the subject of an internal deposit of hard matter, which lines 

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

 Vital Action. AVhen 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 

 peculiar 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 re- 

 main 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 



