VARYING DURATION OF DIFFERENT PARTS. Oil 



consequent loss of functional power, affords us some idea of the limit to the 

 duration of the life of the tissue. It may be stated, then, as a general pro- 

 position, that the interstitial change, which the whole structure of the body is 

 continually undergoing, in its normal or physiological condition, is due to the 

 regularly-occurring death and reproduction of its component cells, of which 

 every one has its own limit of duration. We uniformly find that those 

 Tissues, in which the most active vital changes are going on (such as the 

 Nervous and Muscular), are those in which the duration of the individual 

 component portions is the least; as is shown by the rapidity of the changes 

 of removal and reposition, which are continually taking place in them. The 

 converse holds good also. Further it may be remarked, and this is a matter 

 of much practical importance, that anything which increases the functional 

 activity of any particular tissue, thus causing it to live faster, diminishes the 

 duration of its life ; as is shown in the increased rapidity of disintegration, 

 which results from the continued exercise of the Muscular and Nervous 

 systems. 



v. There is yet another phase, under which Cellular life presents itself as 

 a natural condition in the lower organisms, and in the early condition of the 

 higher; but which constitutes a morbid state in the adult condition of the 

 latter. This is when cells reproduce themselves with extreme rapidity, 

 neither the primary nor secondary cells undergoing any further transforma- 

 tion, and the duration of each individual being limited by the development 

 of its progeny within it, causing its own distension and final rupture or dis- 

 appearance. The growth of the lower Fungi offers a striking example of this 

 in the Vegetable kingdom ; and the early processes of development in the 

 Ovum of the highest Animals, exhibit the same character. Every cell, as it 

 is generated, proceeds at once to the work of multiplication, for which it 

 seems specially destined; and thus it is subject from the first to the law of 

 Reproduction. It is this which distinguishes the Fungoid diseases ; which 

 derive the character designated by the Surgeon as malignancy, simply from 

 their tendency to propagation, and his want of power to control it. It seems 

 probable that many other changes of structure are due to a corresponding 

 cause. 



812. The duration of the existence of the individual Cells in corresponding 

 parts, is further subject to variation, in accordance with the period of life of 

 the entire organism. Thus all the tissues, even those most consolidated, are 

 undergoing continual changes in the young animal, in which the processes of 

 decay and renewal go on much faster than in the adult ; and in the adult, than 

 in the aged person. Even the cells of the Bony structure, which in the adult 

 are almost permanent, and in the aged person are subject to extremely little 

 change, are liable in the infant to an early decomposition ; their places being 

 filled up by others, of which the form adapts itself to the growth of the struc- 

 ture. This may be partly accounted for by the imperfect degree, in which, 

 so long as the entire organism is undergoing rapid increase, the normal struc- 

 ture is developed in any one portion of it; for the degree of consolidation 

 being less, the tendency to decay will naturally be greater. But this explana- 

 tion is not in itself sufficient ; and we must be content for the present to 

 regard it as a general law (which may ultimately prove to be but a result of 

 some more general principle) that the duration of the existence of individual 

 cells increases, cseteris paribus, with the advance of life. At the same time, 

 their functional activity diminishes. They may be said to live more slowly. 

 The dull perceptions, and slow and feeble movements, of the aged man, form 

 a striking contrast with the acute sensibility, and the rapid and vigorous mus- 

 cular actions, of the child; and the same change may be noticed in the organic 

 functions. Hence it may be stated as a general law, that the vital activity of 



