506 OF NUTRITION. 



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 deficiency of the 

 required stimuli 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 Muscu- 

 lar and Nervous Tissues, instances have already been given ( 221 and 382). 

 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 

 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 propo- 

 sition, that the interstitial change which the whole structure of the body is, in 

 its normal or physiological condition, continually undergoing, 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 Tis- 

 sues 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 any thing 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 demand for nourishment, which is set 

 up as a consequence of the continued exercise of the Muscular or Nervous 

 system, and which, being far greater than can be required for such increase of 

 their amount as results from that exercise, necessarily indicates that a corre- 

 sponding removal of effete matter, resulting from the death of the cells, has 

 taken place. 



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 transformation, 

 and the duration of each individual being limited by the development of its 

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

 ance. 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 gene- 

 rated, proceeds at once to the work of multiplication, for which it seems 

 specially destined ; and thus it is subject from the first to the ]aw of Repro- 

 duction. 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 pro- 

 bable that many other changes of structure are due to a corresponding cause. 



646. 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- 



