VARIOUS CAUSES OF DEATH. 351 



the former, as the deficiency of supply is in the latter. This is seen, 

 for example, when death results from a sudden and violent shock, which 

 destroys the vitality of the whole system alike ( 604) ; molecular death 

 being here consentaneous with somatic. 



629. But as each component part of the Animal fabric has an indi- 

 vidual life of its own, so must it have a limited duration of its own ; the 

 period of termination of its vital activity, or its death, being quite inde- 

 pendent of that of the body at large, excepting in so far as the opera- 

 tions of the latter are requisite to afford it a constant supply of appro- 

 priate nutriment, and to maintain its temperature at the proper eleva- 

 tion. It is perfectly compatible, on the other hand, with the Life of 

 the entire organism, that certain parts of it should be continually in 

 course of decay and renewal ; and, in fact, we find that the most im- 

 portant parts in the vital functions are performed by tissues whose indi- 

 vidual duration is comparatively brief, but which are renewed as fast as 

 they degenerate. We have a well-marked example of this in the case 

 of the leaves of trees, which are the chief agents in the preparation of 

 the nutritious fluid, at whose expense the permanent tissues of the trunk 

 and branches are generated ; and although there is nothing in the Ani- 

 mal body at all comparable to the complete exuviation which commonly 

 takes place in the Plant at the close of the season of vegetative activity, 

 yet there is a continual death and separation of parts that have per- 

 formed their function, which in the end makes up a much larger aggregate. 

 Thus there is scarcely a less complete renewal of the epidermis in Man, 

 in the course of twelve months, than there is in Serpents, Frogs, &c., 

 which throw it off periodically ; the only difference being, that in the 

 one case the whole is exuviated and renewed at once, whilst in the other 

 there is a continual interchange. In the exuviation of the antlers of 

 the Deer, and of the milk-teeth of all Mammalia, we have very marked 

 examples of this limitation of the life of individual parts, even in the 

 highest Animals ; and as a general proposition it may be stated, that 

 every part must degenerate, when it has gone through the whole series 

 of changes in which its Life consists, and that it must then either die 

 and decay, or must be so altered in its constitution, as to be able to 

 remain inactive without further change. 



630. Hence we see that the duration of vital activity must be, cceteris 

 paribus, in the inverse ratio of its energy ; that is, the life of any part, 

 or of the entire organism, must be shortened by, any excess of func- 

 tional activity ; whilst it may be prolonged by such a degree of repose, 

 as does not involve an impairment of its nutrition. We see this most 

 remarkably exemplified in the case of cold-blooded animals ; the dura- 

 tion of whose lives, after they have sustained some fatal injury (such as 

 the removal of the heart or of the lungs), or are placed in any other 

 circumstances incompatible with its continuance, is in the inverse pro- 

 portion to the elevation of the temperature to which they are exposed, 

 and therefore to the degree of their vital activity ( 128). Now although 

 this variation is comparatively little observable in the rate of life of that 

 portion of the fabric of warm-blooded animals which is concerned in 

 their organic functions (the temperature to which it is subjected being 

 nearly constant), it. is clearly seen in those organs whose functional 



