456 



SECRETION. 



economy. The cells of the woody stem have 

 a long and almost indefinite duration, espe- 

 cially after they have become consolidated by 

 the filling-up of their cavities with resinous or 

 sclerogenous secretions ; but those of the 

 leaves, which are much more actively con- 

 cerned in the vital operations, have a short 

 and limited term of existence. The " fall of 

 the leaf" is not the cause of the death and 

 decay of the organ, but its result ; for the de- 

 composition of its tissues is already far ad- 

 vanced, when its detachment occurs : its 

 functions have been fulfilled ; its term of life 

 is expired ; and it is cast off, to be replaced 

 by a new development of cellular parenchyma, 

 which in its turn will discharge the same im- 

 portant function, that of preparing the ma- 

 terials for the growth of the more permanent 

 parts of the fabric. This kind of passive 

 change is more constantly going on in the 

 animal body than is usually supposed, espe- 

 cially during the period of its growth and in- 

 crease. A good illustration is afforded by the 

 deciduous or milk teeth. " We trace each of 

 these developed from its germ, and, in the 

 course of its own development, separating a 

 portion of itself to be the germ of its suc- 

 cessor ; then each, having attained its due 

 perfection, retains for a time its perfect 

 state, and still lives though it does not grow. 

 But at length, coincident!}', not consequently, 

 as the new tooth comes, the deciduous tooth 

 dies ; or rather, its crown dies, and is cast 

 out like a dead hair ; while its fang with the 

 bony sheathing, and the vascular and nervous 

 pulp, degenerates, and is absorbed. It is 

 here especially to be observed, that the de- 

 generation is accompanied by some spon- 

 taneous decomposition of the fang, for it 

 could not be absorbed unless it was first so 

 changed as to be soluble. And it is degeneration, 

 not death, which precedes its removal ; for 

 when a tooth fang really dies, as that of the 

 second tooth does in old age, then it is not 

 absorbed, but is cast out entire, as a dead 

 part. Such, or nearly such, it seems almost 

 certain, is the process of assimilation every- 

 where ; these may be taken as types of what 

 occurs in other parts, for these are parts of 

 complex organic structure and composition ; 

 and the teeth-pulps, which are absorbed as 

 well as the fangs, are very vascular and sensi- 

 tive, and therefore, we may be nearly sure, 

 are subject to only the same laws as prevail in 

 all equally organised parts."* All the epidermic 

 and epithelial structures, including the secre- 

 tory substance of glands, are continually un- 

 dergoing the same change, by the exuviation 

 of the old cells when their term of life is ac- 

 complished, and by the production of new ones; 

 the durability being different according to the 

 particular endowments of the part, but also 

 varying with changes in the supply of blood, 

 which increase or decrease its vital activity. 

 Generally speaking, those parts which live 

 most slowly are those of which the chration 

 is the greatest, and in which there if con- 



Pflget, Lee-turns oil Nutrition, 



sequently the least frequent change. Of the 

 exuviation of epidermic structures en manse 

 a process altogether comparable to the fall of 

 the leaf we have striking examples in the 

 entire desquamation of serpents, the moulting 

 of the plumage in birds, and the shedding of 

 the hair in the mammalia ; and in the shedding 

 of the antlers of the stag, we have an example 

 of the exuviation of a highly organised and 

 vascular part, which periodically dies, and 

 which, being external, is cast off entire. 

 " What means all this," says Mr. Paget *, 

 " but that these organs have their severally 

 appointed times, degenerate, die, are cast 

 away, and in due time are replaced by others, 

 which in their turn are to be developed to 

 perfection, to live their life in the mature 

 state, and in their turn to be cast off? " There 

 can be little doubt that a similar change is 

 continually taking place, with more or less 

 activity, in every part of the internal struc- 

 ture ; the products of decay, however, not 

 being at once thrown off, because there is no 

 direct means of getting rid of them ; but being 

 received back into the current of the circula- 

 tion, to be eliminated by instruments ex- 

 pressly provided for that purpose. 



Now, this interstitial change must take 

 place constantly, during the whole life of the 

 entire structure ; but its activity varies ac- 

 cording to certain conditions to which the 

 fabric is subjected. One of the most im- 

 portant of these conditions is heat. It is well 

 known that the tendency to decomposition, 

 which is characteristic of organic compounds, 

 is dependent upon the heat to which they are 

 subjected : thus a compound which passes 

 rapidly into decomposition at J00, shall be 

 much less prone to decay at 60, and shall be 

 permanent at 32. And again, the vital ac- 

 tivity of the several parts of the organised 

 fabric is so dependent upon (he same stimulus, 

 that a very moderate depression of tempera- 

 ture serves to reduce it, or even to suspend it 

 altogether. Now, when the activity of a part 

 is thus reduced, so that it lives more slowly, 

 its duration is proportionally increased, and 

 interstitial change and renewal are scarcely 

 required. We have obvious examples of this 

 in the activity of all the functions in warm- 

 blooded as compared with cold-blooded ani- 

 mals ; in the superior energy of all the vital 

 operations of birds, whose temperature is 10 

 or 12 above that of the Mammalia ; and, on 

 the other hand, in the torpor of cold-blooded 

 animals, and of hy'.ernating Mammalia, when 

 the temperature of their bodies is depressed 

 nearly to the freezing point. In the state of 

 greatest activity, all parts of the body live 

 fast ; their duration is proportionally dimi- 

 nished ; interstitial death and decomposition 

 are continually taking place ; the results of 

 this decomposition have to be got rid of from 

 the body ; and a corresponding demand is set 

 up for nutrient materials, to be applied to the 

 renovation of the structure. On the other 

 hand, a reduction of temperature, which di- 



* Loc. fit. 



