418 OF NUTRITION. 



cise of the formative powers which are concerned in the building up of the 

 organism, but also from the degeneration and decay which are continually 

 taking place in almost every part of it, and the effects of which, if not antago- 

 nized, would speedily show themselves in its complete disintegration. As 

 each component cell of the organism has to a certain degree an independent 

 life of its own, so has it also a limited duration ; and its duration usually 

 bears an inverse ratio to its functional activity. This is particularly striking 

 when we compare the ratio of change in the organisms of cold-blooded ani- 

 mals at low and at high temperatures ; for they live slowly, need little nu- 

 triment, give off but a small amount of excretory products, and require a 

 long time for the performance of the reparative processes, under the former 

 condition ; but live fast, require a comparatively large supply of nutriment, 

 give off a far greater amount of carbonic acid and other excretions result- 

 ing from the " waste " of tissue, and exhibit a far more rapid reparation of 

 injuries, in the latter state. The constantly high temperature of Man, as of 

 other warm-blooded animals, prevents this difference from being displayed 

 in him in a similar manner ; but it is well seen when we contrast his differ- 

 ent tissues with each other, and study their respective histories. For whilst 

 there are some (i) which appear to pass through all their stages of growth, 

 maturation, and decline, within a limited period, there are others (n) whose 

 existence seems capable of almost indefinite prolongation, and others (in) 

 again, which are liable to have a period put to their life at any time by the 

 direction of their vital force into other channels. 



I. Of those belonging to the first category, which are actively concerned 

 in the purely vital operations of the organism, a characteristic example is 

 presented by the Ovule ; which, if not fertilized within a limited period after 

 its maturation, speedily declines and decays; and the same law of limited 

 duration doubtless extends to a large proportion of such tissues as are actively 

 concerned in the maintenance of the organic functions ; as for example, the 

 Corpuscles of the Blood, the Epithelial cells of many glands which are in- 

 strumental in the process of Secretion, the cells forming the parenchyma of 

 the Absorbent and Vascular Glands, and many others. 



n. The contrary extreme to this may be found in those tissues whose func- 

 tions are rather physical than vital ; and especially in such as undergo con- 

 solidation by the deposit of solidifying matter, either in combination with 

 the animal membrane or fibre, or in its interstices. Such tissues are more 

 withdrawn from the general current of vital action ; and there seems to be 

 no definite limit to the duration of some of them, except such as is imposed 

 by the chemical and mechanical degradation to which they may be subjected. 

 This appears to be the case with the simple Fibrous tissues, especially the 

 yellow, even in their soft or uuconsolidated state ; but it is far more obvious 

 in the dentine and enamel of Teeth, which are formed by the combination 

 of calcareous salts with an animal matrix, and which retain their condition 

 apparently unchanged through the whole remainder of life, under circum- 

 stances which show that if any nutritive action takes place in them, its 

 amount must be extremely small. In the dentinal structures of the young, 

 however, there is obviously a determinate limit of existence ; as is shown by 

 the exuviation, at a certain definite epoch, of the first set of teeth, which 

 exuviation is usually preceded by the death and partial disintegration of 

 their texture. In Hair, Nails, and other Epidermic appendages, again, whose 

 substance, when once it has undergone consolidation by the deposit of horny 

 matter, may remain unchanged for centuries, we must recognize the same 

 principle of indefinite duration, in connection with the cessation of vital ac- 

 tivity ; the chemical constitution of these textures, moreover, being such as 

 renders them but little prone to be acted upon by ordinary decomposing 



