FORMATIVE POWER OF INDIVIDUAL PARTS. 549 



like manner exhibit a deficiency of the posterior lobes, or of the corpus callosum, 

 or of some other part whose formation normally takes place in the latter months 

 of intra-uterine life, although the parts already produced may have continued 

 to grow at their usual rate. Numerous instances of the same kind might be 

 cited, but these must suffice. 



589. The demand for Nutrition arises, however, not merely from the exercise 

 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 antagonized, would 

 speedily show themselves in its complete disintegration. We have seen ( 114) 

 that as each component cell of the organism has to a certain degree an inde- 

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

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

 striking, when we compare the ratio of change in the organism of cold-blooded 

 animals at low and at high temperatures ; for they live slowly, need little nutri- 

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

 time for the performance of the reparative processes, under the former condi- 

 tion ; but live fast, require a comparatively large supply of nutriment, give off 

 a far greater amount of carbonic acid and other compounds resulting 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 man- 

 ner ; but it is well seen when we contrast' his different tissues with each other, 

 and study their respective histories. For whilst there are some which appear 

 to pass through all their stages of growth, maturation, and decline, within a 

 limited period, there are others whose existence seems capable of almost inde- 

 finite prolongation, and others, again, which are liable to have a period put to it 

 at any time, by the direction of their vital force into other channels. Of those 

 belonging to the first category, 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 ( 148), 

 the epithelial cells of mucous membranes, which minister to absorption in one 

 situation ( 461) and to secretion in another ( 235), the cells forming the paren- 

 chyma of the ductless glands (482,iv.),and many others. The contrary extreme 

 to this may be found in those tissues whose functions are rather physical than vital; 

 and especially in such as undergo consolidation by the deposit of solidifying matter, 

 either in combination with the animal membrane or fibre, or in its interstices. 

 Such tissues are withdrawn from the general current of vital action, and there 

 seems to be no definite limit to their duration, except such as is imposed by the 

 chemical and mechanical degradation to which they are subjected. This appears 

 to be the case with the simple fibrous tissues, especially the yellow, even in 

 their soft or unconsolidated state; but it is far more obvious in the osseous sub- 

 stance, which is chiefly formed by the combination of calcareous salts with the 

 fibrous animal basis. So, again, in the dentine and enamel of teeth, we have 

 examples of tissues that have once undergone a similar consolidation, retaining 

 their condition unchanged through the whole remainder of life, under circum- 

 stances which show that, if any nutritive change take place in them, its amount 

 must be extremely small. Yet it is curious to observe that, both in the osseous 

 and dentinal structures of the young, there is obviously a determinate limit of 

 existence ; as is shown in the rapid disappearance of a considerable part of the 

 lamellse first formed in the cartilaginous matrix ( 265), as also in the death 

 and removal which continue to take place in the inner and older portions of the 

 shaft of a round bone during the whole period of its increase ( 267); and in 



