FOKMATIVE POWER OF INDIVIDUAL PARTS. 419 



agencies. The limit of existence seems more determinate, however, in Bone; 

 for not only do we find that, in the first development of this substance, a 

 considerable part of the tissue originally generated by the consolidation of 

 its osseous or cartilaginous matrix speedily disappears, and that during the 

 whole period of growth of the shaft of a round bone, there is a continual 

 removal of its inner and older portions, whereby the medullary cavity is 

 progressively enlarged ; but there is strong evidence that, even after the bone 

 has attained its full dimensions, a replacement of old Haversian systems by 

 new is continually in progress. 



in. In the case of the Muscular and Nervous tissues, however, we trace 

 the operation of causes that differ from any of those already specified. These 

 tissues are doubtless subject, like all others that are distinguished by their 

 vital activity, to the law of limited duration ; for we find that, when not 

 called into use, they undergo a gradual disintegration or wasting, which is not 

 adequately repaired by the nutritive processes. But their existence as living 

 structures appears to be terminable at any time, by the exercise of their 

 functional powers; for the development of muscular contractility or of ner- 

 vous force seems to involve, as its necessary condition, a metamorphosis (so 

 to speak) of the vital power which was previously exercising itself in the 

 nutritive operations; and the materials of these tissues, now reduced to the 

 condition of dead matter, undergo those regressive changes which speedily 

 convert them into excrementitious products. But the very manifestation of 

 their peculiar vital endowments determines an afflux of blood towards the 

 parts thus called into special activity; and from this it comes to pass, that 

 the nutrition of these textures is promoted, instead of being impaired by the 

 losses to which they are thus subjected ; so that their constant exercise occa- 

 sions an augmentation, rather than a diminution of their substance, a due 

 supply of the requisite materials being always presupposed. 



335. Thus it comes to pass, that during the whole period of active life, a 

 demand for Nutrition is created by every exertion of the vital powers, but 

 more especially by the evolution of the Nervous and Muscular forces. The 

 production and application of these, indeed, may be considered as the great 

 end and aim of the Human organism, so far at least as the individual is con- 

 cerned ; the whole apparatus of Organic life being subservient to the building 

 up and maintenance of the Nervo-muscular apparatus, and of those parts of 

 the fabric (e. g., the bones, cartilages, fibrous textures, etc.), which it uses as 

 its mechanical instruments. Thus the activity of all the Organic operations, 

 when once the full measure of growth has been attained, is mainly determined 

 by that of the Animal functions ; and as the " rate of life " of all the parts 

 which minister to the former will be proportioned to the energy with which 

 they are called upon to perform their functions, their duration will diminish 

 in the same proportion, and hence occasion will arise for their continual re- 

 newal. 1 But since, in the attainment of the adult condition, the productive 



1 Such an excellent illustration is afforded by the phenomena of Vegetation, of the 

 doctrines here propounded, that it scarcely appears desirable to pass it by in this place, 

 although it has been elsewhere more fully referred to (PRINC. OF COMP. PHYS., \\ 265, 

 358). The leaves of plants serve, like the absorbing and assimilating cells of Animals, 

 for the introduction and elaboration of the nutritive materials which are to be applied 

 to the extension of the fabric ; the more permanent and inactive parts of which are 

 thus generated at the expense of materials prepared by the vital operations of the 

 more transitory and energetic. Now there is an obvious limit to the duration of the 

 leaf-cells; but this limit is not precisely one of time, being rather dependent upon the 

 completion of their series of vital actions. Thus, although we are accustomed to look 

 upon the " fall of the leaves " (which is nothing else than an exuviation consequent 

 upon death) as a phenomenon of regular seasonal recurrence, and to regard their re- 

 placement by a new growth as occurring at a not less constant interval, yet experience 



