550 OF NUTRITION. 



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

 viation is usually preceded by the death and disintegration of their own texture 

 ( 289). In hair, nails, and other epidermic appendages, which, when once their 

 component cells have undergone consolidation by the deposit of horny matter 

 in their interior, may remain unchanged for centuries, we must recognize the 

 same principle of indefinite duration, in connection with the annihilation of vital 

 activity; the chemical constitution of these substances, moreover, being such 

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

 agencies. 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 ( 313, 347). 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 nervous force seems 

 to involve, as its necessary condition, the 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 excrementi- 

 tious 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 occasions an augmentation, rather than a diminution, 

 of their substance a due supply of the requisite materials being always pre- 



590. 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 concerned ; 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, &c.) 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 j 

 and as the " rate of life" of all the parts which minister to the former will be 

 in proportion to the energy with which they are called upon to perform their 

 functions, their duration will diminish in the same proportion, and hence occa- 

 sion will arise for their continual renewal. 1 But since, in the attainment of the 



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

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

 although it has been elsewhere more fully referred to (" Princ. of Phys., Gen. and Comp.," 

 \\ 494, 554). 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 ap- 

 plied 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 replacement by a new 

 growth as occurring at a not less constant interval, yet experience shows that these 

 intervals are entirely regulated by temperature ; for if one of the ordinary deciduous 

 trees of temperate climates be transferred to a tropical climate, it will live much faster, 



