MUTUAL DEPENDENCE OF THE VITAL ACTIONS. 359 



pounds, having the same chemical composition with its own substance ; and it is 

 for the reception of these, for their preparation, and for their maintenance in 

 the requisite state of purity, that a large part of the fabric of the Animal is 

 destined. But if we could imagine its several tissues to be supplied with nutri- 

 ment in any other manner, and maintained in other respects in their normal 

 circumstances (as regards warmth, air, &c.), we have every reason to believe 

 that their independent vitality would manifest itself by their continued devel- 

 opment, and by the regular exhibition of their ordinary properties. An ap- 

 proach to this condition is made, in the experiment of entirely detaching a limb 

 from the body, but keeping up the circulation of blood through it, by means of 

 tubes connecting its main artery and vein with those of the stump. Notwith- 

 standing the prejudicial effect of such severe injuries, the persistence of the 

 muscular irritability in the separated part ( 322), is a sufficient proof of the 

 continuance of the normal actions of nutrition, although of course in a dimin- 

 ished degree. And the occasional reunion of a member which has been entirely 

 separated, when decomposing changes have not yet commenced in it, most clearly 

 shows, that nothing but the restoration of its current of blood is requisite for 

 the preservation of its vitality, and that its powers of growth and renovation are 

 inherent in itself, only requiring a due supply of the nutrient material, with 

 certain other concurrent conditions. 



367. In every living structure of a complex nature, therefore, whilst we wit- 

 ness a great variety of actions, resulting from the exercise of the different powers 

 of its several component parts, we at the same time perceive that there is a cer- 

 tain harmony or co-ordination amongst them all, whereby they are all made to 

 concur in the maintenance of the Life of the organism as a whole. And if we 

 take a general survey of them, with reference to their mutual relations to each 

 other, we shall perceive that they may be associated into groups ; each consist- 

 ing of a set of actions, which, though different in themselves, concur in effecting 

 some positive and determinate purpose. These groups of actions are termed 

 Functions. Thus, one of the most universal of all the changes necessary to the 

 continued existence of a living being, is the exposure of its nutritious fluid to 

 the air ; by the action of which upon it, certain alterations are effected. For 

 the performance of this aeration, simple as the change appears, many provisions 

 are required. In the first place, there must be an aerating surface, consisting 

 of a thin membrane, permeable to gases ; on the one side of which the blood 

 may be spread out, whilst the air is in contact with the other. Then there 

 must be a provision for continually renewing the blood which is brought to this 

 surface ; in order that the whole mass of fluid may be equally benefited by the 

 process. And, in like manner, the stratum of air must also be renewed, as fre- 

 quently as its constituents have undergone any essential change. We include, 

 therefore, in speaking of the " function of respiration/' not only the actual 

 aerating process, but also the various changes which are necessary to carry this 

 into effect, and which obviously have it for their ultimate purpose. 



368. On further examining and comparing these Functions, we find that they 

 are themselves capable of some degree of classification. Indeed, the distinction 

 between the groups into which they may be arranged, is one of essential import- 

 ance in Animal Physiology. If we contemplate the history of the Life of a 

 Plant, we perceive that it grows from a germ to a fabric of sometimes gigantic 

 size generates a large quantity of organized structure, as well as many organic 

 compounds, which form the products of secretion, but which do not undergo or- 

 ganization and multiplies its species, by the production of germs similar to that 

 from which it originated ; but that it performs all these complex operations, 

 without (so far as we can perceive) either feeling or thinking, without conscious- 

 ness or will. All the functions of which its Life is composed, are, therefore, 

 grouped together under the general designation of Functions of Organic or Ve- 



