Page:Encyclopædia Britannica, Ninth Edition, v. 17.djvu/744

686 NUTRITION The total income and expenditure of energy of an average man in twenty-four hours is thus calculated to correspond to the amount of heat required to raise 595 Ibs of water from the temperature of melting ice to that of boiling water. Where, it will be asked, does this transformation of energy chiefly have its seat The answer to this question is that it is firstly in the muscles, then in the glands of the body. At all times, whether in rest or activity, heat is evolved, but the quantity increases in the case both of glands and of muscles as they pass from the former into the latter condition. In the resting body, it has been remarked, the losses of energy are represented by the loss of heat, for the mechanical work done within it takes the form of heat within the body itself. It is difficult, nay, impossible, to calculate the amount of energy which in the first instance takes the form of mechanical work in the body, and which is always transformed into heat. The case of the heart is one in which, however, an approximate calculation can be made. Upon fairly reliable data it has been calculated that the work expended by the heart of a man in twenty- four hours amounts to not less than 627,768 foot-pounds, an amount of work which is equivalent to nearly 45 pound- units of heat, and which represents the energy evolved as heat in the complete combustion of about 386 grains of carbon. These calculations enable one to form some idea of the magnitude of the nutritive processes which have their seat in the muscular substance of the heart ; and to a less degree the same is the case with all the other muscles which are engaged in so-called opus vitale, that is, in the performance of internal work absolutely essential to the continuance of the life of the organism. The proportion of the total energy of the body which takes the form of mechanical work varies within very wide limits. Assuming that the conversion of potential into kinetic energy in equal times were a constant quantity, it would follow that all external work done diminished the amount of heat set free from the body. To a certain extent it is probable that such a relation exists. It is, however, to be remembered that when external work has to be done there is invariably an increased consumption of organic constituents of food, and therefore an increased store of available energy. The working animal consumes more O and produces more C0 2 than the resting animal. Different animals, like steam-engines of different construction, vary in the proportion borne by the external work they are capable of performing to the total energy which becomes kinetic. Experiments made with the separate muscles of animals, no less than observations on the relation between the external mechanical work done by and the total heat evolved in the body of animals, have, however, shown that animals are more economical machines than the most perfect steam-engines. Whilst the latter cannot convert more than one-eighth of their available energy into work, the animal may yield as much as one-fifth of its energy in the form of available external work. VI. GROWTH, DECAY, AND DEATH. In the adult body in a state of health the income of matter balances as nearly as possible the expenditure, and therefore the weight of the body and its dimensions remain nearly constant, and the same is approximately true of the different organs and tissues which compose the body, as well as of the anatomical elements which enter into their formation. Even when the conditions to which the organ ism is subjected undergo great variations as, for instance, when from a state of rest it passes into a state of great activity, or when the temperature of the medium which it inhabits undergoes great changes and when, to meet these variations in external circumstances, the rate of the exchanges of matter has to undergo great fluctuations, the weight of the body remains nearly constant. This remarkable result is one which doubtless depends upon a great many factors which are for the most part hidden from us. In the growing body matters are, however, very different ; in it normally the income must be always so much in excess of the expenditure that an accumulation of capital may occur, and that the body may increase in weight and in dimensions. The growing body is always the seat of a more active exchange of matter than the fully-developed body, i.e., weight for weight, it requires more food and oxidizes it more rapidly. The increase of weight and of dimensions occurs in part by addition of matter to, and increase in weight and in dimensions of, individual organs and their constituent anatomical elements ; in part, however, it is due to an actual multiplication of anatomical elements occurring at a rate which greatly exceeds that which occurs in the adult body. To what an extent this multiplication occurs and how it gradually diminishes is evident when we reflect that the whole organism was originally derived from a single cell, the ovum. Upon what depends this tendency to multiplication of anatomical elements, and this tendency to increase in size of individual anatomical elements or of organs, until a certain approximate limit has been attained, is absolutely unknown. We know to a certain extent that the process of growth depends upon and is influenced by certain cir cumstances, as amount of food, temperature, blood-supply to the particular organ, and so on, but yet the knowledge is wanting which would tell us why, when a certain limit has been attained, the processes of income and expendi ture balance and growth ceases. That the nervous system has a great influence upon nutrition and therefore upon growth is certainly known, and in great measure explicable. By its control over the blood -supply to organs and tissues the nervous system exerts a remarkable influence, as well as by its relations more or less direct upon the anatomical elements through nerve -fibres, Avhich, inasmuch as they appear to influence in a direct manner the nutrition of anatomical elements, have been surmised to exist, and have received the name of "trophic fibres." Yet, even apart from the influence exerted by the nervous system, it appears to be a property of individual anatomical elements that, consistently with health, they shall only develop at a certain rate and attain a certain magnitude. Scarcely less mysterious than the primary causes of growth is the fact that every organism having arrived at maturity remains in a state of apparent structural and functional integrity for a term which possesses an approxi mately constant mean value for each species, and then sooner or later necessarily passes into a condition of gradually lessening efficiency, which ultimately terminates in death. Were the animal organism a machine undergoing a constant though very slow process of waste, its decay and ultimately its death Avould be more obvious than they actually are. The organism, however, differs from the machine in that its matter is continually the seat of change, and that during long periods (i.e., during healthy adult life) the processes of gain and loss of matter appear to be going on with perfect evenness and equality. Why, then, the ultimate deteriora tion resulting in waste and then in death 1 The answer is that the organ as a whole unquestionably does suffer by work, and, though at first the degree of impairment is so slow as to be imperceptible, it doubtless is a continuous process. A general impairment of the mechanism of the body as