Thus, all terrestrial changes are incidents in the course of cosmicalequilibration. It was before pointed out (§69), that of the incessantalterations which the Earth's crust and atmosphere undergo, those which arenot due to the action of the moon and to the still-progressing motion ofthe Earth's substance towards its centre of gravity, are due to the still-progressingmotion of the Sun's substance towards its centre of gravity. Here it is tobe remarked that this continuance of integration in the Earth and in theSun, is a continuance of that transformation of sensible motion into insensiblemotion which we have seen ends in equilibrium; and that the arrival in eachcase at the extreme of integration, is the arrival at a state in which nomore sensible motion remains to be transformed into insensible motion --a state in which the forces producing integration and the forces opposingintegration have become equal. §173. Every living body exhibits, in a four-fold form, the processwe are tracing out -- exhibits it from moment to moment in the balancingof mechanical forces; from hour to hour in the balancing of functions; fromyear to year in the changes of state that compensate changes of conditions;and finally in the arrest of vital movements at death. Let us consider thefacts under these heads.
The sensible motion constituting each visible action of an animal, issoon brought to a close by some opposing force within or without the animal.
When a man's arm is raised, the motion given to it is antagonized partlyby gravity and partly by the internal resistances consequent on structure;and its motion, thus suffering continual deduction, ends when the arm hasreached a position at which the forces are equilibrated. The limits of eachsystole and diastole of the heart, severally show us a momentary equilibriumbetween muscular strains that produce opposite movements; and each gush ofblood has to be immediately followed by another because the rapid dissipationof its momentum would otherwise soon bring the circulating mass to a stand.
As much in the actions and reactions going on among the internal organs,as in the mechanical balancing of the whole body there is at every instanta progressive equilibration of the motions at every instant produced. Viewedin their aggregate, and as forming a series, the organic functions constitutea dependent moving equilibrium, a moving equilibrium of which the motivepower is ever being dissipated through the special equilibrations just exemplified,and is ever being renewed by the taking in of additional motive power. Theforce stored up in food continually adds to the momentum of the vital actions,as much as is continually deducted from them by the forces overcome. Allthe functional movements thus maintained are rhythmical (§85); by theirunion compound rhythms of various lengths and complexities are produced;and in these simple and compound rhythms, the process of equilibration, besidesbeing exemplified at each extreme of every rhythm, is seen in the habitualpreservation of a constant mean, and in the re-establishment of that meanwhen accidental causes have produced divergence. from it. When, for instance,there is a great expenditure of muscular energy, there arises a reactivedemand on those stores of energy which are laid up in the form of consumablematter throughout the tissues: increased respiration and increased circulationaid an extra genesis of force, that counterbalances the extra dissipationof force. This unusual transformation of molecular motion into sensible motion,is presently followed by an unusual absorption of food -- the source of molecularmotion; and the prolonged draft on the spare capital in the tissues, is followedby a prolonged rest, during which the abstracted capital is replaced. Ifthe deviation from the ordinary course of the functions has been so greatas to derange them, as when violent exertion produces loss of appetite andloss of sleep, an equilibration is still eventually effected. Providing thedisturbance is not such as to destroy life (in which case complete equilibrationis suddenly effected), the ordinary balance is by-and-by re-established: the returning appetite is keen in proportion as the waste has been large;while sleep, sound and prolonged, makes up for previous wakefulness. Noteven when some extreme excess has wrought a derangement that is never whollyrectified, is there an exception to the general law; for in such cases thecycle of the functions is, after a time, equilibrated about a new mean state,which thenceforth becomes the normal state of the individual. And this processexemplifies in a large way what physicians call the vis medicatrix naturae.
