Everything which relates to the maintenance of health in the animal body is included in the word "Hygiene", or in the term which the late Dr. Chas. J. B. Williams preferred, "Hygienics". A perfectly accurate definition of health can hardly be formulated in words, at least, which will convey the exact idea existing in the mind of the physiologist. A sufficiently clear view of the condition is, however, present to the common intelligence without the verbal formula. Every horse-owner, for example, knows whether his animals are well or ill, and usually he will be able to give an intelligent explanation of his reasons. In general terms health may be said to consist in the regular performance of the functions which are relegated to the various structures and organs of the body; these are simple or complex according to the position which the living being occupies in the animal world, and it is interesting to note that a vast number of organisms only visible with the aid of optical appliances live in a condition of ceaseless activity and perform their functions of respiration, circulation, nutrition, and locomotion - in fact everything which contributes to the completeness of organic life - in the most perfect way by the aid of very simple apparatus, so long as the conditions in which they are placed remain favourable to their existence. For example, myriads of living organisms, animals and plants, are found in stagnant water, and so long as the medium in which they live remains unchanged their activity continues. Should any serious alterations occur in the conditions of the medium on which their life depends, they become inert, all their functions cease, and the simple tissues of which they are composed become shrivelled, and the once active, living creatures are to all appearance dead. It is only necessary, however, in a large number of instances, to supply, to apparently dead creatures, the medium, water, which is favourable to their life, to enable them to resume their form and functions. The illustration is one of the most simple that can be offered of the physiological maxim that life depends upon the correspondence of the organism with its environment; when the correspondence ceases, either from failure on the part of the organism, or on the part of the environment, then life or health, or both, can no longer be maintained.
The science of Hygienics may be shortly defined as the maintenance of the relation which exists between the organism and its surroundings, and the important question arises in the case of the higher animals: What are the conditions which have to be maintained in order that the organism may perforin its functions?
In considering the surrounding circumstances or environment in which an animal lives, it has to be borne in mind that the organism itself may be at fault, while the conditions of life may be in perfect order. When, therefore, it is assumed that if the conditions of life are maintained in perfect order the organism will remain in health, it must also be possible to affirm, with equal reason, that the organism was in a perfectly normal state to begin with. It is hardly necessary to add that in the greater number of cases this perfect corresisondence does not exist, and, to use a common expression, there may be faults on both sides. It becomes necessary, therefore, to take into consideration the fact that what may be called perfect surroundings absolutely adapted for the preservation of health in one animal may prove to be quite insufficient to secure the same results in the case of another. Indeed, the favourable conditions in the first case may have a tendency to induce disease in a subject which, from previous habit, or as a consequence of the influences of heredity, may be predisposed to contract certain disorders. A single illustration will make this proposition clear. In the case of one animal, the power to sustain extreme cold or extreme heat without suffering may be developed in a high degree. In another animal the system may be particularly obnoxious to cold or heat, and such an animal may suffer from catarrhal diseases which the first animal would entirely escape.