It has been indicated above that disease is due to an interference from without with the normal physiological processes, or to an abnormal construction or tendency of the living tissues. The existence of such interference or tendency does not abolish the normal functions, but rather, in many cases stimulates them to increased activity. The healthy tissues are continually engaged in dealing with external forces, and are generally successful in so doing; and in cases where the external forces have for the moment obtained the supremacy, the tissues are frequently stimulated to exceptional vigour, so as to overcome the interfering agent. As already indicated, many of the phenomena of disease are really due to the reaction of the-tissues against the agent producing the disease. There is therefore on the part of nature an attempt to get rid of the disease, a natural tendency towards recovery. This tendency is expressed in the aphorism vis medicatrix naturae, which implies that the living structures are endowed with a power of overcoming disease.

It will follow from this that recovery is more apt to occur when the disease is due to the direct action of the external forces. In all morbid conditions due to the physical forces, such as wounds, fractures, burns, etc., the tissues at once set to work to repair the injury,-and in most cases with considerable success. In the treatment of such diseases the endeavour is made to place the parts under as favourable circumstances as possible for nature to do its part. In the case of diseases due to infective agents, again, we see usually an effort on the part of nature to overcome the morbid agent. According to one theory the elevation of temperature, which is characteristic of fever, is a means of destroying the morbific agent, and at any rate, in most cases, the normal forces after a time reassert themselves and the patient recovers. In some cases, on the other hand, the disease keeps on advancing with very little check on the part of the tissues. It will be found that when this is the case there has generally existed a special susceptibility to the disease, and that the attack of the morbific agent has, as it were, been invited by the special condition of the person. If, after the morbid process has been established, the susceptibility remains, the disease is likely to remain and extend. In tuberaulosis this seems peculiarly the case, and it is only by profoundly altering the conditions of life that we are able, as a general rule, to bring about an arrest of the process, without the actual removal of the morbific agent. It may, perhaps, be hoped that in the case of infective diseases, medicinal agents may be discovered which will directly attack the morbific agent, but except in the case of syphilis and malarial fevers, such a discovery has not yet been made.

The tendency to recovery is much less in diseases due to causes in the tissues themselves. Malformations may be partially rectified during the processes of development and growth, but recovery will rarely be complete. Tumours generally continue their growth without reference to the activities of the tissues. The diseases due to decadence of the tissues are seldom the subjects of spontaneous recovery.