The intermediary mechanism of this hypertrophy has not been identified, but the similarity between the products of activity and those of autolysis is suggestive and it may well be that the normal sizes of organs which accompany normal amounts of work are de termined by the soluble substances set free in active katabolism. So, too, if functional activity is reduced, the bulk of correspond ing tissue becomes smaller. An unused muscle soon wastes, and in consonance with the quietude of old age most of the important organs shrink. If the arm is cut off at the elbow, the muscles which normally move the lower arm wither, the bones to which they are attached grow smaller, and the nerve cells in the spinal cord which actuate them may disappear.
Two points need further mention: ( ) The removal of one kidney has no detectable ill-effect on an animal or man; one kidney is evidently enough to do the ordinary excretory work which is required. But the kidneys and other organs are designed to dis charge, and are capable of discharging, much more than the or dinary amount of function ; a muscle can at any moment do more than its average work and indeed more than it has ever been called upon to do before. The possession of this reserve force is an obvious teleological necessity for survival, and so precise is the response to an increase of average work that it is maintained in hypertrophied as well as in normal organs (2) If the increase of work is too large, hypertrophy does not occur, i.e., the stimulus to hypertrophy is an amount of extra function within the range of the reserve force of the organ. This is a well recognized prin ciple in the training of voluntary muscles for athletic or aesthetic purposes.
All these circumstances of the response to work seem best to be explained on the hypothesis that an organ is made up of a number of units, each of which requires a restorative period of rest after each of the periods of activity, which are necessary for its maintenance. To fulfil the ordinary amount of function only
a proportion of units is active at any one time; as their activity proceeds they become, in accordance with the general law, pro gressively less excitable by the stimulus which rouses their activity and eventually cease to respond. Their function is then taken over by another set of units which, after a period of rest, are more responsive. In this way a larger number of units is kept in working order than is usually required. The maximum possible work is the amount which can be achieved by all the units work ing simultaneously: this is the limit of the reserve force, and if it is habitually reached no units have time for the necessary rest and restoration, the organ is worn out and cannot make hyper trophic growth.
These reparative responses are not the inevitable result of the application of the appropriate stimulus : the reaction may not occur if it is unnecessary or undesirable. If the portal vein bring ing blood to the left half of the liver is tied, the left lobe pro gressively atrophies and the right lobe hypertrophies so that at any time the total quantity of liver substance remains about normal. But if the right lobe is injured by tying its bile duct, it does not hypertrophy and the left lobe does not atrophy, though it receives blood only through the hepatic artery. The atrophy is conditioned by the total functional requirements of the body for liver tissue and does not necessarily follow the reduction in blood supply. If rabbit's blood is injected into the circulation of a normal rabbit, the excess of liquid is got rid of in a few hours, and the excess of red corpuscles in ten days or so and the normal position is regained. If such a transfusion is repeated, the excess red cells are destroyed in two or three days and the animal can be brought by practice to a habit of disposing of them promptly. If such a trained animal is bled and then transfused with an equal quantity of blood from another rabbit, no destruction of the injected blood occurs. The difference is that the extra blood is not wanted by the normal animal while the bled animal has need of it to carry oxygen : in the former it is destroyed, in the latter it is kept and used.