It has already been pointed out that the body is not entirely dependent upon external agencies for the production of the sugar which it needs. When the supply is inadequate, it manufactures glycogen for itself out of the other constituents of the diet. It can, indeed, make it at the expense of its own proteins. If a dog which has been caused to do muscular work, without a sufficiency of carbohydrate food, until (as judged from a control experiment) all glycogen has disappeared from its liver, be placed under the influence of a narcotic drug, which arrests the activity of its muscles, glycogen reappears. =Dietetics.=—Even those who are most ignorant of the science of physiology flatter themselves that they have one piece of information: “The whole of the body is renewed once in every seven years.” I cannot trace the origin of this sapient apothegm, which for generations has passed current. If seven weeks or seventy years were the period allowed for the renewal of the tissues, the statement would be equally near the truth. Judging from the rate at which they are destroyed, it is unlikely that blood-corpuscles live for more than five or six weeks. Hairs are shed about two years after they first appear above the surface. On attaining this age a hair drops off and a new one takes its place. The superficial cells of the skin are shed in great numbers every day, and their place taken by younger cells which come up from the deeper layers. The cells of many glands would seem to have a comparatively short term of life. On the other hand, some tissue-elements are far more permanent. By the time a child is a year old all its nerve-cells are in position. They last as long as the individual lives. When the statement with regard to the renewal of the tissues is understood as meaning, not that the cells are destroyed and replaced by new ones, but that within a period of seven years all the molecules which enter into their protoplasm are extruded from the body and replaced by molecules received as food, the assertion verges on the transcendental. It is unlikely that we shall ever obtain data against which it can be checked.