Another distinguished Hunterian Professor A. Keith, also gave two lectures in January, 1918, on the “Introduction of the Modern Practice of Bone-grafting,” which, in its modern form, he assigns to the credit of Sir William MacEwen. He lays great stress on the important work performed in such cases by the osteoblasts without whose living and formative action these results could not be obtained. He explains how necessary it is that these living elements should be stimulated into action by _work_. They thrive only so long as they have work to do. Another surgeon, Ollier, “wondered why the fragments of bone which he had succeeded in raising from slips of periosteum planted beneath the scalp or amongst muscles ceased to grow and tended to disappear. These bony grafts withered because they were not subjected to the strains and stresses which rouse the activity of osteoblasts.” MacEwen, “by a fortunate chance, planted his tibial grafts in a situation where they soon became subjected to muscular strains and stresses. In a short time bony fragments gathered from the legs of six boys became intrinsic parts of the humerus of a seventh; from the moment of primary union the bone cells of the graft were brought under the stimulating impulses of the biceps and triceps. Osteoblasts are the obedient slaves of muscles; _muscular dominance is their breath of life_.” (Italics not in the original.) “Wolff was the first to devote thirty years of constant work and observation to prove that the shape and structure of growing bones and adult bones depend on the stresses and strains to which they are subjected. By altering the lines of stress the shape of a bone can be changed.” Wolff’s law is simply this: “Osteoblasts at all times build and unbuild, according to the stresses to which they are subjected.” Professor Keith says further: “We are driven, as I have pointed out in a previous lecture, to look for the primary cause, not in the bones, but in the muscles, particularly in those which are tonically and constantly in action so long as we are standing.” A terse expression of Wolff’s law is quoted from Dr. John B. Murphy, of Chicago: “The amount of growth in a bone depends upon the need for it.” A remarkable illustration of a similar process is given in the construction of sponges by the scleroblasts and it is stated: “The soft walls of this sponge are constantly exposed to the force of moving waters, and we shall see that the spicule-builders--the scleroblasts--are endowed with the same properties as osteoblasts--the powers of fashioning and depositing the elements of the skeleton so that the sponge can best resist the forces to which it is habitually exposed.”