THE EVOLUTION OF A BURSA 189 



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 tbe 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." 



One more important quotation from this lecture will suffice. 

 " No one who has watched the behaviour of scleroblasts and marked 

 the design in their workmanship can doubt that they have acquired 

 certain characteristic qualities, chief of which is a sensitiveness to 

 vibrations — to stresses. We see them build tbe same form of 

 spicules as their ancestors, and therefore must suppose that their 

 building quality is a gift of inheritance. We see them alter their 

 mode of building as stresses change ; we must therefore suppose 

 that their inherited powers can be changed by the circumstances 

 under which they work." 1 



In regard to the action of the scleroblasts of sponges I have 

 only to point out that the cautious words of Professor Keith on the 



1 Hunterian Lecture on "The Introduction of the Modern Practice of 

 Bone-grafting." Royal College of Surgeons of England, January, 1918. 

 Reported in the Lancet, February 9th and 16th, 1918. 



