VI BRAT I OK 



217 



cells become enlarged and multinucleated. Histologists call these 

 cells osteoclasts. An adjustment to meet altered conditions may 

 be seen when a bone is broken and allowed to set badly, so that 

 its parts lie somewhat out of their former positions. Tension 

 and compression lines do not now coincide with the trabecular 

 structure. It has been shown by Wolff and others that in a few 

 weeks, not only has an alteration taken place at the seat of fracture, 

 but the entire trabecular system, right to the ends of the hone, has 

 undergone remodelling to suit the nezo incidence of forces. More 

 recent work on bone grafting has amply demonstrated the 

 astonishing rapidity with which reconstruction of the trabecular 

 meshwork takes place. One must remember that in spite of its 

 rigidity, bone is plastic. Physical chemists have proved that 

 when an inorganic constituent separates as a definite phase from 

 a colloidal matrix, the new phase is at first liquid. We may, 

 therefore, infer that the new 

 trabeculae are more or less liquid 

 when formed. The action of force 

 upon them will tend to set them 

 along the lines of that force, e.g., 

 straws set along the direction of 

 the wind. They are practically 

 " carded " into position, where 

 being in equilibrium they will tend 

 to " solidify " in mass. 



One may easily see this process 

 of " carding " in action in the siliceous sponges. These sponges 

 not only orient themselves in water so as to reduce their 

 resistance to the steady dominant flow of their environment, 

 but their siliceous skeleton which supports the softer tissues 

 has an internal structure like bone with the silica laid down 

 on stress lines so arranged as to take the load. Altering the 

 direction of the stream of water and thus setting up stresses 

 different from the previous ones, causes the laying down of 

 a new skeleton with an absorption of the old one. Chladni's 

 experiments (Fig. 53) demonstrate that fine grains are deposited 

 on a horizontal vibrating plate in certain patterns or figures, 

 depending on the nature of the vibration produced in the plate. 

 That is, the plate does not vibrate in all its parts at the same rate. 

 Some points or nodes are practically stationary, and it is at these 

 nodes that most grains are found. Dendy and Nicholson observed 

 that siliceous particles were first laid down on the nodal points of 

 the sponge. The basis, then, of each line of stress outlined in 

 silica is a series of points which remain stationary when the sponge 



tliladni's exix-rimeiit. 



