LUBRICATION 179 



This internal structure is altered to meet alterations in the 

 incidence of stress. For example, during the first twenty years 

 of life when the body is growing and the bone lengthening, constant 

 alterations in internal and external structure have to be made. 

 The unnecessary parts are decalcified and the fibrous tissue under- 

 goes alteration. During this process some of the fibrous tissue 

 cells become enlarged and multinucleated. Histologists call 

 these cells osteoclasts. The remaining fibrous cells afterwards 

 become bone marrow. 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 bone, has 

 undergone remodelling to suit the new 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. 

 There they are in equilibrium and will tend to " solidify " in 

 that position. 



(5) Lubrication. Certain cartilage cells have a peculiar function, 

 that of acting as a lubricant between rubbing surfaces. One of 

 the most worrying problems of the engineer is to prevent " heating 

 up " of moving surfaces. This he attempts to do by interposing 

 a fine uniform film of oil between surfaces where -friction is apt 

 to take place. The particles of the oil film act as microscopic ball 

 bearings over which the moving surfaces slide with the minimum 

 of friction. The motor cyclist knows how essential it is to have 

 the right amount of the right grade of oil in the right place. In 

 spite of all precautions however, " seizing " does take place. The 

 film of oil is rubbed away just at the point where it is most required. 

 Only one machine has, as yet, been designed which has a perfect 

 lubricating system, and that is the animal body. In the body 

 there are many rubbing surfaces. At joints, bone works against 



