218 THE CIVIL ENGINEERS OF THE BODY 



vibrates on the impact of a stream of water. There is quite a 

 body of evidence that a similar vibration-theory may hold good 

 as well for the work of the osteoblasts as for that of their ancestral 

 relatives the scleroblasts. Compare, for example, the interlacing 

 spiral lines in which calcium is deposited (Fig. 51, b) in the femur 

 with the similar figure obtained when sand is scattered on the 

 surface of a rectangular bar clamped horizontally at one end and 

 caused to vibrate. The nodal lines are really lines separating two 

 parts which are vibrating in opposite phases and so correspond to 

 the stress lines. If now we dust the plate with two powders, one 

 heavy, say, blue sand, the other light like lycopodium, we will have 

 a pattern in blue on the nodal lines and a pattern in white at the 

 parts of maximum vibration. This deposition of the light powder 

 is due to the formation of small vortices in the fluid (gas or liquid) 

 near the vibrating body sweeping the powder to the centre of the 

 vortex (Faraday). Bone is subject to longitudinal, transverse and 

 to torsional vibrations, and has in it all the factors to produce 

 vibration figures. 



Fractures. There are several ways by which a piece of black- 

 board " chalk " can be broken. Let us consider three of them. 

 {a) If we hold it by both ends and apply force directly at right 

 angles to the long axis of the chalk, it will snap transversely at the 

 point of contact of the distorting force. (6) An oblique fracture 

 results from smacking the chalk vertically on the table. The 

 force of compression applied along the long axis leads to an 

 extension at right angles to this (cf. rubber cube, p. 207). The 

 fracture is only indirectly due to the impact on the tabic, but is 

 directly due to the bulging being beyond the elastic limit of the 

 material, (c) A twisting force applied to the chalk produces a 

 spiral fracture. These types of fracture can all be produced in 

 bone, (a) Fractures caused by direct violence are transverse and 

 are located at the point of incidence of the force. (6) Oblique 

 fractures are a sign of indirect violence, e.g. fracture of the clavicle 

 from a fall on the outstretched arm. (c) Spiral fractures are 

 produced when the body is twisted with the limb fixed. 



(5) Lubricating Cells. 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 vip " 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. 



