180 THE CIVIL ENGINEERS OF THE BODY 



bone : tendons run like Bowden wires in sheaths, and yet the 

 healthy animal body moves noiselessly and without " heating up ' 

 or " seizing " at any speed. 



(a) Joints. There are, counting great and small, 230 joints 

 in the human body varying in degrees of magnitude and import- 

 ance. The ends of the two opposed bones in a joint are coated 

 with a thin layer of cartilage. This cartilage, in the adult, is 

 what is left of the scaffolding of bone. As we have seen, it is 

 elastic and acts as a resilient buffer. The surface is always 

 covered, in health, with a film of synovial fluid. 



This synovia is kept in place by being enclosed with the joint 

 in a flaccid membrane or joint capsule (Fig. 32). The synovial 

 fluid results from the destruction of the cartilage cells on the 

 rubbing surface of the joint. In this way the supply of lubricant 

 is absolutely automatic. The more the joint surfaces move on 

 each other, the greater is the destruction of the cartilage cells and 

 the more plentiful is the supply of synovia. 



Two other points require our attention, (i) How is the supply 

 of synovia kept up and (ii) what happens to the waste fluid, 

 (i) The cartilage is constantly, like epithelium, growing. The 

 young cells take their origin in the layer next to the bone and push 

 their way up towards the outer surface of the articular cartilage. 

 Every cell destroyed to form synovia has its place taken by a cell 

 from the layer below and so on. Growth and destruction exactly 

 balance one another, (ii) The waste synovia is drained into the 

 blood stream through certain warty structures which project into 

 the crevices of the joints. 



In chronic rheumatism the cartilage cells fail to form synovia. 

 The articular cartilages become dry and leathery and the joints 

 may be heard creaking as they move. The underlying bone, 

 stimulated by the unaccustomed friction, thickens and throws out 

 gnarled processes. The supply of lubricant has failed, and until 

 someone discovers just how cartilage can be converted into 

 synovia and what controls the process nothing can be done to 

 prevent " seizing " of the joints. 



(b) Tendon sheaths. Muscles are attached to bone by sinews 

 or tendons and these cordy structures work in sheaths. The 

 inner surface of the protecting sheath as well as the outer surface 

 of the tendon is endowed with a lubricating substance similar to 

 that of the joints. 



There is one outstanding point of interest about the lubrication 

 system of the body and that is its nourishment. As far as is 



