82 THE ENGINES OF THE HUMAN BODY 



That implies a large number of surfaces which move or 

 rub on each other — a great number of joints. Counting 

 both great and small, there are altogether 25 joints con- 

 nected with each lower limb ; 50 joint bearings therefore 

 have to be kept lubricated during each step. In the 

 backbone, which we have seen is also set in motion when 

 we walk, there are 46 bony surfaces which rub and glide 

 the one upon the other. They, too, have to be kept 

 oiled. Then in connexion with the ribs and breast-bone, 

 movements take place at 84 joints — all of them quite 

 small — with every breath we take. There is no rest at 

 these joints ; they are in motion night and day. There 

 are only four joints connected with the head, two where 

 it is poised on the backbone and two where the lower 

 jaw is jointed to the base of the skull, just in front of the 

 ear passages. Sometimes, especially if we have had to 

 chew very tough substances or have been talking and 

 laughing too much, we may hear the joints of the lower 

 jaw actually creak, as if they were short of lubricant. 

 Further, in each upper limb there are 23 joints. All told 

 there are 230 joints in the human body, varying in 

 degrees of magnitude and of importance. A swimmer 

 sets all of these in motion — one after the other — as 

 he plies his limbs and moves his body. We are in 

 reality multi-articular machines. There is no need to 

 emphasise the necessity of a perfect lubricating system 

 for the human body, seeing how extensive the rubbing 

 surfaces are. 



We can best understand the lubricating system which 

 Nature has invented and applied in our bodies by a single 

 example — such as that offered to us in the ankle-joint. 

 In fig. 23 this joint has been laid open by a vertical cut 

 of the leg and foot, one which exposes the tibia — the 

 chief lever of the leg — and the manner in which that 

 lever is jointed to the ankle-bone or astragalus. We may 

 look on the tibia as a spoke of a wheel and the astragalus 

 as its axle. The tibia is a spoke which can rotate only 

 within a limited part of a circle ; when the foot rests on 

 the ground and the tibia turns forward a little way, the 



