20 



THE MECHANISM OF LIFE 



the ball-and-socket joints between humerus and scapula, and 

 femur and jjelvis, and so on. There is always a checking action 

 of some kind which restricts the extent to which one segment of 

 the skeleton can move on another ; thus the movement of the head 

 is checked by muscles (and the relaxation of these when a man 

 goes to sleep causes the head to drop, or fall to one side); the 

 olecranon process (or " funny bone ") in the elbow checks the 

 movement when the whole arm comes straight; the patella (or 

 knee-cap) has the same function with regard to the movement of 

 the lower leg on the thigh, and the slipping aside of the patella is 

 the cause of the knee " going out of joint." In other cases the 

 movement is checked by tendons, or ligaments (as in the case of 

 the separate vertebrae). 



The Motor Organs.— Now clothe the skeleton with muscles, 

 which are so arranged as to pull on the bones in the ways that the 

 latter are free to move because of the nature of the joints. This 

 arrangement of muscle and bone is the subject matter of anatomy, 

 and it can be treated in enormous detail; but the reader will easily 

 see that, from a mechanical point of view, it is all very simple. 

 We have, in fact, systems of levers, with the various powers : thus 



Fig. 4. — The Various Lever Movements of the rooT. 



Here 1 is a lever of the first order, 2 of the second, and 3 of 

 the third order, and, with various modifications, these are typical 

 of the mechanisms of movement of the parts of the limbs and 

 body. 



