IN HUMAN PHYSIOLOGY. 147 



7. What class of lever do we use when we raise ourselves 

 from a stooping position ? 



The third class. See second question. If we are 

 attempting to lift a heavy burden, the bones act on the 

 principle of the toggle-joint. " When one stoops to take 

 a heavy weight upon his back or shoulder, he puts both 

 the knee and the hip-joints into the condition that the 

 toggle-joint is when it is bent; and then as he straightens 

 up, the weight is raised by an action of the joints pre- 

 cisely similar to that of the toggle-joint in machinery. 

 In the case of the knee, the straightening of the joints is 

 done by the muscles on the front part of the thigh, that 

 draw up the knee-pan with the tendon attached to it. 

 This is using the principle of the toggle-joint in pressing 

 upward. It is also sometimes used in pressing downward. 

 In crushing anything with the heel, we give great force to 

 the blow on the principle of the toggle-joint, by flexing 

 the knee and straightening the limb as we bring down the 

 heel upon the thing to be crushed. In pushing anything 

 before us, we bend the elbow as preparatory to the act, 

 and then thrust the arm out straight, thus exemplifying 

 the toggle-joint. The horse gives great force to his kick 

 in the same way. The great power exerted by beasts of 

 draught and burden is to be referred very much to the 

 principle of the toggle-joint. When a horse is to draw a 

 heavy load, he bends all his limbs, especially the hinder 

 ones, and then as he straightens them, he starts the load. 

 In this case the ground is the fixed block of the mech- 

 anism, the body of the horse to which the load is attached 

 is the movable one, and his limbs are so many toggle- 

 joints. By this application of the principle, we see 

 draught horses move very heavy loads." HOOKER'S Physi- 



