1 6 PHYSIOLOGY. 



Flexion of the Forearm. Take the bones of the arm that are 

 articulated (if there is not an artificial hinge at the elbow, one can 

 readily be made of wire) ; put a strong rubber band in place of the 

 biceps muscle ; fasten this to the head of the humerus by cords, and 

 by the lower end to the radius, where the rough place, an inch or so 

 from the elbow joint, shows the insertion of the tendon. Have the 

 rubber stretched so that when not held it will flex the forearm. This 

 will serve to show the action of the biceps, though we must be careful 

 to bear in mind that the muscle does not pull the arm up because it has 

 been stretched, as is the case with the rubber. In the case of the 

 muscle, we know that the live muscle has the power of shortening when 

 stimulated, and in this respect is totally unlike the rubber. The live 

 cells, or units, act in concert. 



Levers. The essentials of a lever are the point about 

 which the lever turns, called the fulcrum, the place where 

 the power is applied, called the power, and the part to be 

 moved, called the weight. In the body, the fulcrum is 

 some joint, the power is the place where the muscle is 

 attached, and the weight is the part to be moved. 



Kinds of Levers. In flexing the forearm, the weight 

 is the hand or the hand and what is in it ; the fulcrum is 

 the elbow joint; and the power is the point where the 

 tendon of the biceps is attached to the radius. This kind 

 of a lever is what the books call a lever of the third class. 

 The triceps, on the back of the arm, pulls on the projection 

 of the ulna (the inner bone of the forearm when the palm 

 is up), back of the elbow. The elbow is here, also, the 

 fulcrum, and the hand (or the object to be pushed by the 

 hand) is the weight. This kind of lever, where the fulcrum 

 is between the power and the weight, is called a lever of the 

 first class. In raising the weight of the body, by stand- 

 ing on tiptoe, we use a lever of the second class. Here 

 the ball of the foot is the fulcrum. The weight is the 

 weight of the whole body, resting on the ankle joint, while 



