The Muscles and the Bones. 



3 



Ball 



Articular Extremity 



classes of levers may be illustrated by different motions 



of the foot. In tapping the toes on the floor while 



the heel is lifted, or in pressing down the ball of the 



foot while running the treadle of 



a sewing machine, we have an 



example of a first-class lever. In 



raising the weight of the body on 



tiptoes, or as the foot is used in 



taking each step, the foot is used 



as a lever of the second class. 



When one lifts a weight with the 



toes, the foot is used as a lever 



of the third class. (See Fig. 19.) 



Advantages of Levers in the Body. - II Medullary Cavity 



If the arm consisted merely of the biceps 

 muscle, suspended from the shoulder, it 

 is evident that its only action would be 

 a straight pull. Suppose the biceps, 

 thus hanging alone from the shoulder, 

 had a hook at its lower end, it could, 

 when it shortened, lift a weight just as 

 far as it shortened, and no farther. It 

 could not swing the weight outward, or 

 push it upward. But from the way in 

 which the biceps is attached to the fore- 

 arm (see Fig. 10), when the muscle 

 shortens an inch it may move the hand Fig. 20 

 a foot. Of course the hand moves much 

 faster, and we have a great gain in speed by reason of this lever arrange- 

 ment. But we cannot lift so heavy a weight at this faster rate, as we 

 could at the elbow. 



Hard Bone 



- Spongy Bone 

 Articular Extremity 



Longitudinal Section of 

 Femur. 



Study of One of the Long Bones. For this, take, preferably, a femur 

 or a humerus. Let us suppose we have a femur. 



i. Observe its shape, cylindrical, somewhat curved, enlarged at 

 the ends. 



