SPECIAL PHYSIOLOGY OF MUSCLES. 63 



the fulcrum. As an illustration of this form of lever may be men- 

 tioned : 



1. The depression of the lower jaw, in which movement the fulcrum 

 is the temporomaxillary articulation ; the resistance, the tension 

 of the elevator muscles ; the power, the contraction of the depressor 

 muscles. 



2. The raising of the body on the toes F being the toes, W the 

 weight -of the body acting through the ankle, P the gastrocnemius 

 muscle acting upon the heel bone. 



In levers of the third order the power is applied at a point lying 

 between the fulcrum and the weight. As examples of this form of 

 lever may be mentioned : 



1. The flexion of the forearm F being the elbow-joint, P the con- 

 tracting biceps and brachialis anticus muscles applied at their 

 insertion, W the weight of the forearm and hand. 



2. The extension of the leg on the thigh. 



When levers are employed in mechanics, the object aimed at is the 

 overcoming of a great resistance by the application of a small force 

 acting through a great space, so as to obtain a mechanical advantage. 

 In the mechanism of the human body the reverse generally obtains 

 viz., the overcoming of a small resistance by the application of a 

 great force acting through a small space. As a result, there is a 

 gain in the extent and rapidity of movement of the lever. The power, 

 however, owing to its point of application, acts at a great mechanical 

 disadvantage in many instances, especially in levers of the third 

 order. 



Postures. Owing to its system of joints, levers, and muscles, the 

 human body can assume a series of positions of equilibrium, such as 

 standing and sitting, to which the name posture has been given. 

 In order that the body may remain in a state of stable equilibrium 

 in any posture, it is essential that the vertical line passing through 

 the center of gravity shall fall within the base of support. 



Standing is that position of equilibrium in which a line drawn 

 through the center of gravity falls within the area of both feet placed 

 on the ground. This position is maintained : 



1. By firmly fixing the head on the top of the vertebral column by 

 the action of the muscles on the back of the neck. 



2. By making the vertebral column rigid, which is accomplished 

 by the longissimus dorsi and the quadratus lumborum muscles. 



