56 HUMAN PHYSIOLOGY. 



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 



f ulcum 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 contracting 

 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 over- 

 coming 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 mechan- 

 ical 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. This having 

 been accomplished, the center of gravity falls in front of the tenth dorsal 

 vertebra; the vertical line passing through this point falls behind the line 

 connecting both hip-joints. In consequence, the trunk is not balanced on 

 the hip-joints, and would fall backward were it not prevented by the con- 

 traction of the rectus femoris muscle and ligaments. At the knees and 

 ankles a similar balancing of the parts above is brought about by the action 

 of various muscles. When the entire body is in the erect or military posi- 

 tion, the arms by the sides, the center of gravity lies between the sacrum 

 and the last lumbar vertebra, and the vertical line touches the ground be- 

 tween the feet and within the base of support. 



