84 TEXT-BOOK OF PHYSIOLOGY 



1. The elevation of the trunk from the flexed position. The axis of move- 



ment, the fulcrum, lies in the hip-joint; the weight, that of the trunk, 

 acting as if concentrated at the center of gravity, which lies close to the 

 tenth dorsal vertebra; the power, the muscles attached to the tuberosity 

 of the ischium. The opposite movement is equally one of the first 

 order, but the relative positions of P and W are reversed. 



2. The head in its movement backward and forward on the atlas. 



In levers of the second order the weight lies between the power and the 

 fulcrum. As illustration of this form of lever may be mentioned: 



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, in which movement the fulcrum is 



the toes, the weight that of the body acting through the ankle, the 

 power the gastrocnemius muscle applied to the heel bone. 

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

 the fulcrum and the weight. As example of this form of lever may be 

 mentioned : 



1. The flexion of the forearm, in which the fulcrum is the elbow- joint, the 



power the biceps and brachialis anticus muscles applied at their points 

 of insertion, the weight that of the forearm and hand. 



2. The extension of the leg on the thigh. 



When levers are employed in mechanic operations, the object aimed at 

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

 acting through a great distance, so as to obtain mechanic advantage. In 

 the mechanism of the human body the reverse generally obtains, viz., the 

 overcoming of a small resistance by the application of a large force acting 

 through a short distance. As a result there is a gain in the extent and rapid- 

 ity of the movement of the lever. The power, however, owing to its point 

 of application, acts at a great mechanic 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 term 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 its 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 of the entire body falls within the base of support. 

 This position is maintained largely by the mechanical conditions of the 

 joints, apparently for the purpose of reducing to a minimum muscular 

 action, so that it can be prolonged for some time without giving rise to 

 fatigue. In the military position, which may be assumed as the normal 

 position, all the joints must be in such a condition of extension and fixation 

 that the body will represent a rigid column resting on the astragalus and 

 supported by the arch of the foot. This is accomplished : 

 i. By balancing the head on the apex of the vertebral column. This is 

 done by the action of the muscles on the back of the neck. The mus- 

 cular effort is, however, very slight, as the center of gravity of the head 

 lies but a short distance in front of the articulation. 



