96 TEXT-BOOK OF PHYSIOLOGY. 



ally obtains, viz., the overcoming of a small resistance by the appli- 

 cation of a large force acting through a short distance. As a result 

 there is a gain in the extent and rapidity 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 : 



1. 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 muscular effort is, however, very slight, as the center of 

 gravity of the head lies but a short distance in front of the 

 articulation. 



2. By making the vertebral column erect and rigid. This is brought 



about by the action of the common extensor muscles of the trunk. 

 In this condition the center of gravity lies just in front of the 

 tenth dorsal vertebra. The head, trunk, and upper extremities 

 are now supported by the hip-joints; and in order that this sup- 

 port may give to the body a certain degree of stable equilibrium, 

 independent of muscular action, the line of gravity falls behind the 

 line uniting the center of rotation of the two joints. In conse- 

 quence the body would fall backward were it not prevented by 

 the tension of the iliofemoral ligament and the fascia lata. 

 The line of gravity, continued downward, passes through the knee- 

 joint posterior to the axis of rotation, and hence the body would now 

 fall backward were it not prevented by the tension of the lateral 

 ligaments and the contraction of the quadriceps femoris muscle. 

 Though the body is supported by the astragalus, the line of grav- 

 ity does not pass through the line uniting the two joints, for in so 

 doing constant muscular effort would be required to maintain stable 

 equilibrium; passing a short distance in advance of this line, there 



