GENERAL PHYSIOLOGY OF MUSCLE-TISSUE 85 



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 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. 



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 support 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 consequence 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 back- 

 ward 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 gravity 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 would be a tendency of the body to fall forward, which is prevented by 

 the extensor muscles of the foot. When the body is in the erect or military 

 position, the center of gravity lies between the sacrum and last lumbar 

 vertebra. Standing is thus an act of balancing, and requires not only the 

 static conditions of joints, but the dynamic conditions of various groups of 

 muscles, and hence is not a position of absolute ease and cannot be main- 

 tained for any length of time without experiencing discomfort and fatigue. 

 Sitting erect is an attitude of equilibrium in which the body is balanced on 

 the tubera ischii, when the head and trunk together form a rigid column. 

 Locomotion is the act of transferring the body as a whole through 

 the environment and is accomplished by the combined action of its own 

 muscles. The acts involved consist of walking, running, jumping, etc. 



