64 HUMAN PHYSIOLOGY. 



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 conse- 

 quence, the trunk is not balanced on the hip-joints, and would 

 fall backward were it not prevented by the contraction 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 position, 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 between the feet and within the 

 base of support. 



Sitting erect is a condition of equilibrium in which the" body is 

 balanced on the tubera ischii, when the trunk and head together form 

 a rigid column. The vertical line passes between the tubera. 



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

 space, and is accomplished by the combined action of its own 

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



Walking is a complicated act, involving almost all the voluntary 

 muscles of the body, either for purposes of progression or for balanc- 

 ing the head and trunk, and may be denned as a progression in a 

 forward horizontal direction, due to the- alternate action of both 

 legs. In walking, one leg becomes for the time being, the active or 

 supporting leg, carrying the trunk and head ; the other, the passive 

 but progressive leg, to become in turn the active leg when the foot 

 touches the ground. Each leg, therefore, is alternately in an active 

 and a passive state. 



Running is distinguished from walking by the fact that, at a given 

 moment, both feet are off the ground and the body is raised in the air. 



While the limits of a compend do not permit of a description of 

 the origin, insertion, and mode of action of the individual muscles 

 of the body, it has been thought desirable to call attention to a few 

 of the principal muscles whose function it is to produce special forms 

 of movement, as well as locomotion. (See Fig. 6.) The erect posi- 

 tion is largely maintained by the fixation of the spinal column and 

 the balancing of the head upon its upper extremity ; the former is 

 accompanied by the erector spincz muscle, named from its function 

 and its fleshy continuations, situated on each side of the vertebral 

 column. Arising from the pelvis and lumbar vertebrae, this muscle 



