GENERAL PHYSIOLOGY OF MUSCLE-TISSUE 85 



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 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 balancing the head and 

 trunk, and may be defined 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 progressing leg, to become iri turn the active leg when 

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

 and in 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. 



THE VISCERAL OR INVOLUNTARY MUSCLE 



The visceral muscle, as the name implies, is found in the walls of hollow 

 viscera, where it is arranged in the form of a membrane or sheet. It is 

 present in the walls of the alimentary canal, blood-vessels, respiratory tract, 

 ureter, bladder, vas deferens, uterus, fallopian tubes, iris, etc. In some 

 situations it is especially thick and well developed e.g., uterus and pyloric 

 end of the stomach; in others it is thin and slightly developed. 



The Histology of the Visceral Muscle-fiber. When examined with 

 the microscope, the muscle sheet is seen to be composed of fibers, narrow, 



