GENERAL PHYSIOLOGY OF MUSCLE-TISSUE. 97 



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 maintained 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 bal- 

 ancing 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 progressing leg, to become in turn the active leg when the foot 

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

 and 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 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 iHs tliin 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, elongated, and fusiform in shape. As a rule, they 

 are extremely small, measuring only from 40 to 250 micromillimeters 

 in length and from 4 to 8 micromillimeters in breadth. The center 

 of each fiber presents a narrow, elongated nucleus. The muscle- 

 protoplasm which makes up the body of the fiber appears to be 

 enclosed by a delicate elastic membrane resembling in some respects 

 the sarcolemma of the skeletal muscle. In some animals the visceral 

 fiber presents a longitudinal striation suggesting the existence of 

 fibrillae surrounded by sarcoplasm (Fig. 39). The fibers are united 

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