GENERAL PHYSIOLOGY OF MUSCLE-TISSUE. 83 



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 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 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 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, 



FIG. 40. Two SMOOTH MUSCLE-FIBERS FROM SMALL INTESTINE OF FROG. X 240. Isolated 

 with 35 per cent, potash-lye. The nuclei have lost their characteristic form through the action 

 of the lye. (Stohr.} 



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 connective-tissueJ 



skeletal muscle. In some animals the 



visceral fiber presents a longitudinal 



striation suggesting the existence of Nucleus. 



fibrillae surrounded by sarcoplasm (Fig. Smooth muscie-fibei 



40). The fibers are united longitudi- intransver 



nally and transversely by a cement FIG. 41. SECTION OF THE CIRCULAR 



material. The muscle is increased in LAYER OF THE MUSCULAR COAT OF THE 



thickness by the superposition of succes- HuMAN 



sive layers. At varying intervals the fibers are grouped into' bundles or 



fasciculi by septa of connective tissue (Fig. 41). Blood-vessels ramify in 



the connective tissue and furnish the necessary nutritive material. 



The visceral muscle receives stimuli from the spinal cord, not directly, 

 however, as in the case of the skeletal muscle, but indirectly through the 



