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TEXT-BOOK OF PHYSIOLOGY 



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. 40). The fibers are united 

 longitudinally and transversely by a cement material. The muscle is 

 increased in thickness by the superposition of successive layers. At vary- 



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.) 



ing 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 

 intermediation of ganglion cells, which may be located at some distance 

 from the muscle or near the walls of the viscera. Non-medullated fibers 

 from the ganglion pass directly into the muscle, where they frequently unite 

 to form a general plexus. From this plexus fine branches take their origin 

 and ultimately become physiologically associated with the muscle-fiber. 



Physiologic Properties. The visceral muscles which have been sub- 

 jected to experiment are mainly those of the stomach, intestine, bladder, 



ureter, and iris. From the results of 

 the experiments which have been pub- 

 lished, it is evident that all visceral 

 muscles possess elasticity, tonicity, irri- 

 tability, and conductivity. 



The elasticity of the bladder mus- 

 cle of the cat was strikingly shown in 

 the experiments published by Dr. Colin 

 C. Stewart. When this muscle was 

 weighted with weights differing by a 

 common increment, it was extended on 

 the addition of each weight, though to a progressively less extent. On the 

 removal of the weights the muscle eventually returned to its former length. 

 The records of the extension were similar to, if not identical with, those of 

 the skeletal muscle. 



The tonicity of visceral muscles is as pronounced in many situations as 

 is the tonicity of skeletal muscles. Each muscle is continuously in a state 

 of contraction intermediate between that of complete contraction and that 

 of relaxation. In how far this is due to local and inherent causes or to 

 stimuli reflected from the nervous system as a result of peripherally acting 

 causes is not in individual instances readily determinable. From time to 



Connective-tissue 

 septum. 



Nucleus. 



Smooth muscle-fiber 

 in transverse section. 



FIG. 41. SECTION OF THE CIRCULAR 

 LAYER OF THE MUSCULAR COAT OF THE 

 HUMAN INTESTINE. (Stohr.) 



