'll6 HISTOLOGY 



spersed elastic fibers. These features, which are essential for understand- 

 ing the action of smooth muscle, are usually difficult to observe in the com- 

 pact tissue of the adult. 



Smooth muscle fibers in the adult are fusiform, cylindrical or slightly 

 flattened cells, varying in length from about 0.02 mm. in some blood 

 vessels to approximately 0.5 mm. in the pregnant uterus. In the intestine 

 they are said to measure about 0.2 mm. Their diameter, through the 

 widest part, is from 4-7 /*,. Owing to the length of these fibers and the 

 fact that they are not perfectly straight, they are seldom wholly included 

 in a single section. Moreover they are usually so closely packed that their 

 outlines are hard to follow. They may be isolated, however, by treating 

 fresh tissue with a 35% aqueous solution of potassium hydrate, or 20% 

 nitric acid. The fibers when shaken apart appear as in Fig. 105. Owing 



FIG. 105. SMOOTH MUSCLE FIBERS FROM THE SMALL INTESTINE OF 

 A FROG. X 240. 



to the readiness with which they may be disassociated, the existence of 

 connections between them has sometimes been overlooked or under- 

 estimated; but it is evident that independent cells, by shortening cannot 

 cause the contraction of a tube. Branching fibers have been isolated 

 from the aorta, and are said to occur also in the ductus deferens and 

 bladder. 



The fibers when sectioned longitudinally (see Fig. 17 7, p. 186) some- 

 what resemble connective tissue, from which they may be distinguished 

 by the staining and texture of their protoplasm and the position of their 

 nuclei, which are within the fibers. With haematoxylin and eosin the mus- 

 cle substance takes a deeper and more purple stain than the connective 

 tissue fibers, and it is not so refractive. In doubtful cases Mallory's con- 

 nective tissue stain may be used, which colors the muscle substance red and 

 the white fibrous tissue blue. 



The nuclei of smooth muscle fibers are elliptical or rod-like bodies, con- 

 taining a characteristic chromatic reticulum and sometimes several nu- 

 cleoli (Fig. 9, A, p. 10). When the muscle fiber contracts, the nucleus 

 shortens and broadens, but according to measurements made by Miss 

 McGill (Anat. Rec., 1909, vol. 3, pp. 633-635) there is no change in its 

 volume. She finds, however, that the chromatin tends to collect at the 

 poles of the contracted nucleus, and states that "the nucleus appears to 

 take an active part in the process of contraction." Frequently spirally 

 twisted or bent nuclei are found in layers of contracted muscle (Fig. 106) 

 and they have been regarded as occupying contracted fibers. It is 



