54 



NORMAL HISTOLOGY. 



and in the walls of the ureter, bladder and urethra. 4. The male reproduc- 

 tive organs in the epididymis, vas deferens, seminal vesicles, prostate and 

 Cowper's glands, and cavernous and spongy bodies of penis. 5. The female 

 reproductive organs in the oviducts, uterus and vagina; in the broad and 

 round ligaments; in the erectile tissue of the external genital organs and in 

 the nipple. 6. The vascular system in the walls of the arteries, veins and 

 larger lymphatics; sometimes in the trabeculae of the larger lymph-nodes; in 

 the capsule and trabeculae of the spleen. 7. The eye in the iris and ciliary 

 region; in the eyelids. 8. The integument in the sweat and some seba- 

 ceous glands: as the minute erector muscles of the hair- follicles and in the 

 skin covering the scrotum and parts of the external genital organs. 



Nonstriated, smooth, pale, unstriped or involuntary muscle, as it is 

 variously designated, consists of structural units known as the fibre-cells. 

 These are delicate spindle, often prismatic, elements whose tapering ends fit 



FIG. 65. Involuntary muscle from intestine ; several isolated fibre-cells are seen above. X 200. 



between the adjacent fibre-cells. They vary greatly in size, measuring from 

 50-225 fi in length and from 3-8 p in width. The fibre-cells found in the 

 skin and the blood-vessels are short (15-20 //) and broad; those in the intes- 

 tinal wall are more elongated (215-220 /;.) and delicate. The largest 

 elements are encountered in the gravid uterus where they may attain a length 

 of 500 /i and a width of 30 p. Occasionally the cells are bifurcated, esper 

 daily among the lower vertebrates. Each fibre-cell consists of protoplasm in 

 which are embedded the nucleus and the contractile fibrilloe/ The nucleus, 

 usually described as rod-shaped from its elongated oval or cylindrical form, 

 is placed frequently somewhat eccentrically with regard to the long axis and 

 nearer one pole than the other. It is rich in chromatin which usually 

 presents a reticular arrangement. Influenced by contrac- 

 tion, the nuclei often exhibit deviations from the typical 

 rod-form. Paired centrosomes have been observed 

 within the cytoplasm close to the side of the nucleus. 

 The contractile fibrillae, convincingly seen only within 

 the large elements of the amphibia, are doubly refract- 

 ing (anisotropic) threads within the cytoplasm. They 

 lie at the periphery of the fibre-cell, closely related to 

 the denser boundary zone, which forms the exterior of 

 the fibre-cell and fulfils the purpose of a limiting 

 membrane, or sarcolemma, although no such definite 

 structure encloses the smooth muscle-cell as in the case of the striated fibre. 

 The individual fibre-cells are held together by an exceedingly delicate 

 investment of connective tissue fibres, both white and elastic, which surround 

 the muscle elements and in cross-sections appear as lines, formerly inter- 

 preted as cement-substance, that pass between and around the fibre-cells. 

 Since the latter are fusiform, their transverse areas, irregularly oval or polyg- 

 onal in outline, vary with the plane of section, being relatively large and 

 nucleated when cut through the middle of the fibre-cell and progressively 

 smaller towards the ends (Fig. 66). 



FIG. 66. Bundles of 

 involuntary muscle in 

 transverse section, show- 

 ing the fibre-cells cut 

 crosswise. X 400. 



