NON-STRIATED OR INVOLUNTARY MUSCLE. 



455 



mucosa and villi ; the muscular tunic from the lower half of the oesophagus to the anus ; 

 in the large excretory ducts of the liver, pancreas, and some salivary glands, as well 

 as in the gall-bladder. 2. The respiratory tract, — in the posterior part of the trachea, 

 encircling bundles in the bronchial tubes as far as their terminal divisions. 3. The 

 uriyiary tract, — in the capsule and pelvis of the kidney, ureter, bladder, and urethra. 



Fig. 468. 



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



4. "Wv^ male generative organs, — in the epididymis, vas deferens, seminal vesicles, 

 prostate body, Cowper's glands, and cavernous and spongy bodies of the penis. 



5. T\^Q. female generative organs, — in the oviducts, uterus, and vagina ; in the broaci 

 and round ligaments ; in the erectile tissue of the external genitals and of the nipple. 



6. The vascular system, — in the coats of the arteries, veins, and larger lymphatics. 



7. The lymphatic glands, — in the capsule and trabeculae of the spleen ; sometimes in 

 the trabeculae of the larger lymph-nodes. 8. The eye, — in the iris and ciliary 

 body ; in the eyelids. 9. The integument, — in the sweat- and some sebaceous 

 glands, as the minute erector muscles of the hair-follicles and in the skin covering 

 the scrotum and parts of the external genitals. 



Structure. — Non-striated, unstriped, pale or involuntary muscle consists of 

 an aggregation of structural units known as the fibre-cells. These are deli- 

 cate spindle, often prismatic, elements which terminate in oblique surfaces at either 

 end for contact with adjacent cells. They vary greatly in size, measuring from .050- 



.225 mm. in length and .003-. 008 mm. 

 Fig. 469. in width. The muscle-cells found in the 



skin and blood-vessels are short (.015- 

 .020 mm.) and broad; those in the in- 

 testinal wall are more elongated (.215- 

 .220 mm.) and delicate. The largest 

 elements are encountered in the gravid 

 uterus, in which they attain a length of 

 .500 mm. and a breadth of .030 mm. 

 Occasionally the cells are bifurcated at 

 ! the ends, especially among the lower ver- 



: tebrates. 



Fig. 470. 







Portions of intestinal muscle-cells, showing nucleus and 

 centrosome (f). Highly magnified. {Lenhossek.) 



Bundles of involuntary muscle in transverse section, 

 showing the fibre-cells cut crosswise. X 400. 



More recent critical examinations of the fibre-cells have demonstrated the 

 existence of greater structural complexity than was formerly recognized.^ According 

 to these later views, each fibre-cell consists of a protoplasmic mass in which lie 

 embedded the yiucleus and the contractile fibrilla. The former is appropriately 



' An e.xhaustive review of the literature and various opinions concerning the structure ot 

 unstriped muscle is given by M. Heidenhain : Ergebnisse der Anatomic und Entwick., Bd. x., 

 1900. 



