456 



HUMAN ANATOMY. 



Fig 









 5-/ 



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J' 



/ 



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Section of uterus, showing bundles of 

 various directions. 



involuntary muscle cut in 



/^ 220. 



described as rod-shaped, being cylindrical with rounded ends. Its position is fre- 

 quently eccentric with regard to the axis of the cell, as well as often somewhat nearer 

 one pole than the other. The nuclei of these muscle-cells are rich in chromatin, 

 which usually presents a reticular arrangement. Under the influence of contraction, 



the nuclei present more or less 

 variation from their typical rod 

 form. Centrosomes (Fig. 469) 

 may be distinguished in favorable 

 preparations lying within the cy- 

 toplasm close to the nucleus (Zim- 

 mermann, Lenhossek). 



The contractile fibriUcB rep- 

 resent differentiated anisotropic 

 threads within the cell-body, in 

 their property of double refraction 

 resembling the fibrillae of striped 

 muscle. They are most conspicu- 

 ous at the periphery of the fibre- 

 cell, where they lie closely related 

 to the condensed boundary zone 

 ( Heidenhain ) which forms the 

 exterior of the fibre and fulfils the 

 purpose of a limiting membrane 

 or sarcolemma, although no such definite structure encloses the muscle-cell as in the 

 case of the striated fibre. The demonstration of contractile fibrillse within the muscle- 

 cells of the higher vertebrates is unsatisfactory on account of the small size of the 

 elements ; in the large cells of the amphibia, especially in the huge elements of the 

 amphiuma, their presence is readily established. Although lying usually within the 

 periphery of the fibre-cell, the existence of a conspicuous axial fibre is seen in certain 

 cases, as in the large isolated muscle-cells within the mesentery of newts. 



The individual elements of unstriped muscle are held together by delicate mem- 

 branous expansions of connective tissue prolonged from the more robust septa investing 

 and uniting the bundles and fasciculi of the fibre- 

 cells. On cross-section (Fig. 470), these inter- 

 cellular membranous partitions appear as delicate 

 lines between the transversely cut cells, which 

 were formerly interpreted as tracts of cement- 

 substance uniting the muscular elements. The 

 appearances of intercellular bridges, described by 

 several authors (Barfuth, de Bruyne, Werner, 

 Bohemann, Apathy) as connecting the adjacent 

 cells, depend probably upon the shrinkage of 

 the latter due to the action of reagents (Stohr, 

 Heidenhain). 



The blood-vessels supplying involuntary 

 muscle are guided in their distribution by the 

 septa of interfascicular connective tissue in which 

 the larger twigs run. The latter give off minute 

 branches which terminate in capillaries that ex- 

 tend between the primary bundles of the muscle- 

 cells. The blood-supply of non-striated muscle 

 is meagre when compared with that of the striped 

 muscles. 



The lymphatics occur closely associated 

 with the muscular tissue in localities in which 

 the latter exists in considerable quantity, as in the wall of the stomach and intestine, 

 the interfascicular connective tissue containing plexuses of lymph-channels. 



The nerves supplying involuntary muscle are intimately related to the sympa- 

 thetic system. The larger trunks form plexuses, in close association with microscopic 



Fig. 472. 



Portion of injected intestinal wall, showing 

 arrangement of blood-vessels supplying invol- 

 untary muscle ; upper longitudinally, lower 

 transversely cut. X 50. 



