72 MUSCULAR TTSSUE [CH. VI. 



canal from the middle of the oasophagus to the internal sphincter 

 ani ; (2.) of the ureters and urinary bladder ; (3.) of the trachea and 

 bronchi ; (4.) of the ducts of glands ; (5.) of the gall-bladder ; (6.) of 

 the vesicuke seminales ; (7.) of the uterus and Fallopian tubes ; (8.) of 

 blood-vessels and lymphatics ; (9.) of the iris, and ciliary muscle of the 

 eye. This form of tissue also enters largely into the composition (10.) 

 of the tunica dartos, the contraction of which is the principal 

 cause of the contraction of the scrotum on exposure to cold. It 

 occurs also in the skin generally, being found surrounding the secret- 

 ing part of the sweat glands and in small bundles attached to the hair 

 follicles ; it also occurs in the areola of the nipple. It is composed of 

 long, fusiform cells (fig. 87), which are not as a rule more than 

 ^o- inch long. Each cell has an oval or rod-shaped nucleus. The 

 cell substance is longitudinally but not transversely striated. Each 

 cell or fibre, as it may also be termed, has a delicate sheath. The 

 fibres are united by cementing material, which can be stained by silver 

 nitrate, and is bridged across by fine filaments passing from cell to cell 

 The nerves in involuntary muscle (both cardiac and plain) do not 

 terminate in end -plates, but by plexuses or networks, which ramify 

 between and around the muscular fibres. 



Development of Muscular Fibres. 



All muscular fibres (except those of the sweat glands, which are 

 epiblastic) originate from the mesoblast. The plain 

 fibres are simply elongated cells in which the nucleus 

 becomes rod-shaped. In cardiac muscle, the likeness to 

 the original cells from which the fibres are formed is not 

 altogether lost, and this is specially the case for the 

 Purkinje's fibres alluded to on the preceding page. 



Voluntary muscular fibres are developed from cells 

 which become elongated, and the nuclei of which mul- 

 tiply. In most striated muscle fibres the nuclei ulti- 

 mately take up a position beneath the cell-wall or 

 sarcolemma which is formed on the surface. Stria- 

 tions appear first along one side, and extend round the 

 fibre (fig. 88), then they extend into the centre. 



During life new fibres appear to be formed in part 

 by a longitudinal splitting of pre-existing fibres ; this 

 FIG. ss. Develop- is preceded by a multiplication of nuclei ; and in part 

 by the lengthening and differentiation of embryonic 



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months. (Ran- QQ[\ S (sarcoplasts) f ound between the fully formed fibres. 

 In plain muscle, growth occurs in a similar way : 

 this is well illustrated in the enlargement of the uterus during preg- 

 nancy. After parturition the fibres shrink to their original size, but 

 many disappear and are removed by absorption. 



