CAUSES AND PHENOMENA OF MOTION. 



15 



Structure. The non-striated muscles are made up of elongated, 

 spindle-shaped, nucleated, fibre cells (Fig. 263), which in their perfect 

 form are flat, from about ^-^5- to j-. 1 ,,-,-,- of an inch broad, and -$fa to -j-J^ of 

 an inch in length, very clear, granular, and brittle, so that when they 



FIG. 263. A, unstriped muscle cells from mesentery of newt, sheath with transverse marking 

 faintly seen. X ISO. B, from similar preparation, showing each muscle cell consists of a central 

 bundle of fibrils (contractile part) connected with the intranuclear network and a sheath with annu- 

 lar thickenings. The cells show varicosities due to local contraction, and on these the annular thick- 

 enings are most marked. X 450. (Klein and Noble Smith.) 



break they often have abruptly rounded or square extremities. Each 

 muscle cell consists of a fine sheath, probably elastic; of a central bundle 

 of fibrils representing the contractile substance; and of an oblong nucleus 

 which includes within a membrane a fine network anastomosing at the 

 poles of the nucleus with the contractile fibrils. The ends of fibres 



FIG. 264. Plexus of bundles of unstriped muscle cells of the pulmonary pleura of the guinea-pig. 

 X 180. (Klein and Noble Smith.) 



are usually single, sometimes divided. Between the fibres is an albumi- 

 nous cementing material (endomysium) in which are found connective- 

 tissue corpuscles, and a few fibres. Theperimyshim is the fibrous con- 

 nective tissue surrounding and separating the bundles of muscle cells. 



(2.) STRIATED OR STRIPED MUSCLE. 



Distribution. The striated muscles include the whole class of vol- 

 untary muscles, the heart, and those muscles neither completely volun- 



