THE STRUCTURE OF THE ELEMENTARY TISSUES. 81 



III. Muscular Tissue. 



There are two chief kinds of muscular tissue, differing both in mi- 

 nute structure as well as in mode of action, viz., (1.) the plain or non- 

 striated, and (2.) the striated. 



Unstriped or Plain Muscle. 



Distribution. Unstriped muscle forms the proper muscular coats 

 (1.) of the digestive canal from the middle of the oesophagus to the in- 

 ternal 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-blad- 

 der; (6.) of the vesiculaa seminales; (7.) of the pregnant uterus; (8.) of 

 blood-vessels and lymphatics; (9.) of the iris, and some other parts of 



Fig. 81. A, Unstriped muscle cells from the mesentery of a newt. The sheath exhibits trans- 

 verse markings, x 1HO. B, From a similar preparation, showing that each muscle cell consists of 

 a central bundle of fibrils, i<' (coiuractile part), connected with Lhe intra-nuclear network, N, and a 

 sheatli with annular thickenings, St. The cells show varicosities due to local contraction, and on 

 these the annular thickenings are most marked, x 450. (Klein and Noble Smith.) 



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 wrinkling and contraction of the scrotum on exposure to cold. 

 Unstriped muscular tissue occurs largely also in the true skin generally, 

 being especially abundant in the interspaces between the bases of the 

 papillae. Hence when it contracts under the influence of cold, fear, 

 electricity, or any other stimulus, the papillae are made unusually prom- 

 inent, and give rise to the peculiar roughness of the skin termed cutis 

 anserina, or goose skin. It occurs also in the superficial portion of the 

 cutis, in all parts where hairs occur, in the form of flattened roundish 

 bundles, which lie alongside the hair-follicles and sebaceous glands. 

 They pass obliquely from without inward, embrace the sebaceous glands, 

 and are attached to the hair-follicles near their base. 



Structure. TJnstriated muscles are made up of elongated, spindle- 

 shaped, nucleated cells (fig. 81), which in their perfect form are flat, 

 from about -^- to y-gVo of an inch broad (7 to 8,u), and -^ to ^fa of an 

 inch (^ to -fa mm) in length very clear, granular, and brittle, so that 

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