CHAPTER XIV. 



THE MUSCULAR SYSTEM. 



I. STRUCTURE OF MUSCULAR TISSUE. 



THERE are two chief kinds of muscular tissue, differing both in 

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

 striated, and (2 ) the striated. The striped form of muscular fibre is 

 sometimes called voluntary muscle, because all muscles under the con- 

 trol of the will are constructed of it. The plain or unstriped variety is 

 often termed involuntary, because it alone is found in the greater num- 

 ber of the muscles over which the will has no power. 



(i.) Unstriped or Plain Muscle. 



Distribution. Unstriped muscle forms the proper muscular coats 

 (1.) of the digestive canal from the middle of the O3sophagus to the in- 



FIG. 271. Vertical section through the scalp with two hair sacs; a, epidermis; 6, cutis; c, mus- 

 cles of the hair-follicles. (Kolliker. ) 



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 vesiculse seminales; (7.) of the pregnant uterus; (8.) of 

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

 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. Un- 

 striped muscular tissue occurs largely also in the cutis generally, being 

 especially abundant in the interspaces between the bases of the papillae. 



