14 HAND-BOOK OF PHYSIOLOGY. 



III. MUSCULAR MOTION. 



Varieties of Muscular Tissue. There are two chief kinds of 

 muscular tissue: (1.) the plain or non-striated, and (2.) the striated, and 

 they are distinguished by structural peculiarities and mode of action. 

 The striped form of muscular fibre is sometimes called voluntary muscle, 

 because all muscles under the direct control of the will are constructed of 

 it. The plain or unstriped variety is often termed involuntary, because 

 it alone is found in the greater number of the muscles over which the 

 will has no power. 



(1.) PLAIK OR UNSTRIPED MUSCLE. 



Distribution. Involuntary muscle forms the proper muscular coats 

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

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

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

 vesiculae seminales; (7.) the pregnant uterus; (8.) of blood-vessels and 

 lymphatics; (9.) the iris, and some other parts. This form of tissue also 



FIG. 262. Vertical section through the scalp with two hair-sacs; a. epidermis; 6, cutis; c, muscles 

 of the hair-follicles. (Kolliker.) 



enters (10. ) largely into the composition of the tunica dartos, and is the 

 principal cause of the wrinkling and contraction of the scrotum on expo- 

 sure to cold. Unstriped muscular tissue occurs largely also (11.) in the cutis 

 (p. 335, Vol. I.), 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 

 pi#minent, 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 (Fig. 228). 



