582 MOTIOX. 



either by a fine continuous dark streak, or by short iso- 

 lated dark lines, or by dark points arranged in a row, or 

 scattered. These fibre-cells, by their union, form fibres- 

 and bundles of fibres (fig. 156). The fibres have no 

 distinct sheath. 



The fibres of involuntary muscle, such as are here 

 described, form the proper muscular coats of the 

 digestive canal from the middle of the oesophagus ta 

 the internal sphincter ani, of the ureters and urinary 

 bladder, the trachea and bronchi, the ducts of glands,, 

 the gall bladder, the vesiculae seminales, the pregnant 

 uterus, of blood-vessels and lymphatics, the iris, and soma 

 other parts. 



This form of tissue also enters largely into the compo- 

 sition of the tunica dartos, and is the principal cause of 

 the wrinkling and contraction of the scrotum on exposure 

 to cold. The fibres of the cremaster assist in some measure 

 in producing this effect, but they are chiefly concerned in 

 drawing up the testis and its coverings towards the inguinal 

 opening. Unstriped muscular tissue occurs largely also- 

 Ffr , 57 .* in the cutis (p. 421), 



being especially abun- 

 dant at the interspaces 

 between the bases of 

 the papilla3. Hence, 

 when it contracts under 

 the influence of cold, 

 fear, electricity, or any 

 other stimulus, the 

 papilltt) are made un- 

 usually prominent, and give rise to the peculiar rough- 



* Fig. 157. Perpendicular section through the scalp, with two hair- 

 sacs ; , epidermis ; b, cutis ; c, muscles of the hair-follicles (after 

 Kolliker). 



