582 MOTION. 



tary muscles and the heart, consists of compound and 

 apparently striped fibres, or tubes including fibres. The 

 difference, however, between these two kinds of muscular 

 tissue is less marked than was formerly supposed ; for 

 both in structure and mode of action they often closely 

 resemble each other. 



The inoluntary or unstriped muscles are made up, 

 according to Kolliker, of elongated, spindle-shaped, nu- 

 cleated fibre-cells (fig. 148), which in their most perfect 

 form are flat, from about -^-^ to ^Vo" f an i nc h broad, 

 and about -$^ to -5^- of an inch in length, very clear, 

 granular, and brittle, so that when they break, they often 

 have abruptly rounded or square extremities. Each fibre- 

 cell possesses an elongated nucleus, and many are marked 

 along the middle, or, more rarely, along one of the edges, 

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

 ted 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. 149), 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 to 

 the internal sphincter ani, of the ureters and urinary 

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

 the gall-bladder, the vesiculse seminales, the pregnant 

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

 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 

 in the cutis (p. 423), being especially abundant at the 



