288 ELEMENTARY PHYSIOLOGY les& 



fications, tend to drive the mucus in which they work, 

 outwards. 



In addition to the air-passages, cilia are found, in the 

 human body, in a few other localities ; but the part which 

 they play in man is insignificant in comparison with their 

 function in the lower animals, among many of which they 

 become the chief organs of locomotion. 



3. The Structure of Unstriated Muscle. — Unstriated 

 (also called " plain " or " smooth ") muscle occurs in the 

 walls of the alimentary canal, the blood-vessels, the 

 bladder, and other organs. It is composed of bands of 

 fibres which are bound together by connective tissue 

 carrying nerves and blood-vessels. The fibres are in 

 reality elongated, flattened, spindle-shaped cells whose 



FlO. 85.— A FiBRE-CEI.I. FROM THE PLAIN, N0N-8TRtATED MuSCULAB 



Coat of the Intestine. 

 /, fibre ; n, nucleus ; p, granular protoplasm around the nucleus. 



length is about 50/i (.Jg inch) and width 6/x (^^^ inch). 

 Somewhere towards the middle of each cell there is an 

 elongated oval or sometimes rod-shaped nucleus, sur- 

 rounded by a small amount of granular protoplasm which 

 is pointed at the ends of the nucleus. 



The substance of the cell is clear and shows no trans- 

 verse striations, although it often shows signs of a very 

 fine longitudinal fibrillation. A number of such fibre- 

 cells ave united together by a minute quantity of cement, 

 or intercellular substance, into a thin flat band, and a 

 number of such bands are bound together by connective 

 I issue into larger bands or bundles. Each fibre is capable 

 of contracting, that is, of shortening and becoming at the 

 same time thicker. In addition to the actual physical 



