286 



ELEMENTARY PHYSIOLOGY 



most part, piled up into large heaps, and tumble down 

 into small heaps before they leave it. The energy which 

 they set free in this tumbling down, is the source of the 

 active powers of the' organism. 



These active powers are chiefly manifested in the 

 form of motion— movement, that is, either of part of the 

 body, or of the body as a whole, which last is termed 

 locomotion. 



The organs which produce total or partial movements 

 of the human body are of three kinds : cells exhibiting 

 amoeboid movements, cilia, and muscles. 



The amoeboid movements of 

 the white corpuscles of the blood 

 have been already described, and 

 it is probable that similar move- 

 ments are performed by many 

 other simple cells of the body 

 in various regions. 



The amount of movement to 

 which each cell is thus capable 

 of giving rise may appear per- 

 fectly insignificant ; neverthe- 

 less, there are reasons for think- 

 ing that these amoeboid move- 

 ments are of great importance 

 to the economy, and may under certain circumstances be 

 followed by very notable consequences. 



2. Ciliated Epithelium and Action of Cilia.— Cilia 

 are filaments of extremely small size, attached by their 

 bases to, and indeed growing out fi'om, the free surfaces 

 of certain epithelial cells ; there being in most instances 

 very many (thirty for instance), but, in some cases, only 

 a few cilia on each cell (Figs. 40, 84). In some of the 

 lower animals, cells may be found possessing only a 

 single ciliuni. They are in incessant waving motion, so 

 long as life persists in them. Their most common form of 

 movement is that each cilium is suddenly bent upon itself, 



Fio. 84.— Columnar Cili- 

 ated Epithelii'm Cells 

 FROM THE Human Nasal 

 Membrane. 



Magnified 300 diameters. 

 (Sharpey.) 



