32 ESSENTIALS OF PHYSIOLOGY. 



a double nerve supply, one set of nerves when stimulated causing con- 

 traction, and the other relaxation of the muscle. 



CILIARY MOVEMENT. 



Cilia are delicate filaments projecting from the border of columnar 

 epithelial cells, and are found in the greater part of the respiratory 

 passages, in the generative organs, and elsewhere. 



Their movement consists in a rapid bending of each cilium, followed 

 by a slow return to the erect position. All the cilia on a ciliated 

 surface bend in the same direction, the movement travelling as a wave 

 over the surface. The movements are repeated ten to twelve times a 

 second, and serve to carry forward solid particles of dust or mucus in 

 the respiratory passages ; in the Fallopian tubes they assist the passage 

 of the ovum. 



Ciliary movement is entirely independent of the central nervous 

 system, and its mode of production is unknown. 



