200 HUMAN PHYSIOLOGY 



If the cells are stimulated by an induction current, the 

 pseudopods are withdrawn. Stimulations which work upon 

 the cell from one side, e.g. chemical influences, may have 

 an orientating effect upon the movements of the cell. The 

 wandering of the leucocytes through the walls of the blcod 

 vessels into the tissues seems to be caused by chemical 

 stimulation falling upon the cell from one side only. 



The constant electric current also orientates the movements of 

 naked protoplasmic bodies, by polarization at the places of 

 entrance and exit. In many amoebae the movements are always 

 towards the kathode. The orientating effects of chemical and 

 electric stimulations are called chematropism and galvanotropism. 



2. Ciliary movements. The epithelial cells of many 

 mucous membranes have, on their free surfaces, cilia which 

 move forward and backward in a definite direction. The 

 movement in one direction takes place with greater velocity 

 than in the opposite direction, hence light particles resting 

 on the surface of the mucous membrane are carried forward 

 in that direction in which the movement is stronger. 



The ciliated epithelial cells of a mucous membrane stand 

 in close physiological relation to each other, so that the 

 movements of all the cilia occur in a definite, orderly 

 manner. The nature of this physiological relationship is 

 not known. 



The activity of the cilia is favored by oxygen and by the 

 feeble alkalinity of the surrounding fluid. 



In man, cilia are found in the mucous membrane of the 

 air passages, uterus, oviducts, and on the ependyma of the 

 cerebral ventricles. The movements of the cilia in the air 

 passages force the mucous and the inhaled dust outward ; the 

 movements of the cilia in the oviducts and uterus serve to 

 move the egg forward. 



The spermatozoa are composed of a head and a long 

 thread-like tail. This tail, by making whiplike or pendu- 

 latory movements (analogous to the cilia of the ciliated cells), 

 propels the spermatozoa forward. These movements are 

 increased by the feeble alkalinity of the medium in which 

 the spermatozoa move ; they are decreased by acid fluids. 



