578 THE NERVOUS SYSTEM. 



thetic nerves and ganglia may be in man, as in the lower 

 animals, the parts through which the ordinary and constant 

 influence of nervous force is exercised on the organic 

 processes ; while the cerebro-spinal nervous centres and 

 their ganglia are so closely connected with the proper 

 sympathetic ganglia, that neither of them can be said to 

 be independent of the other ; each, as a rule, and under 

 ordinary circumstances, governing its own domain, but 

 always liable to be influenced by the other. 



CHAPTER XVII. 



CAUSES AND PHENOMENA OF MOTION. 



THE most evident vital motions observable in the bodies 

 of animals, are performed in one or other of the following 

 ways : first, by means of the oscillatory motion or vibration 

 of microscopic cilia, with which the surfaces of certain 

 membranes are beset ; and secondly, by the contraction of 

 fibres which either have a longitudinal direction and are 

 fixed at both extremities, or form circular bands : the con- 

 traction or shortening of the fibres bringing the parts to 

 which they are fixed nearer to each other. There are, 

 besides, various molecular movements allied to those which 

 need not here be considered. 



CILIAHY MOTION. 



Ciliary motion consists in the incessant vibration of fine, 

 pellucid, blunt processes, about ^Voir ^ an * nc k l n e> 

 termed cilia (figs. 153, 154), situated on the free extremi- 

 ties of the cells of epithelium covering certain surfaces of 

 the body. 



The distribution and structure of ciliary epithelium 

 and the microscopic appearances of cilia in motion have 

 been already described (p. 33). 



