MOTION. 579 



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. 



CHAPTEE XVII. 



CAUSES AND PHENOMENA OF MOTION. 



THE so-called vital motions observable in the bodies of 

 animals, are performed almost exclusively 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 longitu- 

 dinal direction and are fixed at both extremities, or form 

 circular bands : the contraction or shortening of the fibres 

 bringing the parts to which they are fixed nearer to each 

 othei*. There are, besides, various molecular movements 

 allied to these which need not here be considered. 



CILIARY MOTION. 



As just remarked, ciliary motion consists in the incessant 

 vibration of fine, pellucid, blunt processes, about -^^ of 

 an inch long, termed cilia (figs. 146, 147), situated on the 

 free extremities 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 (pp. 42, 43). 



Ciliary motion seems to be alike independent of the 

 will, of the direct influence of the nervous system, and of 



