169 



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CHAP. XX. 



VOLUNTARY MOTION. 



THE processes of every living system, like those of inanimate 

 nature, are carried on with motion. 



" By ceaseless action all that is subsists." * 



It is implied in the circulation, secretion, nutrition, and absorp- 

 tion of the minutest vegetable and animal ; and, generally, when 

 observation is possible, the solids, no less than the fluids within 

 them, are seen in these functions to move. b Some contend, though 

 without proof, that the nervous functions are performed with 

 motion of a vibratary kind. The evident motion of the brain 

 from circulation and respiration, and the very much larger quan- 

 tity of blood constantly passing through the brain and other 

 nervous parts than mere nutrition (unless each functional act causes 

 waste of solids) can require, and this in proportion to activity of 

 function, show that in one sense motion is indispensable even to 

 nervous function. The other functions by which fresh substance 

 from without is obtained, the blood purified, the new animal 

 originated, and indeed all those other functions and modes of 

 function which distinguish animals from vegetables, take place 

 with manifest and considerable motion, and, though vegetables 

 have not the power of locomotion, the leaves and flowers of 

 many of them move rapidly and considerably. Now motions of 

 the leaves, flowers, and vessels of plants are evidently the result 

 of life, and are inexplicable by mere gravity, electricicy, &c. No 

 peculiar known structure is united with their movements. Some 



* Cowper. Task. 



6 We witness vegetable fluids passing along surfaces and through cellular 

 structure ; and fluids in some adult animals through such a structure ; and in all 

 before a heart exists, or even vessels at the spot. Many declare that particles 

 move spontaneously not only in blood (Dr. Tiedemann, 1. c. cclxv.), but in the 

 juices of plants (cclxxxvii.), (cccclxxxiii. sqq.) (alsodlxx.). Perhaps some of 

 these motions are to be explained by the absorption and emission of fluid, some 

 by evaporation, some by chemical processes altering the position of particles, and 

 some by extraneous impulse. In many animals we shall see that the movements 

 of fluids upon surfaces arise from the vibration of hair-like projecting bodies, 

 termed cilia. 



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