CHAPTER FIFTH. 



OF MOTION. 

 SECTION I. 



APPARATUS OF MOTION. 



205. THE power of voluntary motion is the second grand 

 characteristic of animals ( 65). Though they may not all have 

 the means of transporting themselves from place to place, 

 there is no animal which has not the power of executing some 

 motions. The oyster, although fixed to the ground, opens 

 and closes its shell at pleasure ; and the little coral animal 

 protrudes itself from its retreat, and retires again at its will. 



206. The movements of animals are affected by means of 

 muscles, which are organs designed expressly for this purpose, 

 and make up that large portion of the body, commonly called 

 flesh. They are composed of a series of bundles, which are 

 readily seen in boiled meat. These bundles are again composed 

 of parcels of still more delicate fibres, called muscular fibres 

 ( 215), which have the property of elongating and contracting. 



207. The motions of animals and plants depend, there- 

 fore, upon causes essentially different. The expansion and 

 closing of the leaves and blossoms of plants, which are their 

 most obvious motions, are due to the influence of light, heat, 

 moisture, cold; and other external agents ; but all the motions 

 peculiar to animals are produced by an agency residing within 

 themselves, namely, the contractility of muscular fibres. 



208. The cause which excites contractility resides in the 

 nerves, although its nature is not precisely known. We only 

 know that each muscular bundle receives one or more nerves, 

 whose filaments pass at intervals across the muscular fibres. 

 It has also been shown, by experiment, that when a nerve 

 entering a muscle is severed, the muscle instantly loses its 

 power of contracting, under the stimulus of the will, or, in 

 other words, is paralyzed. 



