4 PRELIMINARY CONSIDERATIONS 



the more ordinary varieties of what may be called 

 extra-organic force^ in contradistinction to that which, 

 originating within the body, may be called intra- 

 organic. And this shall be the subject of our present 

 chapter. 



As we might anticipate, the parts of the 

 plant or animal which respond most readily to extra- 

 organic agents, are those which are simple and rudi- 

 mentary in their character ; for in the higher textures, 

 such as muscle, the influence of this class of agents is 

 greatly masked by the workings of inherent vitality. 

 In the simple tissues, however, there is no obscurity. 



In the sensitive plant, for example the cellular 

 cushions which move the leaves are found to expand 

 under the influence of light and warmth, and to 

 contract on the withdrawal of these agents ; and with 

 few exceptions, and these but apparent, the same law 

 is observable in all similar organs. 



The simpler fabrics of animal bodies, which are 

 furthest removed from the control of the nervous 

 system, and in this respect most nearly allied to 

 the tissues of the plant, are also found to move in 

 obedience to an external impulse. The subcutaneous 

 areolar web is shrunk and puckered in winter, so as 

 to give rise to the appearance called " cutis anserina," 



