52 THE DATA OF BIOLOGY. 



it is subject. Though these re-actions are not direct, but 

 seem to be remote consequences of changes wrought by 

 external agencies on the organism, they are yet incidents in 

 that general re-distribution of motion which these external 

 agencies initiate; and as such must here be noticed. 



§ 21. To these known modes of motion, has next to be 

 added an unknown one. Heat, Light, and Electricity are 

 emitted by inorganic matter when undergoing changes, as 

 well as by organic matter. But there is manifested in some 

 classes of living bodies a kind of force which we cannot 

 identify with any of the forces manifested by bodies that are 

 not alive, — a force which is thus unknown, in the sense that 

 it cannot be assimilated to any otherwise-recognized class. I 

 allude to what is called nerve-force. 



This is habitually generated in all animals, save the lowest, 

 by incident forces of every kind. The gentle and violent 

 mechanical contacts, which in ourselves produce sensations 

 of touch and pressure — the additions and abstractions of 

 molecular vibration, which in ourselves produce sensations of 

 heat and cold, produce in all creatures that have nervous 

 systems, certain nervous disturbances : disturbances which, 

 as in ourselves, are either communicated to the chief nervous 

 centre, and there arouse consciousness, or else result in mere 

 physical processes set going elsewhere in the organism. In 

 special parts distinguished as organs of sense, other external 

 actions bring about other nervous re-actions, that sliow them- 

 selves either as special sensations or as excitements which, 

 without the intermediation of distinct consciousness, beget 

 actions in muscles or other organs. Besides neural 



discharges following the direct incidence of external forces, 

 others are ever being caused by the incidence of forces 

 which, though originally external, have become internal by 

 absorption into the organism of the agents exerting them. 

 For thus may be classed those neural discharges which result 

 from modifications of the tissues wrought by substances car- 



