THE RE-ACTIONS OF ORGANIC MATTER ON FORCES. 49 



nervous stimulants, greatly increase the violence and rapidity 

 of the electric discharges. 



But whether general or special, and in whatever manner 

 produced, these evolutions of electricity are among the 

 re-actions of organic matter, called forth by the actions to 

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

 but seem rather to be remote consequences of those 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 bo 

 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 a kind of force mani- 

 fested in some classes of living bodies, 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 with 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 mole- 

 cular 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 constitute consciousness, or else result in 

 merely physical processes that are set going elsewhere in the 

 organism. In special parts distinguished as organs of sense, 

 other external actions bring about other nervous re-actions ; 

 that show themselves either as special sensations, or as ex- 

 citements which, without the intermediation of consciousness;, 



