50 THE DATA OF BIOLOGY. 



beget actions in muscles or other organs. Besides 



neural discharges that follow the direct incidence of external 

 forces, there are others 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 that from 

 moment to moment result from modiiications of the tissues, 

 wrought by substances carried to them in the blood. That 

 the unceasing change of matter which oxygen and other 

 agents produce throughout the system, is accompanied by a 

 genesis of nerve-force, is shown by various facts ; — by the fact 

 that nerve-force is no longer generated, if oxygen be with- 

 held, or the blood prevented from circulating ; by the fact that 

 Avhen the chemical transformation is diminished, as during 

 sleep with its slow respiration and circulation, there is a 

 diminution in the quantity of nerve-force ; in the fact that an 

 excessive expenditure of nerve-force, involves excessive re- 

 spiration and circulation, and excessive waste of tissue. To 

 these proofs that nerve-force is evolved in greater or less quan- 

 tity, according as the conditions to rapid molecular change 

 throughout the body, are well or ill fulfilled ; may be added 

 proofs that certain special molecular actions, are the causes 

 of these special re-actions. The effects of alcohol, ether, 

 chloroform, and the vegeto-alkalies, put beyond doubt the 

 inference, that tlie overthrow of molecular equilibrium by 

 chemical affinity, when it occurs at certain places in the body, 

 results in the overthrow of equilibrium in the nerves pro- 

 ceeding from these places — results, that is, in the propagation 

 through these nerves, of the change called a nervous dis- 

 charge. Indeed, looked at from this point of view, 

 the two classes of nervous changes — the one initiated from 

 M'ithout and the other from within — are seen to merge into 

 one class. Both of them may be traced to metamorphosis of 

 tissue. There can be little doubt that the sensations of 

 touch and pressure, are consequent on accelerated changes of 

 matter, produced by mechanical disturbance of the mingled 



