THE RE-ACTIONS OP ORGANIC MATTER ON FORCES. 53 



ried to them in the blood. That the unceasing change of 

 matter which oxygen and other agents produce throughout 

 the system, is accompanied by production of nerve-force, is 

 shown by various facts; by the fact that nerve-force is no 

 longer generated if oxygen be withheld or the blood pre- 

 vented from circulating; by the fact that when the chemical 

 transformation is diminished, as during sleep with its slow 

 respiration and circulation, there is a diminution in the 

 quantity of nerve-force; by the fact that an excessive ex- 

 penditure of nerve-force involves excessive respiration and 

 circulation, and excessive waste of tissue. To these proofs 

 that nerve-force is evolved in greater or less quantity, accord- 

 ing 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 the vegeto-alkalies put 

 beyond doubt the inference that the overthrow of molecular 

 equilibrium by chemical affinity, when it occurs in certain 

 parts, causes excitement in the nerves proceeding from those 

 parts. Indeed, looked at from this point of view, the 



two classes of nervous changes the one initiated from with- 

 out and the other from within are seen to merge into one 

 class. Both of them may be traced to metamorphosis of 

 tissue. The sensations of touch and pressure are doubtless 

 consequent on accelerated changes of matter, produced by 

 mechanical disturbance of the mingled fluids and solids com- 

 posing the parts affected. There is abundant evidence that 

 the gustatory sensation is due to the chemical actions set up 

 by particles which find their way through the membrane 

 covering the nerves of taste ; for, as Prof. Graham points out, 

 sapid substances belong to the class of crystalloids, which 

 are able rapidly to permeate animal tissue, while the colloids 

 which cannot pass through animal tissue are insipid. Simi- 

 larly with the sense of smell. Substances which excite this 

 sense are necessarily more or less volatile; and their vola- 

 tility being the result of their molecular mobility, implies 

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