23. The relations of properties of nerves may be inquired into 

 upon grounds of analogy, and by the mode of investigation many 

 times repeated in every case in which a function is displayed.* 

 Thus it may be inquired after in the instances of secietion before 

 enumerated, and in the parts of the nervous system, with relation to 

 each other: this latter examination will refer principally to the 

 brain and spinal marrow; and, with but few exceptions, and those 

 almost equivocal, will respect the animal and intellectual pheno- 

 mena, rather than those of the organic life, with which we are at 

 present engaged. 



24. Can we then, it will be asked, define in abstract no cri- 

 terion by which the seats of dependent functions might be assigned, 

 and the functional dependent distinguished in every case from the 

 assimilating properties? No such criterion exists: we may say 

 those phenomena depend only upon the properties which assimilate 

 in their seat, which phenomena take place merely as a result of 

 the relation between blood and the living spirit of the seat. Thus 

 much might be set down as an abstract, general criterion; but the 

 investigation we have indicated must be applied in every instance 

 before we can decide the question of dependence, &c. in the examples 

 in which this question might arise. Thus, it cannot be said either 

 that secretion is pioduced wholly by the assimilating life of its seat, 

 or that secretion is a process dependent upon properties obtained 

 from a distant sphere, because to the former assertion it would be 

 replied, the secretion of gastric fluid ceases upon the division of the 

 eighth pair of nerves; and to the latter it would be objected, that a 

 wound will suppurate below the place of the division of an 

 axillary plexus. 



25. After discussing certain relations subsisting between the 

 properties of the nervous system, the more difficult inquiry must be 

 suggested here, as on former occasions, concerning the identical 

 nature of the properties engaged in these relations. Our causation 

 teaches us that for every diversity of effect a different cause is to be 

 assigned: this axiom is incontrovertible, and therefore the different 

 phenomena of the nervous system are to be referred to so many 

 different agencies. It has long been a favourite design with certain 

 physioligists to evince the identity of electricity with the properties 

 of the nerves; but the inferences deduced from the facts prove such 

 physiologists to be but very indifferent reasoners. Electricity will 

 produce muscular contraction, after certain functions, characteristic 

 of the living state, have ceased : so also will irritation of the muscles 

 of an amputated limb with a scalpel excite their contraction. From 

 these facts, there is as much reason to pronounce life to be a scalpel, 

 as electricity to be life. Electricity will excite powerful actions of 

 the muscles after death ; other substances, perhaps mechanical ones, 

 will excite weaker contractions ; the former is therefore to be con- 



* The functional instances have formed the subjects of preceding pages, 

 fend need not hereto be repeated. 



