ACTIVITY OF THE NERVOUS GANGLIA AND CORDS. 577 



the nature of the inquiry admits of. Thus in the determination of the func- 

 tions of a particular nerve-trunk, it should be shown that a certain cil'rrt is 

 conistuiilli/ produced by its excitation (under the conditions laid down in the 

 preceding paragraph), and that a corresponding interruption in the action to 

 which it is hence inferred to be subservient, takes place when its continuity 

 has been interrupted ; by this double proof, the Glosso-pharyngeal and the 

 Pneumogastric are shown to be the principal, but not the sole, exeitors of the 

 movements of Deglutition and Inspiration respectively. But the evidence 

 afforded solely by the interruption of a particular function, after the division 

 of a certain nerve, or the destruction or removal of a nervous centre, is by 

 no means so satisfactory ; for this may be occasioned rather by the general 

 effects of the operation, than by the simple lesion of the nervous apparatus. 

 In order to get rid, so far as possible, of this source of fallacy (which par- 

 ticularly affects experiments upon the Encephalic centres, and upon the in- 

 fluence of the nerves upon the viscera), it is desirable to perform comparative 

 experiments, in which the general injury shall be as nearly as possible the 

 same, ami the only difference shall lie in the lesion of the nervous system; 

 and to subtract from the entire result all that can be thus shown to be attribu- 

 table to the general disturbance produced by the operation. But even then, 

 it may happen that the function is only suspended for a time, by the shock 

 which lias been induced by the injury to the nerve; and if it should be sub- 

 sequently renewed without any reunion of the trunk, \\e have the most con- 

 vincing proof that whatever degree of participation the nerve may have in 

 it, the action is not essentially dependent upon the integrity of that portion 

 of the nervous apparatus. Such \ve have seen to be the case, in regard to the 

 relation of the Pneumogastric nerves to the secretion of gastric fluid in the 

 walls of the stomach ( 113). 



468. All our positive knowledge of the functions of the Nervous System 

 in general, save that which results from our own consciousness of what passes 

 within ourselves, and that which we obtain from watching the manifestations 

 of disease in Man, is derived from observation of the phenomena exhibited 

 by animals made the subjects of experiments; and in the interpretation of 

 these, great caution must be exercised. In the first place it must be con- 

 stantly borne in mind, that, except through the movements consequent upon 

 them, we have no means of ascertaining, whether or not particular changes 

 in the Nervous System, whose character we are endeavoring to determine, 

 are attended with sensation ; since we have no power of judging whether or 

 not this has been excited, save by the cries and struggles of the animal made 

 the subject of experiment. Now although such cries and struggles are ordi- 

 narily considered as indications of pain, yet it is not right so to regard them 

 in every instance ; and the only unequivocal evidence is derived from ob- 

 servation of the corresponding phenomena in the Human subject; since we 

 can there ascertain, by the direct testimony of the individual affected, what 

 impressions produce sensation, and what excite movements independently of 

 sensation. Further, we are not justified in assuming that Consciousness is ex- 

 cited by an irritation, still less that Intelligence and Will are called into 

 exercise by it, merely because movements evidently tending to get rid of its 

 source are performed in respondeuce to it. We know that the contractions 

 of the heart and alimentary tube are ordinarily excited by a stimulus, with- 

 out any sensation being involved ; and these movements, like all that are 

 concerned in the maintenance of the Organic functions, have an obvious 

 design, when considered either in their immediate effects, or in their more 

 remote consequences. The character of adaptivcness, then, in Muscular 

 movements excited by external stimuli, is no proof that they are performed 

 in obedience to sensation ; much less that they have a voluntary character. 



