ZOOLOGY. 351 



of sense around us and the fact of sensation within us. The chasm in 

 our knowledge we do not pretend wholly to fill. At the same time, 

 so many facts bearing on the question have been brought to light by 

 the progress of physical science on the one side, and by physiology 

 on the other, and so much has been added by the mental analyst, 

 likewise, from his peculiar point of view, that the distance between 

 the outer world and our own inner consciousness has been vastly 

 diminished, and the mystery driven back to that one point of connec- 

 tion between the brain and the human soul which no analysis appears 

 likely fully to solve. Let us attempt, then, to strip away all that is 

 mixed up with sensation naturally, and all that is added to it by our 

 subsequent mental activity, so as to analyze the bare fact itself, and 

 reduce it to its simplest elements. 



Looking to the physical and external parts of the process, we must 

 consider, first of all, what it is that the nerves convey from the world 

 without to the mind within. Let us take as an example the sense of 

 hearing, as presenting the greatest degree of simplicity. We know, 

 from the investigation of physical science, that the sole medium of 

 sound is the atmosphere. Where there is no atmosphere there can be 

 no sound, and where the atmosphere is perfectly still, perfect silence 

 is the necessary result. The real cause of sound, therefore, exter- 

 nally considered, is found in the motion of the atmosphere ; and the 

 variations in the acuteness or gravity of sound, we know by experi- 

 ment, arise from the greater or less rapidity of the oscillations- The 

 deepest note which the human ear appears capable of perceiving as 

 a continuous sound is that produced by sixteen oscillations in a sec- 

 ond ; the acutest, that which is produced by about forty-eight thou- 

 sand oscillations in the same time. The differences in the quality of 

 sounds arise, in like manner, from the peculiar way in which the at- 

 mosphere is affected by the object that sets it in motion, and the corre- 

 sponding peculiarity of the atmospheric waves that reach the ear. 

 What we really sensize, therefore, through the ear is simply the 

 motion of the atmosphere, and nothing more. The human ear is an 

 apparatus beautifully formed for receiving the vibrations on which 

 all sound depends, and the auditory nerve conveys them, in some 

 manner, to the sensorium. With regard to the way in which this lat- 

 ter effect is brought about, we have as yet very little insight. The 

 soft texture of the nerves, and the manner in which they are imbed- 

 ded in the surrounding materials, would naturally suggest a total 

 inaptitude for propagating vibrations in the ordinary sense of that 

 term. It seems more probable that the flow of life through the body 

 is accompanied with a constant thrill and movement in every part 

 of the nervous system, forming what is technically termed the coenaBS- 

 thesis, or common sensibility, so that the outward oscillations do not 

 so much originate wholly new vibrations as enter into conflict with 

 the nervous action already going on, and give it that peculiar deter- 

 mination which is necessary to create any given sensation in the mind. 

 This is perhaps as far as it is possible to go in our analysis of the 

 physical process. How the vibration of the air comes into conflict 

 with the living thrill of the nerve, and how the result of this conflict 

 reaches the mind, we are at present unable to comprehend. It is 

 one of those hidden secrets of nature which science has not yet been 

 able to unfold. 



