168 REPORT 1861. 



superior larj-ngcal nerve, which are clistribtited to the laryngeal nincous mem- 

 brane, which seem to act in a reflex manner on the diaphragm — stopping its action 

 if the action be too great, as from impure or pungent chloroform acting on the 

 membrane, or jiossibly from idiosyncrasj' ; as it has been a long time observed, in 

 France especially, tliat it is dangerous to administer chloroform where irritable 

 larynx exists, or empliysema or other extensive lung-disease. That such irritation, 

 under other circumstances, of other branches of the eighth pair produces permanent 

 closure of the glottis till relieved by tracheotomy — a very formidable remedy no 

 doubt, but one never to be lost sight of in accidents from chloroform." 



On tlie Physical and PJu/siolojical Processes involved in Sensation. 

 By J. i). MoBELL, 3I.A., LL.D. 

 When an appropriate stimulus is applied to any of the organs of sense, a feeling 

 is produced in the mind which is termed, in the language of mental science, a sen- 

 sation. A pin driven into any of the nerves which extend themselves immediately 

 under the surface of the shin produces paiu, — a ray of light falling on the retina 

 produces vision, — a sapid substance put into the mouth produces taste, and so forth. 

 Now it has always been a puzzle amongst mental philosophers to understand how 

 it is that wo can come to a consciousness of external objects at all. Tlieorics without 

 number have been fonued, from the time of Plato downwards, to bridge over the 

 gulf which lies between matter and consciousness, between objects of sense around 

 us and the fact of sensation within us. This chasm in oiu* Itnowledge we do not 



gretend wholly to fill. At the same time, so many facts bearing on the question 

 ave been brought to light by the progi-ess of ^physical science on the one side and 

 by pliysiology 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 o^vn inner consciousness has been vastly diminished, and the mystery driven 

 back to that one point of connexion 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 sub- 

 sequent mental activity, so as to analyse 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 v.'orld without to 

 the mind within. Let us take as an example the sense of hearing, as presenting the 

 greatest degTee of simplicity. "N^'e knoAV, from the investigation of physical science, 

 that the sole medium of sound is the atmosphere. Wliero there is no atmosphere, 

 there can be no sound; and Vv'hei'e the atmosphere is perfectly si ill, perfect silence 

 is the necessary residt. The real cause of sound, therefore, exlenialty considered, 

 is found in the motion of the atmosphere ; and the variations in tlie acuteness or gTa- 

 vity of soimd, we knov/ by experiment, 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 second; the acntcst, 

 that which is produced by about 48,000 oscillations in the same time. The differences 

 in the quality of sounds arise, in like manner, from the pcculiarway in which the atmo- 

 sphere is aftected by the object tliat sets it in motion, and the corresponding pecu- 

 liarity of the atmospheric waves tliat reach tlie ear. What we really sensize, there- 

 fore, 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 auditoiy nerve conveys them, in some manner, 

 to the sensorium. With regard to the way in which this latter effect is brought 

 about we have as yet veiy little insight. The soft texture of the nerves, and the 

 manner in which they are imbedded 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 accom- 

 panied M'ith a constant thrill and movement in every part of the nervous sj^stem, 

 forming what is technically termed the canasfhcsis, 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 de- 

 termination 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. IIow 



