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Communications from the following gentlemen were received too late
for publication this month : Charles Batham, M. D. Messrs. W. H. Leadbeater, J. Ring, A. Fogo,
G. Nassé Hill, J. Smith, Apothecarius, an Essex Practitioner,
. S. L. and T. "We have been informed that the Practitioner of Hackney, mentioned
in the Monthly Magazine, and alluded to in the account of Stramo. nium in this Number of our Journal, as having the merit of intro.
ducing the practice of smoking that plant in cases of asthma, is Mr. • Toulmin. The notice of Mr. Dumbell's Liverpool Lint, of which we have heard . a good character, was mislaid, and did not arrive in time for in
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· To the Editors of the Medical and Physical Journal.
Theory of Sensation.
S Sensation is one of the most striking characteristics of animal life ; and as painful or uneasy sensation forms a prominent feature in almost all diseases :so a knowledge of the real state of the living power and organ upon which it acts, under, 'impressions that cause sensation, cannot fail to be highly interesting both in a medical and philosophical point of view.
In the following discussion an explanation of these has been attempted, which from the manner in which the enquiry has been conducted, will, it is hoped, not be found liable to any very material objections, care having been taken to avoid ail hypotheses, and to advance nothing which is not either founded on universally received data, or supported by proofs derived from the unyariable phenomena of life. · Before offering a new explanation of the cause of Sensation, it may be proper to notice the opinions which have been als ready advanced. I shall do this as briefly as possible. .
The only Theories of Sensation that I am acquainted with are the following. First, That it is caused by impressions made on the extremities of nerves, and communicated through. them to the sensorium commune; and secondly, that it is truly an action of the sensorial power upon the fibres of the delicate vessels in which the nerves terminate, and that this action resembles muscular contraction.
Of the first of these opinions little requires to be said ; it professes to teach us nothing of the state of the organ or part at which sensation is felt, under impressions; and seems to imply that the nerves are not endowed with vitality as well as the sensorium commune, a doctrine that is not proved, and certa inly not altogether consonant to facts. The second opia No. 148.)
nion, that of Dr. Darwin, is more determinate; he has slated his doctrine clearly and explicitly, and has endeavoured to prove it by reference to facts. I do not know the general opinion of his Theory of Sensation ; but if I am not mistaken, . it is defective in this circumstance, that it does not inform us by what means the stirnulus of light or other cause of Sensation induces the sensorial power to act on the fibres of the organ. This action of the vital power causing contraction of these fibres, certainly implies that previous thereto it bad received soine intelligence of the presence of the stimulus, and if so, it is quite superfluous to suppose contraction of fibres to be necessary to Sensation, since the Sensation must be already communicated before the action can take place.
In all philosophical investigations it ought never to be for. gotten, not only that we are to admit no more causes of things than such as are both true and sufficient to explain the appearance, but that assigned causes, if true, ought always to produce the same effects in similar circumstances. Hence if a fibrous action, resembling the contraction of muscles, be in any instance not only a cause of Sensation but the Sensation itself, then I say that muscular contraction ought in every
instance to be felt; otherwise the Theory is imperfect. But · the contraction of muscles is in general not attended with Sensation in the fibres acting. In speaking we have no Sensation in the muscles which move the tongue. By attending to it we know, indeed, tbat our tongue moves, but that knowledge is derived from the Sensation produced in those parts of the mouth which the tongue touches, or in the tongue itself at those parts of it where contact with the teeth, gums, or palate takes place, and not in the fibres of the muscles that move that organ. When I will to lift my hand off this table I know that it moves obedient to volition, because I am informed of the motion by my sense of sight, or by my hand or arm touching some other part of my body, &c. ; but I have no sensation in the muscles which acted in performing that intention of the will, and could not have known that these muscles did move if I had not been previously informed of the fact, or received conviction of it at the time, by applying my other hand and feeling a motion there. The same is equally true of all other muscles subject to the power of voli. tion. The cause of any apparent deviations from this rule will, I trust, be explained to the entire satisfaction of the reader hereafter. With regard to the involuntary muscles, it must be obvious to every observer that the same want of Sensation obtains during their motions or actions. For in eommon cases we have no Sensation from the action on the hcart and arteries, of the muscles of respiration, of the intes:
tinal canal, or fron, the motion of the lacteal and lymphatic systems; all of which perform their various functions with out sensation in the power acting, or fibre acted upon.
From these facts I conclude that muscular contraction, although it may be in many, as it evidently is in some in. stances, an exciting cause, is not in one instance the sole cause of Sensation ; and that some condition of the vital power and organic fibre, different from muscular contraction, must be present when Sensation is excited. What tlrat con. dition is we will now endeavour to point out and demonstrate by the phenomena of Sensation.
In order to convey at once to the mind of my readers a clear idea of my views, I shall begin by stating what has appeared to me, after an attentive review of the phenomena of Sensation in health, and as far as my opportunities have per.. mitted me, in diseases, to be the fundamental principle of Sensation ; and then mention the phenomena in proof of that principle, which may be expressed as follows :
Sensation is felt when an action attempted by the vital
power is in any degree interrupted or obstructed.: or in other words, when an action performed is less than the power exerted.
Proofs from the Phenomena.
FIRST GENERAL PHENOMENON. .
the abstraction or defect of those matters which are
FIRST INSTANCE, A greater or less quantity of disengaged caloric* is essentially necessary to most, if not all, the actions of the living body. When more or less of that quantity is abstracted, it follows that a greater or less degree of interruption to the actions previously going on in the part from which it is taken must ensue. But when caloric is abstracted in unusual quan
* I shall always use this word when the matter of heat is meant, to distinguish it from heat which I shall employ to express the Sensation only which caloric excites.
tily, an immediate Sensation of cold is felt. Hence it is 'evi. dent this Sensation must be attended with interrupted action, and that the interruption is rially the cause of the Sensation appears for the following reasons :
ist. When the actions interrupted are restored by the reapplication of caloric, the Sensation of cold instantly ceases.
2d. When new actions (to which the abstraction of caloric is a stimulus) are successfully performed by the same power, and the former actions are discontinued, the Sensation ceases although the abstraction of caloric continues.
Thus, when a person enters the cold bath, he feels a very powerful Sensation of cold, and the temperature of his body falls. Commonly in a very short time the Sensation diminishes greatly, or goes entirely off, always as I have found by experiment before the temperature of the body arrives at the natural standard, but not till a progressive rise is indicated by the thermometer. In experiments of this kind, care ought always to be taken to distinguish between the Sensation of cold and the shivering which is sometimes a consequence of it : for in my experiments I have always found that the Sen. sation of cold was dininished by the shivering, and that when the sbivering ceased before the temperature of the body had arrived at, or was rapidly ascending to the natural standard, the cold Sensation recurred with the effect of inducing a return of shivering. In general the temperature of the body rises gradually to the natural standard, which is a proof that the actions by which caloric is evolved are greater in the centre of the body at least, although the surface and extremities feel colder to another person—for the abstraction of caloric continuing greater than common, it must require an increased evolution of caloric to support the temperature. But since the actions of the surface to which caloric is necessary cannot be performed so casily while the greater abstraction continues, it is probable that the Sensation of cold ceasing or diminishing on the surface of the body is owing to the lesser exertions of the vital poker on the surface, and greater in the centre or other parts of the system. A fact noticed by Dr. Currie (whose beautiful experiments related in the Medical Reports are well know!) and which I myself have experienced, makes this amount almost to certainty : namely, that after the Sensation of cold had gone greatly or entirely off; and the temperature of the body was increasing ; if the Sensation returned, a new fall of temperature speedily took place. This seems to show that the power which performed the actions. on the surface that were interrupted by the abstraction of caloric on entering the bath, had really been employed in new actions in other parts of the system, while the sensation of cold was