850 OF SENSATION, AND THE ORGANS OF THE SENSES. 



excitement, is to stimulate the individual to remove himself from what would 

 be injurious in its effects upon the system. Thus, the pain resulting from vio- 

 lent pressure on the cutaneous surface, or from the proximity of a heated body, 

 gives warning of the danger of injury, and excites mental operations destined to 

 remove the part from the influence of the injurious cause : and this is shown by 

 the fact that loss of sensibility is frequently the indirect occasion of severe 

 lesions the individual not receiving the customary intimation that an injurious 

 process is taking place. 1 Instances have occurred, in which severe inflammation 

 of the membrane lining the passages has resulted from the effects of ammoniacal 

 vapors, introduced into them during a state of syncope the patient not receiving 

 that notice of the irritation which, in an active condition of his nervous system, 

 would have prevented him from inhaling the noxious agent. 



853. It is a general rule, with regard to all sensations, that their intensity is 

 much affected by Habit; being greatly diminished by frequent and continual 

 repetition. This is not the case, however, with regard to those sensations to 

 which the attention is peculiarly directed; for these lose none of their acuteness by 

 frequent repetition; on the contrary, they become much more readily cognizable 

 by the mind. We have a good example of both facts in the effects of sounds 

 upon sleeping persons ( 843). The general law, then, seems to be, that Sen- 

 sations, not attended to, are blunted by frequent repetition ; and this may per- 

 haps be connected with certain other general facts, which lie under the obser- 

 vation of every one. It is well known that th vividness of sensations depends 

 rather on the degree of change which they produce in the system, than on the 

 absolute amount of the impressing force ; and this is alike the case with regard 

 to the special and the ordinary sensations. Thus, our sensations of heat and 

 cold are entirely governed by the previous condition of the parts affected ; as is 

 shown by the well-known experiment of putting one hand into hot water, the 

 other into cold, and then transferring both to tepid water, which will seem cool to 

 one hand, and warm to the other. Every one knows, too, how much more we 

 are affected by a warm day at the commencement of summer, than by an 

 equally hot day later in the season. The same is the case in regard to light 

 and sound, smell and taste. A person going out of a totally dark room into 

 one moderately bright, is for the time painfully impressed by the light, but soon 

 becomes habituated to it ; whilst another, who enters it from a room brilliantly 

 illuminated, will consider it dark and gloomy. Those who are constantly ex- 

 posed to very loud noises become almost unconscious of them, and are even 

 undisturbed by them in illness ; and the medical student well knows that even 



1 The following case, recorded in the "Journal of a Naturalist," affords a remarkable 

 instance of this general fact. The correctness of the statement having been called in ques- 

 tion, it was fully confirmed by Mr. Richard Smith, the late senior surgeon of the Bristol 

 Infirmary, under whose care the sufferer had been. " A travelling man, one winter's even- 

 ing, laid himself down upon the platform of a lime-kiln, placing his feet, probably numbed 

 with cold, upon the heap of stones, newly put on to burn through the night. Sleep over- 

 came him in this situation ; the fire gradually rising and increasing, until it ignited the stones 

 upon which his feet were placed. Lulled by the warmth, the man slept on ; the fire in- 

 creased until it burned one foot (which probably was extended over a vent-hole) and part 

 of the leg above the ankle entirely off, consuming that part so effectually, that a cinder- 

 like fragment was alone remaining and still the wretch slept on ! and in this state was 

 found by the kiln-man in the morning. Insensible to any pain, and ignorant of his mis- 

 fortune, he attempted to rise and pursue his journey, but missing his shoe, requested to 

 have it found ; and when he was raised, putting his burnt limb to the ground to support 

 his body, the extremity of his leg-bone, the tibia, crumbled into fragments, having been 

 calcined into lime. Still he expressed no sense of pain, and probably experienced none ; 

 from the gradual operation of the fire, and his own torpidity during the hours his 

 foot was consuming. This poor drover survived his misfortunes in the hospital about a 

 fortnight; but the fire having extended to other parts of his body, recovery was hopeless." 

 See also \ 349, note. 



