1164 



TOUCH. 



siologist in the employment of them ; and 

 they will be accordingly employed in our 

 present enquiry. 



The external sensations received through 

 this medium all require the absolute contact* 

 of the body which excites them, with the im- 

 pressible part ; and in this respect, therefore, 

 there is an exact correspondence between 

 the general sensibility and the sense of taste. 

 The force with which the impression is 

 made, is judged of by the intensity of the sen- 

 sation and the depth to which it is felt; whilst 

 the size of the object is in some degree es- 

 timated by the extent of surface over which 

 its contact is perceived. Both these estimates, 

 however, are extremely vague, when they 

 depend only upon the general sensibility of the 

 body ; we shall hereafter see how much more 

 precise they become, when the impressions 

 are received through the skin, which is the 

 special organ of touch, and when they are 

 aided and corrected by the muscular sense. 

 Still we find that the different modes in 

 which external objects affect even the general 

 sensibility, produce a very marked diversity 

 in the character of the sensations. Thus the 

 feeling called forth by a prick is of a very 

 different kind from that which arises from a 

 blow; both, again, are very distinct from that 

 produced by a steady uniform pressure ; and 

 all these are very different from the sensation 

 of stretching or tearing. In the case of a 

 prick, we have an intense impression made 

 on the sensible part ; but this is limited to a 

 very small spot ; whilst the impression of a 

 blow is made with a like intensity over a 

 much larger area.f In the case of steady 

 pressure, on the other hand, the impression 

 is made over a large surface, but is less in- 

 tense ; whilst in stretching and tearing, the 

 mechanical condition of the nerves of the 

 part operated on is affected in a manner pre- 

 cisely opposite to that in which it is acted on 

 by pressure. The sense of shock, derived 

 from a blow extending over a large surface, 

 is very distinct from that of the mere blow, 

 and may be experienced when little or no 

 pain is "felt, as from the "wind "of a shot; 

 and nearly allied to this is the peculiar jar 

 which is experienced, when the shock is 

 transmitted in such a manner as especially 

 to affect the solid casing of the nervous 

 centres, as when we jump from a height, 

 without sufficient care to alight in such a 



* There might seem to be an exception to this 

 statement in regard to heat and cold, which are 

 perceived when their source is at any distance that 

 allows its radiations to affect the temperature of the 

 body itself. But, as will presently be shown, it 

 does not appear that the sense of temperature forms 

 part of the general sensibility of the body; the 

 power of appreciating heat or cold, as such, being 

 limited to the special organs of touch. 



f There can be no doubt that a multitude of 

 pricks made at the same moment, in sufficiently close 

 proximity to prevent them from being separately 

 distinguished, would be felt as a blow ; this being the 

 only sensation which is received from the numerous 

 incisions simultaneously made by the scarificator used 

 in cupping, when it is applied to the parts of the 

 jkin endowed with the least discriminating power. 



manner as to deaden the concussion by the 

 elasticity of the feet, &c. A succession of 

 slight impulses rapidly following one another, 

 gives rise to the peculiar sensation of thrill; 

 which, though most readily excited when the 

 impressions are made upon the skin, may be 

 received through the internal organs also 

 as when we hold a vibrating body between 

 the teeth. 



These and other well-marked differences 

 in the kind of information received through 

 the general sensibility of the body, seem ob- 

 viously attributable rather to diversities in 

 the mode in which the impressions are made, 

 than to any differences in the nature of the 

 impressions themselves, the character of the 

 fibres receiving them, or the endowments of 

 the ganglionic centres in which these fibres 

 terminate. And hence we seem to have a 

 greater right to conclude, that the various 

 affections of general sensibility, which are 

 usually ranked under the common head of 

 internal sensations, however unlike they may 

 be one to another, are really derived from 

 impressions which do not mutually differ so 

 essentially as do those of the special senses 

 from each other and from the general sense. 

 Of this kind are the sensations of hunger and 

 thirst, the "besoin de respirer," the genital 

 sense, the sensation which calls for the expul- 

 sion of the urine and fasces, nausea, the feelings 

 of oppression and of" sinking" at the stomach, 

 burning, itching, tingling, formication, and 

 others. However different these are from 

 each other, when but slightly or moderately 

 excited, they all merge, when more strongly 

 called forth, into the simple consciousness of 

 pain ; and it is further common to them all, 

 that the nerves through which they are ex- 

 cited are the same as those which commu- 

 nicate the impressions that excite tactile 

 sensations, and that mechanical irritation of 

 any of these nerves occasions pain. To the 

 foregoing may be added the " muscular sense," 

 whereby we are informed of the degree of 

 effort put forth by a muscle; and the sense 

 of " fatigue," which seems to be a modifica- 

 tion of this. We are not conscious either of 

 effort or fatigue in the actions of any of the 

 muscles over which the will has no control, 

 such as the heart, the muscular walls of the 

 intestinal canal, &c. ; nor in the case of those 

 whose actions are purely rhythmical, as those 

 of respiration. It is, in fact, only when the 

 will is exerted, either in increasing the force 

 of their contractions, or in antagonising them 

 by the operation of some other muscles, that 

 we become conscious of effort ; and we shall 

 hereafter see that this sense is most important, 

 in enabling us to proportion the amount of 

 force we exert to the resistance to be over- 

 come, and in guiding us with regard to the 

 direction in which exertion is required. A 

 sense of effort and of fatigue seems also to be 

 excited in the sensorium by the mental opera- 

 tions of which the cerebrum is the instrument ; 

 especially when these mental operations are 

 no longer spontaneous, or carried on with 

 facility, but when they require a more or less 



