first, all impressions made upon the same part agree in this, whatever be their differences as to kind and degree, that they enter by the same nerves, and at the same part of the brain, and spinal marrow. Secondly, we impress a great variety of sensations ourselves by our hands, according as they are hot or cold, by friction, scratching, &c. and most impressions from foreign bodies will bear some resemblance to some of these. Thirdly, we often see upon what part impressions from foreign bodies are made. Fourthly, when they leave permanent effects, as in wounds, burns, &c. we always examine by feeling, where the impression was made.
Now from all these things laid together it follows, that in itchings from an internal cause, and in impressions where neither our hand or eye give us any information, we shall, however, be able to determine at once with tolerable accuracy what external part is affected, and to put our hand upon it, so as to confirm our present judgment, and render our future judgment, and voluntary power, more certain and ready. We shall also do this most readily in those parts which we see and feel most frequently, the hands for instance; less so, cæteris paribus, in those we seldom see or feel; and least so, where we never see the part, and seldom touch it. At least this seems to result from the theory. But it is to be observed, that the fact ought to be tried chiefly in children. For in adults the several degrees approach more to perfection, i.e. to an equality among themselves.
Here we may observe, first, that as we never see or feel the internal parts, such as the lungs, heart, stomach, intestines, liver, kidneys, bladder, &c. we can have no direct information in the manner explained under the foregoing proposition.
Secondly, Since all pains diffuse an increase of vibrations into the neighbouring parts, the increased vibrations in the external parts, arising from internal pains, will be a gross general direction, so as to determine the seat of the pain within gross limits, in respect of superior and inferior, anterior and posterior, right and left.
Thirdly, Pressing the external parts, so as to augment or alleviate the internal pain, must contribute also.
Fourthly, Since all the internal parts in the thorax and abdomen receive branches from the intercostal nerve, which communicates with each vertebral pair, it follows that the internal pains will send vibrations up to the spinal marrow, which will enter in at the same parts of it, as the vibrations from external pains in the neighbourhood. At the same time it appears from the many ganglions, plexuses, and communications of nerves, in the thorax
