2. Where, in addition to being _felt_ upon the tongue, the substance excites sensation in the _olfactory nerves_, as by lead, tin, copper, &c. 3. Where, besides being _felt_, there are peculiar sensations of _taste_, expressive of the properties of bodies, as salt, sugar, tartaric acid, &c. 4. Where, besides being _felt_ and _tasted_, there is an _odour_ characteristic of the substance, and essential to the full development of its flavours, as in cloves, lemon-peel, caraway-seed, and aromatic substances generally. 1004. _Why do we feel?_ Because there are distributed to various parts of the body fine nervous filaments, which have for their special duty the transmission to the brain of impressions made upon them _by contact_ with substances. [Verse: "The works of the Lord are great, sought out of all them that have pleasure therein."--PSALM CXI.] 1005. _In what parts of the body does the sense of touch more especially reside?_ In the points of the fingers and in the tongue. By laying a piece of paper upon a table, and upon the paper a piece of cloth, on the piece of cloth a bit of silk, and on the bit of silk a piece of leather, so that the edge of each would be exposed to the extent of half-an-inch, it would be possible by the touch to tell when the finger passed successively over the leather, silk, cloth, or paper, and arrived on the table. Those impressions of touch must have been communicated, with their extremely nice distinctions, to the sensitive nerves that lie underneath the skin, and must have been transmitted all the way through the arm to the brain, although the touch itself was so light as scarcely to be appreciable with regard to the force applied. A hair lying on the tongue will be plainly perceptible to the touch of the tongue; and the surface of a broken tooth will often cause the tongue great annoyance, by the acute perception it imparts of the roughness of its surface. The toes are also highly sensitive, though their powers of touch are seldom fully developed. Persons who have lost their arms, however, have brought their feet to be almost as sensitive as fingers. Blind persons increase, by constant exercise, their powers of touch to such a degree that they are able to read freely by passing their fingers over embossed printing; and they have been known to distinguish _colours_ by differences in their grain, quite unappreciable by other persons. 1006. _Why is feeling impaired when the hands are cold?_ Because, as the blood flows slowly to the nerves, they are less capable of that perception of touch which is their _special sense_. The skin contracts upon the nervous filaments, and _impairs the contact_ between them and the bodies which they touch. 1007. _Why do the fingers prick and sting when they again become warm?_