Of the Nerves of Touch.

317

not to have two surfaces that might admit of separation, -much in the same manner that we suppose space to be infinite, since the imagination can always frame the idea of extension beyond the greatest assignable distance.

The nerves which minister to the sense of touch are the posterior roots of the spinal nerves, the large division of the fifth, the nervi vagi, and the glossopharyngeal

nerves.

The body, the neck and occiput, and the limbs are supplied by the spinal nerves; the head and fauces by the fifth; the pharynx and œsophagus by the nervi vagi and glossopharyngeal nerves.

It is remarkable that the nerves of touch have ganglions near their origin. It is to this class of nerves, that physiologists have commonly but vaguely attributed the unconscious influence, which is exercised by the nervous system over nutrition. Recent observation and experiment concur in supporting the justness of this conjecture.

In an interesting case of anaesthesia mentioned by Dr. Yellowly, in which, though sensation was almost wholly extinguished in the fore-arms and hands and in the legs and feet, yet the power of voluntary motion was not much diminished, it was observed that an elevated temperature more readily produced vesication of the skin than in healthy persons; that is to say, the palsy of the nerve which experiment has proved to be the nerve of touch, had deprived the part of its physical capacity of resisting heat.

I have already mentioned M. Magendie's experiments of dividing the fifth nerve in the cranial cavity, both at the ganglion of Gasser and at a part nearer to the brain, and the injurious effect in each case upon the structure

318 Influence of the Nerves of Touch over Nutrition.

of the eye; extending in the former to destructive ulceration, in the latter producing a partial opacity of the cornea only.

What renders these experiments of more than usual value, is the light which they have directly thrown upon pathology and surgery. In my Anatomical Commentaries I have described a case, which was treated by Dr. Macmichael, and which presented the anomalous circumstance of inflammation of the eye combined with palsy of the fifth nerve, that admitted of a satisfactory explanation by reference to Magendie's experiments on animals; and M. Serres has subsequently published a case of a very similar nature, and of the highest interest for the patient died, and the opportunity was taken to examine the change which had ensued in the fifth nerve. The sentient portion of the fifth together with the ganglion was found discoloured, softened, and loaded with a quantity of serosity; this change extended to the origin of the nerve, and was the more distinct, as it happened that the muscular portion of the fifth was unaffected.

In this case M. Serres observed that in addition to the change produced in the eye (a thickening namely and opacity of the cornea with adhesion of the iris to its surface) other parts had likewise suffered in their texture. The mucous membrane of the tongue was softer and more spongy on the affected side; and though the gums were in a scorbutic state on both sides, yet this appearance was far marked upon the side which had been insensible I m.

more

1 Medico-chirurgical Trans. vol. iii. p. 94.

m

Magendie. Journal de Phys. tom. v. p. 248.

SECTION 3.

Of the Organ of Taste.

The organ of taste is situated at the commencement of the digestive canal, and appears originally intended to provide us with the means of distinguishing wholesome food.

The apparent seat of taste is the tongue and palate : the same surfaces have an exquisite sense of touch; and an attentive examination shows that the latter occupies a larger surface than the former, and is indeed the only sense with which the palate is endowed.

Upon the surface of the tongue again the sense of taste is very partially distributed. The mucous membrane which covers it is marked by a vast variety of little elevations. For an inch from its root the tongue is covered with mucous follicles: before these, fourteen or fifteen broad papillæ, termed papillæ conicæ, are found, that are contained in fossula, being adherent by their apices and presenting a broad cupped surface level with the dorsum of the tongue; seven of these advance on either side from the centre to the edges of the tongue, the whole remaining surface of which is covered with oval papillæ that proceed in ranks parallel to the papillæ conicæ; these are termed papillæ conoïdeæ: at the edges of the tongue some of similar fabric seemingly to the last assume a shred-like appearance, and are called papillæ filiformes: while a fourth class remains, that are interspersed among the papillæ conoïdea; they are termed papillæ fungiformes: the largest of these are found upon the dorsum of the tongue, where they exceed the papillæ conoïdeæ

320

Impressions included under the term Taste.

in size; they are smaller but more numerous along the sides and towards the tip of the tongue.

Of these papillæ the last alone belong to taste; they are vascular and erectile, and may be observed to shoot up upon the surface of the tongue, when it is touched by a sapid substance.

In order that a substance may excite a sensation of taste, it must be presented to the tongue in a liquid state : to promote this object, when a solid is placed in the mouth, the saliva is observed to flow abundantly; its sapid qualities are perceived in proportion as it dissolves: in like manner an aëriform fluid is tasted as soon as the moisture of the mouth becomes impregnated with it.

Various substances after exciting the sense of touch on the fauces, and that of taste upon the tongue, are capable of producing a third impression which is popularly referred to the palate, but is really felt upon the sentient membrane of the nostrils: the fume of certain ́ kinds of food ascends into the cavities of the nose, and produces a third and distinct sensation: in administering medicine to children, it is well known that the greater part of what is disagreeable in its flavour may be avoided, by closing the nostrils while the draught is swallowed : and by repeating this experiment upon various articles of food, it is easy to ascertain how much of their flavour depends upon one sense, and how much is appreciated by the other. Hence it is that the senses of taste and smell have been often compared as having a resemblance, the odour of many substances being supposed to resemble their flavour; while the fact is, that the flavour of such bodies consists in their scent, and that the two impressions, which are compared, are identical.

Of the Nerves of the Tongue.

321

It follows from what has been said that substances taken into the fauces may be such as either,

1. to excite sensations of touch alone; of this nature are rock-crystal, sapphire, or ice:

2. or to be felt upon the tongue, and in addition to excite sensation in the nostrils, as for instance tin and other odorous metals:

3. or to be felt upon the tongue, and in addition to excite sensations of taste, as for instance sugar and salt:

4. or finally, to be felt upon the tongue, to be tasted by the tongue, and in addition to excite sensation in the nostrils, as for instance bread, manna, and other substances.

It may be remarked in addition that some substances of a penetrating nature, such as peppermint, appear to produce another distinct impression, the seat of which seems to be the pharynx.....

Sensations of taste are not perfect, until the mouth is closed and the tongue pressed against the palate, by which means the sapid liquid is brought into more exact contact with the surface of the tongue and perhaps forced into the texture of its mucous membrane, at the same time that its fumes are driven through the posterior fauces into the cavities of the nostrils.

The tongue is supplied, by the ninth nerve, which is distributed through its muscular texture: by the gustatory, a branch of the ganglionic portion of the third division of the fifth, which is distributed not merely to the muscles of the tongue, but to its mucous surface likewise and to two of the salivary glands: by the glossopharyngeal nerve, which gives branches to the surface of the root of the tongue.

After the division of the ninth nerve on both sides in

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