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In the act of expiration accompanying mastication, especially the instant after deglutition, the odorous particles are carried into the cavities of the nose, and affect the sense of smell, or make their odour apparent. This effect is what we term flavour. Some bodies, as cinnamon, have hardly any taste, but a flavour, in other words an odour, brought out by mastication.

16. Smell, like taste, is an important instrument in the discrimination of material bodies, and therefore serves a high function in guiding our actions and in extending our knowledge of the world. Man does not exemplify the highest development of this organ. The order of ruminants, certain of the pachydermatous animals, and above all the carnivorous. quadrupeds, excel the human subject in the expansion given to the membrane of the nose, and in a corresponding sensibility to odours. The scent of the dog is to us almost miraculous; it directs his pursuit, and tells him his whereabouts. It may act the part of sight in enabling him to retrace his steps or to find out his master.

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SENSE OF TOUCH.

1. Physiologists, in describing the senses, usually commence with Touch. This,' say Messrs. Todd and Bowman, 'is the simplest and most rudimentary of all the special senses, and may be considered as an exalted form of common sensation, from which it rises, by imperceptible gradations, to its state of highest development in some particular parts. It has its seat in the whole of the skin, and in certain mucous membranes, as that of the mouth, and is therefore the sense most generally diffused over the body. It is also that which exists most extensively in the animal kingdom; being, probably, never absent in any species. It is, besides, the earliest called into operation, and the least complicated in its impressions and mechanism.'

It may be well admitted that Touch is less complicated than Taste, in whose organ four different kinds of sen

sations may be said to meet, the tactile being one of them. It may be further said of touch, that the mode of action (mechanical contact or pressure), appears to us the most simple of any. Nevertheless, Touch is an intellectual sense of a far higher order than these. It is not merely a knowledge-giving sense, as they all are, but a source of ideas and conceptions of the kind that remain in the intellect and embrace the outer world. The notions of the size, shape, direction, distances, and situation of external bodies may be acquired by touch, but not by either taste or smell.

But this last assertion must be accompanied by an important explanation. Touch, considered as a source of ideas such as those, is really not a simple sense, but a compound of sense and motion; and it is to the muscular part of the sense, or to the movements of the touching organs, that these conceptions owe their origin and their embodiment, as we have endeavoured to show in the previous chapter. The superiority of touch to taste and smell, in this view, therefore, consists in its union with movement and muscular sensibility; and the same advantage pertains to sight. The contact of solid bodies with the surface of the body gives occasion to the exercise of movement, force, and resistance, and to the feelings and perceptions consequent on these: which cannot be said to any extent of smell, nor of taste properly so-called.

A second feature marking the superiority of the sense of Touch, and qualifying it to furnish intellectual forms and imagery, is the distinctness or separateness of the sensations felt over the different parts of the skin. The sensations of the different parts of the surface of smell, would seem all to fuse into one stream of sensibility; it is not possible ever to refer a smell to any one portion of the membrane more than another. But the sensations of the skin are conveyed by distinct nervous filaments; each little area of skin has a separate nerve, and an independent communication with the nerve centres, whereby we can, after a little education, refer each sensation to the spot where the contact is made. The

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stimulus on one finger is not, at any part of the course of the nerve, confounded with the stimulus on another finger; the back can always be distinguished from the breast, the right side from the left, and so on. I shall afterwards endeavour to show that this localization of touches has to be learned by practice; but the very possibility of it rests upon the distinctness and independence of the nerve filaments. This is an extremely important fact, and makes the great difference between touch and what is sometimes called 'common sensation,' or the sensibility diffused over all the internal organs and tissues. There is no such distinguishing sensibility in the stomach, or the lungs, or the liver; at all events, the distinctness of the nerves in those parts is very low in degree, just sufficient to enable us to refer a pain to the lungs, the liver, or the stomach, without indicating the particular region or sub-division. The skin is therefore marked by a great exaltation of the common sensibility of the body, not as regards intensity of feeling, but as regards distinctiveness of locality.

2. Having made these preliminary remarks, we commence as usual, with the objects, or external agents concerned in the sense of Touch. These are principally the solid substances of the outer world. Gases do not act on the touch unless they are blown with great violence. Liquids also give very little feeling, if they are of the same warmth as the body. The sensations of a bath are confined to heat or cold. It is manifest that an even, equal pressure, such as fluids give, is not sufficient to impress the tactile nerves. The asperities and inequalities of solid surfaces, by pressing intensely on some points and not at all on others, are requisite for this purpose.

The hard unyielding nature of the mineral constituents of the earth's crust, metals, rocks, &c., is well fitted to excite the touch. The woody fibre of the vegetable world has a compactness next in degree to the solid minerals. The soft and yielding class of solids impress the surface in a totally different manner and these differ among themselves accord

ing as they recover their form after pressure, or not; whence the distinction of elastic and non-elastic. When the substance is moved over the skin, the asperities come to be felt more acutely, and hence the further distinction into rough and smooth surfaces. In treating of the sensations themselves, we shall attend to these qualities more minutely.

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3. The sensitive organ or surface is the Skin, or common integument of the body, the interior of the mouth, and the tongue. The parts of the Skin are its two layers, its papillæ, the hairs and nails, its two species of glands,-the one yielding sweat, the other a fatty secretion,-with blood vessels and nerves. I shall quote a few extracts from the anatomical description of those parts. Of the two layers, the outermost is the cuticle, epidermis, or scarf skin. It forms a protective covering over every part of the true skin, and is itself quite insensible and non-vascular. The thickness of the cuticle varies in different parts of the surface, measuring in some not more than, and in others from to of an inch. It is thickest in the palms of the hands and soles of the feet, where the skin is much exposed to pressure, and it is not improbable that this may serve to stimulate the subjacent true skin to a more active formation of epidermis; but the difference does not depend solely on external causes, for it is well marked in the fœtus.

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Many of the cells of the cuticle contain pigment, and often give the membrane more or less of a tawny colour, even in the white races of mankind; the blackness of the skin in the negro depends entirely on the cuticle. The pigment is contained principally in the cells of the deep layer, and appears to fade as they approach the surface, but even the superficial part possesses a certain degree of colour.

'The true skin, cutis vera, derma, or corium, is a sentient and vascular texture. It is covered and defended by the insensible and non-vascular cuticle, and is attached to the parts beneath by a layer of cellular tissue, named "subcutaneous," which, excepting in a few parts, contains fat, and has therefore been called also the "panniculus adiposus."

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The connexion is in many parts loose and movable, in others close and firm, as in the palmar surface of the hand and the sole of the foot, where the skin is fixed to the subjacent fascia by numerous stout fibrous bands, the space between being filled with a firm padding of fat. In some regions of the body, the skin is moved by muscular fibres, which, as in the case of the orbicular muscle of the mouth, may be unconnected to fixed parts, or may be attached beneath to bones or fasciæ, like the other cutaneous muscles of the face and neck, and the short palmar muscle of the hand.'

The upper or free surface of the true skin is marked in various places with larger or smaller furrows, which also affect the superjacent cuticle. The larger of them are seen opposite the flexures of the joints, as those so well known in the palm of the hand and at the joints of the fingers. The finer furrows intersect each other at various angles, and may be seen almost all over the surface; they are very conspicuous on the back of the hands. These furrows are not merely the consequence of the frequent folding of the skin by the action of muscles or the bending of joints, for they exist in the foetus. The wrinkles of old persons are of a different nature, and are caused by the wasting of the soft parts which the skin covers. Fine curvilinear ridges, with intervening furrows, mark the skin of the palm and sole; these are caused by ranges of the papillæ, to be immediately described.'

‘Papillæ.—The free surface of the corium is beset with small eminences thus named, which seem chiefly intended to contribute to the perfection of the skin as an organ of touch, seeing that they are highly developed where the sense of touch is exquisite, and vice versa. They serve also to extend the surface for the production of the cuticular tissue, and hence are large-sized and numerous under the nail. The papillæ are large, and in close array on the palm and palmar surface of the fingers, and on the corresponding parts of the foot. There they are ranged in lines forming the curvilinear ridges seen when the skin is still covered with its thick epi

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