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In certain cases, the delicate fibrous sheath, or neurilemma, of the nerve which enters the papilla, enlarges in the papilla into an oval swelling, which' is called a tactile corpuscle (see Lesson XII.). These corpuscles are found in the papillæ of those localities which are endowed with a very delicate sense of touch, as in the tips of the fingers, the point of the tongue, &c.

9. It is obvious, from what has been said, that no direct contact takes place between a body which is touched and the sensory nerve,-a thicker or thinner layer of epithelium, or epidermis, being situated between the two. In fact, if this layer is removed, as when a surface of the skin has been blistered, contact with the raw surface gives rise to a sense of pain, not to one of touch properly so called. Thus, in touch, it is the epidermis, or epithelium, which is the intermediator between the nerve and the physical agent, the external pressure being transmitted through the horny cells to the subjacent ends of the nerves, and the kind of impulse thus transmitted must be modified by the thickness and character of the cellular layer, no less than by the forms and mber of the papillæ.

10. Certain very curious phenomena appertaining to the sense of touch, are probably due to these varying anatomical arrangements. Not only is tactile sensibility to a single impression much duller in some parts than in others-a circumstance which might be readily accounted for by the different thickness of the epidermic layer--but the power of distinguishing double simultaneous impressions is very different. Thus, if the ends of a pair of compasses (which should be blunted with pointed pieces of cork) are separated by only one-tenth or one-twelfth of an inch, they will be distinctly felt as two, if applied to the tips of the fingers; whereas, if applied to the back of the hand in the same way, only one impression will be felt ; and, on the. arm, they may be separated for a quarter of an inch, and still only one impression will be perceived.

Accurate experiments have been made in different parts of the body, and it has been found that two points can be distinguished by the tongue, if only one-twentyfourth of an inch apart; by the tips of the fingers if

one-twelfth of an inch distant; while they may be one inch distant on the cheek, and even three inches on the back, and still give rise to only one sensation.

II. The feeling of warmth, or cold, is the result of an excitation of sensory nerves distributed to the skin, which are probably distinct from those which give rise to the sense of touch. And it would appear that the heat must be transmitted through the epidermic or epithelial layer, to give rise to this sensation ; for, just as touching a naked nerve, or the trunk of a nerve, gives rise only to pain, so heating or cooling an exposed nerve, or the trunk of a nerve, gives rise not to a sensation of heat or cold, but simply to pain.

Again, the sensation of heat, or cold, is relative rather than absolute. Suppose three basins be prepared, one filled with ice-cold water, one with water as hot as can be borne, and the third with a mixture of the two. If the hand be put into the hot-water basin, and then transferred to the mixture, the latter will feel cold; but if the hand be kept awhile in the ice-cold water, and then transferred to the very same mixture, it will feel

warm,

Like the sense of touch, the sense of warmth varies in delicacy in different parts of the body.

The cheeks are very sensitive, more so than the lips; the palms of the hands are more sensitive to heat than their backs. Hence a washerwoman holds her flat-iron to her cheek to test the temperature, and one who is cold spreads the palms of his hands to the fire.

12. The organ of the sense of TASTE is the mucous membrane which covers the tongue, especially its back part, and the hinder part of the palate. Like that of the skin, the deep, or vascular, layer of the mucous membrane of the tongue is raised up into papillæ, but these are large, separate, and have separate coats of epithelium. Towards the tip of the tongue they are for the most part elongated and pointed, and are called filiform; over the rest of the surface of the tongue, these are mixed with other larger papillæ, with broad ends and narrow bases, called fungiform; but towards its root there are a number of large papillæ, arranged in the figure of a V with its point backwards, each of which

is like a fungiform papilla surrounded by a wall. These are the circumvallate papillæ (Fig. 61, C.p.). The larger of these papilla have subordinate small ones upon their surfaces. They are very vascular, and they receive nervous filaments from two sources, the one the nerve

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Fig. 61.-THE MOUTH WIDELY OPENED TO SHOW THE TONGUE AND

PALATE. Uv. the uvula ; Tn. the tonsil between the anterior and posterior pillars of

the fauces; C.p. circumvallate papillæ ; F.p: fungiform papillæ. The minute filiform papillæ cover the interspaces between these. On the right side the tongue is partially dissected to show the course of the filaments of

the glossopharyngeal nerve, VIII. called glossopharyngeal, the other the gustatory, which is a branch of the fifth nerve. (See Lesson XI. $ 18.) The latter chiefly supplies the front of the tongue, the former its back and the adjacent part of the palate : and there is reason to believe that it is the latter region which is more especially the seat of the sense of taste.

The great majority of the sensations we call taste, however, are in reality complex sensations, into which smell and even touch largely enter. When the sense of smell is interfered with, as when the nose is held tightly pinched, it is very difficult to distinguish the taste of various objects. An onion, for instance, the eyes being shut, may then easily be confounded with an apple.

13. The organ of the sense of SMELL is the delicate mucous membrane which lines a part of the nasal cavities, and is distinguished from the rest of the mucous membrane of these cavities-firstly, by possessing no cilia ; secondly, by receiving its nervous supply from the olfactory, or first, pair of cerebral nerves, and not, like the rest of the mucous membrane, from the fifth pair.

Each nostril leads into a spacious nasal chamber, separated, in the middle line, from its fellow of the other side, by a partition, or septum, formed partly by cartilage and partly by bone, and continuous with that partition which separates the two nostrils one from the other. Below, each nasal chamber is separated from the cavity of the mouth by a floor, the bony palate (Figs. 62 and 63); and when this bony palate comes to an end, the partition is continued down to the root of the tongue by a fleshy curtain, the soft palate, which has been already described. The soft palate and the root of the tongue together, constitute, under ordinary circumstances, a moveable partition between the mouth and the pharynx, and it will be observed that the opening of the larynx, the glottis, lies behind the partition ; so that when the root of the tongue is applied close to the soft palate no passage of air can take place between the mouth and the pharynx. But in the upper part of the pharynx above the partition are the two hinder openings of the nasal cavities (which are called the posterior nares) separated by the termination of the septum ; and through these wide openings the air passes, with great readiness, from the nostrils along the lower part of each nasal chamber to the glottis, or in the opposite direction. It is by means of the passages thus freely open to the air that we breathe, as we ordinarily do, with the mouth shut.

Each nasal chamber rises, as a high vault, far above the

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FIG. 62.-VERTICAL LONGITUDINAL Sections of the NASAL CAVITY. The upper figure represents the outer wall of the left nasal cavity; the lower figure

the right side of the middle partition, or septum ($p.), of the nose, which forms the inner wall of the right nasal cavity. I, the olfactory nerve and its branches ; V, branches of the fifth nerve ; Pa. the palate, which separates the nasal cavity from that of the mouth; S.T. the superior turbinal bone ; M.T. the middle turbinal; I.T. the inferior turbinal. The letter I is placed in the cerebral cavity; and the partition on which the olfactory lobe rests, and through which the filaments of the olfactory nerves pass, is the cribriform plate.

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