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The cutaneous surface varies in its sensibility to temperature in different parts of the body, and depends, to some extent, upon the thickness of the skin, exposure, habit, etc.; the inner surface of the elbow is more sensitive to changes in temperature than the outer portion of the arm ; the left hand is more sensitive than the right; the mucous membrane, less so than the skin.

Excessive heat or cold has the same effect upon the sensibility; the temperatures most readily appreciated are those between 50° F. and 115° F.

The sensations of pain and tickling appear to be conducted to the brain, also, by nerves different from those of touch; in abnormal conditions the appreciation of pain may be entirely lost, while touch remains unimpaired.

THE SENSE OF TASTE. The Sense of Taste is localized mainly in the mucous membrane covering the superior surface of the tongue.

The Tongue is situated in the floor of the mouth ; its base is directed backward, and connected with the hyoid bone, by numerous muscles, with the epiglottis and soft palate ; its apex is directed forward against the posterior surface of the teeth.

The substance of the tongue is made up of intrinsic muscular fibers, the linguales ; it is attached to surrounding parts, and its various movements performed by the extrinsic muscles, e. 8., stylo-glossus, genio-hyo-glossus,

etc.

The mucous membrane covering the tongue is continuous with that lining the commencement of the alimentary canal, and is furnished with vascular and nervous papillæ.

The papillæ are analogous in their structure to those of the skin, and are distributed over the dorsum of the tongue, giving it its characteristic rough

ness.

There are three principal varieties :

1. The filiform papillæ are most numerous, and cover the anterior twothirds of the tongue; they are conical or filiform in shape, often prolonged into filamentous tufts, of a whitish color, and covered by horny epithelium.

2. The fungiform papillæ are found chie at the tip and sides of the tongue; they are larger than the preceding, and may be recognized by their deep red color.

3. The circumvallate papillæ are rounded eminences, from eight to ten in number, situated at the base of the tongue, where they form a V-shaped

a

figure. They are quite large, and consist of a central projection of mucous membrane, surrounded by a wall, or circumvallation, from which they de. rive their name.

The Taste Beakers, supposed to be the true organs of taste, are flasklike bodies, ovoid in form, about go of an inch in length, situated in the epithelial covering of the mucous membrane, on the circumvallate papillæ. They consist of a number of fusiform, narrow cells, and curved so as to form the walls of this flask-like body; in the interior are elongated cells, with large, clear nuclei, the taste cells.

Nerves of Taste.—The chorda tympani nerve, a branch of the facial, after leaving the cavity of the tympanum, joins the third division of the fifth nerve between the two pterygoid muscles, and then passes forward in the lingual branches, to be distributed to the mucous membrane of the anterior two-thirds of the tongue. Division or disease of this nerve is followed by a loss of taste in the part to which it is distributed.

The glosso-pharyngeal enters the tongue at the posterior border of the hyo-glossus muscle, and is distributed to the mucous membrane of the base and sides of the tongue, fauces, etc.

The lingual branch of the trifacial nerve endows the tongue with general sensibility ; the hypoglossal endows it with motion.

The nerves of taste in the superficial layer of the mucous membrane form a fine plexus, from which branches pass to the epithelium and penetrate it; others enter the taste beakers, and are directly connected with the taste cells.

The seat of the sense of taste has been shown by experiment to be the whole of the mucous membrane over the dorsum of the tongue, soft palate, fauces, and upper part of the pharynx.

The Sense of Taste enables us to distinguish the savor of substances introduced into the mouth, which is different from tactile sensibility. The sapid quality of substances appreciated by the tongue are designated as bitter, sweet, alkaline, sour, salt, etc.

The Essential Conditions for the production of the impressions of taste are (1) a state of solubility of the food; (2) a free secretion of the saliva, and (3) active movements on the part of the tongue, exciting pressure against the roof of the mouth, gums, etc., thus aiding the solution of various articles and their osmosis into the lingual papillæ. Sapid substances, when in a state of solution, pass into the interior of the taste beakers, and come into contact, through the medium of the taste cells, with the terminal filaments of the gustatory nerves.

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