Nerves can be seen entering these bodies, and are in all probability directly connected with the modified epithelial cells of

Drawing of upper surface of the tongue, showing the position of the papillæ. 1 and 2. Circumvallate papillæ. 3. Fungiform papillæ. 4. Filiform papillæ. (Sappey.)

which they are made up. The relation of the glosso-pharyngeal nerves to these taste buds has been shown by the fact that in the

rabbit (in which animal they are crowded together in a special organ so as to be easily found) they degenerate, and in a few months disappear, after one of these nerves has been cut.

The genuine taste sensations are very few. Much of what we commonly call taste depends almost exclusively upon the smell of the substance, and we habitually confuse the impressions derived from these two senses.* * The different tastes have been divided into four, viz., sweet, sour, bitter and salty, under some

(Cadict.)

a, fibrous tissue of papilla; d and c, epithelial covering of papilla; b, taste buds. On the right, a, b, show the separate cells of a taste bud.

one or other of which headings all our tastes, properly so called, would naturally fall. Though this classification has no just claim to being a chemical one, it is interesting to know that each taste pretty well corresponds to a distinct group of substances chemically allied one to the other. Thus, acids are sour, alkaloids are

*Many of the comestibles, the taste of which we most prize, have really no taste, but only a smell which we habitually confound with taste, having mingled the experience obtained from the two senses. Thus, if the draft of air be carefully excluded from the nose, wine, onion, etc., may easily be proved to have no taste. Hence the familiar rule of holding the nose adopted in taking medicine with a nasty "taste."

bitter, the soluble neutral salts of the alkalies are salty, and polyatomic alcohols, as glycerine, grape sugar, etc., are sweet.

These substances probably act on the nerve terminals as chemical stimuli, because they must be in solution to be appreciated. If solid particles be placed on the tongue they must be dissolved in the mouth fluid before they can excite the taste organs.

In order to explain the appreciation of the different tastes, we may imagine that there are different kinds of terminals, each of which is or is not influenced by various substances as they possess a special sweet, sour, bitter or salt energy. From these different terminals pass fibres bearing impulses to certain central cells, each of which is capable of exciting a sweet, sour, bitter or salty sensation, as the case may be.

SENSE OF SMELL.

The numerous delicate nerves which pass from the olfactory bulb to the mucous membrane of the upper and part of the middle meatus of the nose form the special nerves of smell. When certain subtle particles we call odors come in contact with the terminals of these nerves they excite impulses which, on arriving in the special centres of the brain, give rise to the impressions of smell.

Anatomically, the relations of the olfactory region are well defined. Its mucous membrane is not covered with motile cilia, as is that of the rest of the nasal cavity, and it is less vascular and peculiarly pigmented, looking yellow to the naked eye when compared with the neighboring membrane. The epithelial cells are elongated into peculiar cylinders, between which lie long thin rods, ending on the surface in free hair-like processes. The deeper extremities of these rod-shaped filaments expand to surround a nucleus, and are then continued into a network of filaments, into which prolongations of the epithelial cells also seem to pass, and in which the delicate fibrils of the olfactory nerve can be traced. The existence of direct communication between the nerves and the rod-shaped filaments and the epithelial cells is satisfactorily established in some animals.

The odorous particles must be in the form of gases, in order to be carried by the air into the olfactory region, and the air must be kept in motion, by sniffing it in and out of the nasal cavity, in order to excite the nerve terminals, which are not influenced by the odors of air absolutely at rest, though it be in contact with the mucous membrane of the olfactory tract.

The extreme delicacy of appreciation of odors by the olfactory nerve terminals is very remarkable. Even in human beings,

region.

a, Epithelial cells and bundles of nerves; b, Glands separated from each other by bundles of nerves, c. (Cadiat.)

whose sense of smell is but poorly developed when compared with that of animals, an amount of odorous substance can be perceived which the finest chemical tests fail to appreciate. Thus, Valentin has estimated that the two-millionth of a milligram of musk is sufficient to excite the specific energy of a man's olfactory apparatus.

No satisfactory classification of odors has been made out. The common division into agreeable and disagreeable smells, or

scents and stinks, is dissimilar in different individuals, and therefore cannot have a physiological basis.

With smell, as with taste, no degree of intensity of stimulation can be said to produce pain, though disgust, nausea, vomiting, and many other nervous operations, may be induced by various smells. The appetites are either excited or annulled by different excitations of the olfactory nerves.