it ceases to dominate the medulla oblongata and the spinal cord. These centres then act more vigorously, and diminish the calibre of the cerebral blood vessels through the action of the vaso-motor nerves, producing a condition of physiological anæmia and sleep; during this state waste products are removed, force is stored up, nutrition is restored, and waking finally

occurs.

THE SENSE OF TOUCH.

The Sense of Touch is a modification of general sensibility, and located in the skin, which is especially adapted for this purpose, on account of the number of nerves and papillary elevations it possesses. The structures of the skin and the modes of terminations of the sensory nerves have already been considered.

The Tactile Sensibility varies in acuteness in different portions of the body; being most marked in those regions in which the tactile corpuscles are most abundant, e. g., the palmar surface of the third phalanges of the fingers and thumb.

The relative sensibility of different portions of the body has been ascertained by means of a pair of compasses, the points of which are guarded by cork, and then determining how closely they could be brought together, and yet be felt at two distinct points. The following are some of the

The sense of touch communicates to the mind the idea of resistance only, and the varying degrees of resistance offered to the sensory nerves enables us to estimate, with the aid of the muscular sense, the qualities of hardness and softness of external objects. The idea of space or extension is obtained when the sensory surface or the external object changes its place in regard to the other; the character of the surface, its roughness or smoothness, is estimated by the impressions made upon the tactile papillæ.

Appreciation of Temperature.—The general surface of the body is more or less sensitive to differences of temperature, though this sensation is separate from that of touch; whether there are nerves especially adapted for the conduction of this sensation has not been fully determined. Under pathological conditions, however, the sense of touch may be abolished, while the appreciation of changes in temperature may remain normal.

This 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 sensation 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 fibres, the linguales; it is attached to surrounding parts, and its various movements performed by the extrinsic muscles, e. g., stylo-glossus, genio-hyoglossus, 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 papilla are analogous in their structure to those of the skin, and are distributed over the dorsum of the tongue, giving it its characteristic roughness.

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 chiefly 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 8 to 10 in number, situated at the base of the tongue, where they form a V-shaped figure. They are quite large, and consist of a central projection of mucous membrane, surrounded by a wall, or circumvallation, from which they derive their name.

The Taste Beakers, supposed to be the true organs of taste, are flasklike bodies, ovoid in form, about the 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 3d division of the 5th 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, exerting pres

sure 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.

THE SENSE OF SMELL.

The Sense of Smell is located in the mucous membrane lining the upper part of the nasal cavity, in which the olfactory nerves are distributed. The Nasal Fossæ are two cavities, irregular in shape, separated by the vomer, the perpendicular plate of the ethmoid bone, and the triangular cartilage. They open anteriorly and posteriorly by the anterior and posterior nares, the latter communicating with the pharynx. They are lined by mucous membrane, of which the only portion capable of receiving odorous impressions is the part lining the upper one-third of the fossæ.

The Olfactory Nerves, arising by three roots from the posterior and inferior surface of the anterior lobes, pass forward to the cribriform plate of the ethmoid bone, where they each expand into an oblong body, the olfactory bulb. From its under surface from 15 to 20 filaments pass downward through the foramina, to be distributed to the olfactory mucous membrane, where they terminate in long, delicate, spindle-shaped cells, the olfactory cells, situated between the ordinary epithelial cells.

The olfactory bulbs are the centres in which odorous impressions are perceived as sensations; destruction of these bulbs being attended by an abolition of the sense of smell.

In animals which possess an acute sense of smell, there is a corresponding increase in the development of the olfactory bulbs.

The Essential Conditions for the sense of smell are, (1) a special nerve centre capable of receiving impressions and transforming them into odorous sensations. (2) Emanations from bodies which are in a gaseous or vaporous condition. (3) The odorous emanations must be drawn freely through the nasal fossæ; if the odor be very faint, a peculiar inspiratory movement is made, by which the air is forcibly brought into contact with the olfactory filaments. The secretions of the nasal fosse probably dissolve the odorous particles.

Various substances, as ammonia, horseradish, etc., excite the sensibility of the mucous membrane, which must be distinguished from the perception of true odors.

THE SENSE OF SIGHT.

The Eyeball. The eyeball, or organ of vision, is situated within the orbital cavity, and loosely held in position by the fibrous capsule of Tenon. It rests upon a cushion of fat, which never disappears, except in cases of extreme starvation; it is protected from injury by the bony orbital walls, and is so situated as to permit an extensive range of vision.

Blood vessels and Nerves. The structures of the eyeball are supplied with blood by the ciliary arteries, which pierce the posterior surface around the optic nerve.

The Ciliary or Ophthalmic ganglion, about the size of a pin's head, situated in the posterior portion of the orbital cavity, receives filaments of communication from the trifacial or 5th nerve, the motor oculi or 3d nerve, and the sympathetic. From its anterior portion are given off the ciliary nerves, which enter the ball posteriorly and are distributed to the structures of which it is composed.

Structure. The form of the eyeball is that of a sphere; it is about one inch in the transverse diameter, and a little longer in the antero-posterior diameter, on account of its having the segment of a smaller sphere inserted into the anterior surface. It is composed of 3 coats; in its interior is contained the refracting apparatus.

The Sclerotic and Cornea together form the external coat of the eye; the former covering the posterior, the latter covering the anterior. The sclerotic is a dense, opaque, fibrous membrane, varying in thickness from the to the of an inch; it is composed of connective tissue and is slightly vascular. Posteriorly it is continuous with the sheath of the optic nerve, and is pierced by that nerve, as well as by the ciliary vessels and nerves; anteriorly its fibres become quite pale, and after passing into the cornea, transparent. It is a protective covering, and gives attachment to the tendons of the muscles by which the eyeball is moved.

The Cornea is a non-vascular, transparent membrane, composed for the most part of connective tissue in which are contained stellate corpuscles filled with a clear fluid. It is covered anteriorly by the basement membrane of the conjunctiva, upon which rests several layers of epithelial cells; posteriorly it is lined by the membrane of Descemet, which is reflected on to the anterior surface of the iris.

The Choroid, the Iris, the Ciliary Muscle and Ciliary Processes, together constitute the middle coat of the eye.

The Choroid coat, about the of an inch in thickness, is both a vas