The wind-pipe commences at the root of the tongue. and descends in front of the oesophagus to the lungs.

Its upper orifice is covered by a valve, called the epiglottis, to prevent the introduction of food, which passes directly over it. Laughter during eating, renders food liable, by the opening of this valve, to pass into the wind-pipe, thus producing violent cough or suffocation. The wind-pipe is composed of cartilaginous rings, united by membranes, and is divided into three parts. The upper portion is called larynx, the middle portion the trachea. The lower part or the bronchia bifurcates from the trachea about the third or fourth dorsal vertebra, and passes, one to each lung, where it terminates in an innumerable number of air-cells, which exist in every part of the lungs. These air-cells are separated only by a thin membrane from an equal number of cells filled with blood communicating with the heart. The lungs then are of a spongy texture, made up of cellular tissue, and these innumerable air and blood-cells. Respiration consists in filling these cells with air. The lungs are thus distended, and the dark venous blood passing in from the heart on the opposite side, give off through the thin intervening membrane its carbonic-acid and takes in return the oxygen from the air. The air then by the contraction of the proper muscles is forced out and the lungs contract in size. Thus the chest rises and falls with the respiration and expiration of air. There are two lungs, one on each side of the chest, embracing the heart, and separated from each other by a membranous partition. They are suspended in the chest by roots composed of the pulmonary arteries and veins, the bronchial tubes, etc. They are covered by a serous membrane, which is reflected over the wall of the chest, forming a shut sack. This membrane is called the pleura, and when inflamed occasions that most agonizing disease pleurisy. The lungs rising and falling at each respiration, the inflamed surfaces rub against each other, occasioning the most acute pain.

Circulation. The heart is the great centre of circulation, the main-spring in the beautiful mechanism of the system, its pulsations only ceasing with death. It is situated in the chest between the right and left lung, has two sides, each of which has two chambers or cavities. The upper is called the auricle, the lower the ventricle. These chambers, the auricle and ventricle, are separated by valves. The right side of the heart is appropriated to the venous blood, the left to the arterial blood. As the venous blood passes into the right auricle, the auricle contracts and forces it through the valve into the right ventricle. The contraction of the ventricle closes the valve, thus preventing the return of blood, and forcing it into the pulmonary artery, through which it is conveyed to the lungs. Here, in the innumerable cells prepared for its reception, it throws off, through the thin membrane separating it from the air cells, its carbonic acid, and receives in return oxygen from the air; changed now from dark and impure venous blood, to bright and life-giving arterial blood, it passes on through the pulmonary veins to the left auricle of the heart. This chamber, contracting, forces the blood into the lower chamber or left ventricle, from whence it is prevented from returning by a similar valvular arrangement to that found in the right heart. The contraction of the ventricle forces the blood into the great aorta. This great artery of the body gives off trunks to the head, the upper and lower extremities and the organs in the abdominal cavity. These branches, dividing and subdividing into innumerable branches, growing smaller and smaller, convey the arterial blood to every part of the system, and at length terminate in a curious net-work of vessels, called the capillary vessels. The arterial system has fulfilled its duty, conveyed the elements of life and nutrition to every part of the system, and the blood is now ready to be conveyed back to the heart, no longer bright and life-giving, but dark and impure. The veins then, commencing in this capillary net-work of vessels, at first innumerable in number, collect the blood, and flowing into each other, form at length two large trunks the ascending and descending vena cava, by which it is returned to the right auricle of the heart, and from thence in the manner already explained, passes on to the lungs. There throwing off its carbonic acid, and receiving in return oxygen from the air, it flows on to the right heart, when it is again ready to be distributed throughout the system. Thus we have the heart the great central point of the system, the arterial blood flowing outward, freighted with life, to every part of the body and the venous blood, charged with impurities, flowing inward to the heart.

Each cavity of the heart holds two ounces, and as it contracts about seventy times in a minute, more than two hogsheads traverse it every hour. And yet performing this mighty labor, it beats on year after year, never tiring, until paralyzed by the hand of death.

For a more minute explanation of the structure of the heart and the circulation, see "Diseases of the Heart," also chapter on Physiology.

The Skin. The skin is composed of two layers of membrane, the cuticle, and true shin. The cuticle has neither blood-vessels nor nerves, and serves as a protection to the true skin. In the inner layers of the cuticle there is a peculiar coloring matter, black in the negro, copper-colored in the indian, and in the white, so transparent as to be scarcely perceptible. The true skin contains, besides arteries, veins, and absorbents, oil glands, perspiratory glands, and nerves. The nerves ramify on the surface, and render the skin sensitive to the touch. The absorbents are small vessels opening on the inner layers of the cuticle, and through these, poisons being rubbed on the skin are conveyed into the system.

From the perspiratory glands, which separate from the blood the perspiration, spiral ducts pass obliquely to the skin. In health, these glands are constantly in action, pouring out through the ducts an enormous amount of matter in the form of sensible and insensible perspiration. For a more full description of the functions of the skin, see chapter on Physiology.

The Teeth. The teeth are divided into two parts. The crown rising above the gum is covered with a fine enamel, to protect it from decay and wear, and to render it more fit to perform its important functions. The root is of a bony substance, and is firmly inserted in the jaw. Communicating to each tooth through the root, is a small nerve. When this is diseased or exposed by the decay of the tooth, that exquisite sensation is produced known as tooth-ache.

The first set, twenty in number, appearing in infancy, are only temporary, and are called milk-teeth. The second, or permanent set, appearing between the ages of six and fourteen are thirty-two in number, sixteen in each jaw.

The four front-teeth in each jaw are called incisors, the next on each side the cuspid or eye-tooth, the next two, bi-cuspids, the next two molars, and the last two on each side of the jaw, wisdom teeth, from their not appearing until about twenty, and then being of short duration. The molars on the upper jaw have three roots, those on the lower two, while the incisors, cuspids and bi-cuspids have each but one root

See also chapter on Diseases of Children.

Vision. The eye is the most perfect and beautiful optical instrument in the world. It consists of a globe, held in its position by means of six muscles attached externally to the sclerotic coat near the cornea, and internally to the bones of the orbit behind the eye.

The sclerotic coat, seen in the white of the eye, is dense and fibrous. It is the strong membrane which invests it, except the transparent part in front This is called the cornea. It is united with the sclerotica in the same way that the crystal, which it resembles in form, is set in a watch.

Within the sclerotica there is a vascular membrane called the choroid coat. It secretes on its internal surface the pigmentum nigrum, or black pigment, giving the dark color to the pupil of the eye, and is of vast importance in vision. It terminates near the cornea, in a white circle, called the ciliary circle. The internal layer of the edge is thrown into folds, the central border of which rests on the crystalline lens. These folds are called ciliary processes.

Starting from the ciliary circle, is a beautifully contrived curtain, with an opening in its centre, dividing the anterior portion of the eye into two chambers, the communication between which being at this opening. This curtain is capable of contraction and dilation, thus enlarging the central opening, and is called the iris. The central opening is called the pupil. The two chambers are filled with an aqueous humor. Behind the pupil is placed a lens, clear as crystal, and called the crystalline lens.

Another and last coat within the choroid is called the retina. The optic nerve passes from the brain through the outer coats and expands on this. This is the seat of vision, and here every object we witness is pictured or daguerreotyped, and the impression thus transmitted to the brain. Within the coats already mentioned and back of the crystalline lens, is a chamber forming the larger portion of the ball of the eye, filled with a humor resembling the aqueous, but more dense, called the vitreous humor. Over the eye are the eyelids, lined on the inside by a delicate membrane, which is also reflected over the ball of the eye, called the conjuctiva. It is liable to become inflamed when it appears blood-shot. It secretes a fluid which lubricates the eye.

At the upper and outer angle of the orbit is the lachrymal gland. It secretes the tears which are poured on the ball of the eye, keeping it constantly moist. They pass off through small openings at the internal angle of the eye into the nose.

We have, then, the cornea collecting and bending inward the rays of light; the aqueous humor transmitting the rays and giving free motion to the iris; the iris contracting and dilating, admitting only the necessary rays; the crystalline lens, the focus concentrating the rays which then, crossing each other, are transmitted through the vitreous humor to the retina, which serves as a daguerreotype-plate, upon which the image is pictured, and the impression through the expanded optic nerve transmitted to the brain. What instrument in the world could be more beautiful or perfect in its construction?