Experiment 118. _To show that the air we expire is warm._ Breathe on a thermometer for a few minutes. The mercury will rise rapidly.

Experiment 119. _To show that expired cur is moist_. Breathe on a mirror, or a knife blade, or any polished metallic surface, and note the deposit of moisture.

Experiment 120. _To show that the expired air contains carbon dioxid_. Put a gla.s.s tube into a bottle of lime water and breathe through the tube. The A liquid will soon become cloudy, because the carbon dioxid of the expired air throws down the lime held in solution.

Experiment 121. "A subst.i.tute for a clinical thermometer may be readily contrived by taking an ordinary house thermometer from its tin case, and cutting off the lower part of the scale so that the bulb may project freely. With this instrument the pupils may take their own and each other's temperatures, and it will be found that whatever the season of the year or the temperature of the room, the thermometer in the mouth will record about 99 F. Care must, of course, be taken to keep the thermometer in the mouth till it ceases to rise, and to read while it is still in position."--Professor H. P. Bowditch.

Experiment 122. _To ill.u.s.trate the manner in which the movements of inspiration cause the air to enter the lungs._ Fit up an apparatus, as represented in Fig. 95, in which a stout gla.s.s tube is provided with a sound cork, B, and also an air-tight piston, D, resembling that of an ordinary syringe. A short tube, A, pa.s.sing through the cork, has a small India-rubber bag, C, tied to it. Fit the cork in the tube while the piston is near the top. Now, by lowering the piston we increase the capacity of the cavity containing the bag. The pressure outside the bag is thus lowered, and air rushes into it through the tube, A, till a balance is restored. The bag is thus stretched. As soon as we let go the piston, the elasticity of the bag, being free to act, Movements of drives out the air just taken in, and the piston returns to its former place.

[Ill.u.s.tration: Fig. 95. Apparatus for Ill.u.s.trating the Movements of Respiration.]

It will be noticed that in this experiment the elastic bag and its tube represent the lungs and trachea; and the gla.s.s vessel enclosing it, the thorax.

For additional experiments on the mechanics of respiration, see Chapter XV.

Chapter IX.

The Skin and the Kidneys.

232. The Elimination of Waste Products. We have traced the food from the alimentary ca.n.a.l into the blood. We have learned that various food materials, prepared by the digestive processes, are taken up by the branches of the portal vein, or by the lymphatics, and carried into the blood current. The nutritive material thus absorbed is conveyed by the blood plasma and the lymph to the various tissues to provide them with nourishment.

We have learned also that oxygen, taken up in the air cells of the lungs, is being continually carried to the tissues, and that the blood is purified by being deprived in the lungs of its excess of carbon dioxid.

From this tissue activity, which is mainly oxidation, are formed certain waste products which, as we have seen, are absorbed by the capillaries and lymphatics and carried into the venous circulation.

In their pa.s.sage through the blood and tissues, the alb.u.mens, sugars, starches, and fats are converted into carbon dioxid, water, and urea, or some closely allied body. Certain articles of food also contain small amounts of sulphur and phosphorus, which undergo oxidation into sulphates and phosphates. We speak, then, of carbon dioxid, salts, and water as waste products of the animal economy. These leave the body by one of the three main channels,--the lungs, the skin, or the kidneys.

The elimination of these products is brought about by a special apparatus called organs of excretion. The worn-out substances themselves are called excretions, as opposed to secretions, which are elaborated for use in the body. (See note, p. 121.) As already shown, the lungs are the main channels for the elimination of carbon dioxid, and of a portion of water as vapor. By the skin the body gets rid of a small portion of salts, a little carbon dioxid, and a large amount of water in the form of perspiration. From the kidneys are eliminated nearly all the urea and allied bodies, the main portion of the salts, and a large amount of water. In fact, practically all the nitrogenous waste leaves the body by the kidneys.

[Ill.u.s.tration: Fig. 96.--Diagrammatic Scheme to ill.u.s.trate in a very General Way Absorption and Excretion.

A, represents the alimentary ca.n.a.l; L, the pulmonary surface; K, the surface of the renal epithelium; S, the skin; o, oxygen; h, hydrogen,; n, nitrogen.

233. The Skin. The skin is an important and unique organ of the body. It is a blood-purifying organ as truly as are the lungs and the kidneys, while it also performs other and complex duties. It is not merely a protective covering for the surface of the body. This is indeed the most apparent, but in some respectes, the lest important, of its functions.

This protective duty is necessary and efficient, as is proved by the familiar experience of the pain when a portion of the outer skin has been removed.

The skin, being richly supplied with nerves, is an important organ of sensibility and touch. In some parts it is closely attached to the structures beneath, while in others it is less firmly adherent and rests upon a variable amount of fatty tissue. It thus a.s.sists in relieving the abrupt projections and depressions of the general surface, and in giving roundness and symmetry to the entire body. The thickness of the skin varies in different parts of the body. Where exposed to pressure and friction, as on the soles of the feet and in the palms of the hands, it is much thickened.

The true skin is 1/12 to ? of an inch in thickness, but in certain parts, as in the lips and ear pa.s.sages, it is often not more than 1/100 of an inch thick. At the orifices of the body, as at the mouth, ears, and nose, the skin gradually pa.s.ses into mucous membrane, the structure of the two being practically identical. As the skin is an outside covering, so is the mucous membrane a more delicate inside lining for all cavities into which the apertures open, as the alimentary ca.n.a.l and the lungs.

[Ill.u.s.tration: Fig. 97.--A Layer of the Cuticle from the Palm of the Hand.

(Detached by maceration.)]

The skin ranks as an important organ of excretion, its product being sweat, excreted by the sweat glands. The amount of this excretion evaporated from the general surface is very considerable, and is modified as becomes necessary from the varied conditions of the temperature. The skin also plays an important part in regulating the bodily temperature(sec. 241).

234. The Cutis Vera, or True Skin. The skin is remarkably complex in its structure, and is divided into two distinct layers, which may be readily separated: the deeper layer,--the true skin, dermis, or corium; and the superficial layer, or outer skin,--the epidermis, cuticle, or scarf skin.

The true skin consists of elastic and white fibrous tissue, the bundles of which interlace in every direction. Throughout this feltwork structure which gradually pa.s.ses into areolar tissue are numerous muscular fibers, as about the hair-follicles and the oil glands. When these tiny muscles contract from cold or by mental emotion, the follicles project upon the surface, producing what is called "goose flesh."

The true skin is richly supplied with blood-vessels and nerves, as when cut it bleeds freely, and is very sensitive. The surface of the true skin is thrown into a series of minute elevations called the papillae, upon which the outer skin is moulded. These abound in blood-vessels, lymphatics, and peculiar nerve-endings, which will be described in connection with the organ of touch (sec. 314). The papillae are large and numerous in sensitive places, as the palms of the hands, the soles of the feet, and the fingers. They are arranged in parallel curved lines, and form the elevated ridges seen on the surface of the outer skin (Fig. 103).

235. The Epidermis, or Cuticle. Above the true skin is the epidermis.

It is semi-transparent, and under the microscope resembles the scales of a fish. It is this layer that is raised by a blister.

As the epidermis has neither blood-vessels, nerves, nor lymphatics, it may be cut without bleeding or pain. Its outer surface is marked with shallow grooves which correspond to the deep furrows between the papillae of the true skin. The inner surface is applied directly to the papillary layer of the true skin, and follows closely its inequalities. The outer skin is made up of several layers of cells, which next to the true skin are soft and active, but gradually become harder towards the surface, where they are flattened and scale-like. The upper scales are continually being rubbed off, and are replaced by deeper cells from beneath. There are new cells continually being produced in the deeper layer, which push upward the cells already existing, then gradually become dry, and are cast off as fine, white dust. Rubbing with a coa.r.s.e towel after a hot bath removes countless numbers of these dead cells of the outer skin. During and after an attack of scarlet fever the patient "peels," that is, sheds an unusual amount of the seal; cells of the cuticle.

The deeper and more active layer of the epidermis, the _mucosum_, is made up of cells some of which contain minute granules of pigment, or coloring matter, that give color to the skin. The differences in the tint, as brunette, fair, and blond, are due mainly to the amount of coloring matter in these pigment cells. In the European this amount is generally small, while in other peoples the color cells may be brown, yellow, or even black. The pinkish tint of healthy skin, and the rosy-red after a bath are due, not to the pigment cells, but to the pressure of capillaries in the true skin, the color of the blood being seen through the semi-transparent outer skin.

[Ill.u.s.tration: Fig. 98.--Surface of the Palm of the Hand, showing the Openings of the Sweat Glands and the Grooves between the Papillae of the Skin. (Magnified 4 diameters.) [In the smaller figure the same epidermal surface is shown, as seen with the naked eye.]]

Experiment 123. Of course the living skin can be examined only in a general way. Stretch and pull it, and notice that it is elastic. Note any liver spots, white scars, moles, warts, etc. Examine the outer skin carefully with a strong magnifying gla.s.s. Study the papillae on the palms. Sc.r.a.pe off with a sharp knife a few bits of the scarf skin, and examine them with the microscope.

236. The Hair. Hairs varying in size cover nearly the entire body, except a few portions, as the upper eyelids, the palms of the hands, and the soles of the feet.

The length and diameter of the hairs vary in different persons, especially in the long, soft hairs of the head and beard. The average number of hairs upon a square inch of the scalp is about 1000, and the number upon the entire head is estimated as about 120,000.

Healthy hair is quite elastic, and may be stretched from one-fifth to one-third more than its original length. An ordinary hair from the head will support a weight of six to seven ounces. The hair may become strongly electrified by friction, especially when brushed vigorously in cold, dry weather. Another peculiarity of the hair is that it readily absorbs moisture.

237. Structure of the Hair. The hair and the nails are structures connected with the skin, being modified forms of the epidermis. A hair is formed by a depression, or furrow, the inner walls of which consist of the infolded outer skin. This depression takes the form of a sac and is called the hair-follicle, in which the roots of the hair are embedded. At the bottom of the follicle there is an upward projection of the true skin, a papilla, which contains blood-vessels and nerves. It is covered with epidermic cells which multiply rapidly, thus accounting for the rapid growth of the hair. Around each papilla is a bulbous expansion, the hair bulb, from which the hair begins to grow.

[Ill.u.s.tration: Fig. 99.--Epidermis of the Foot.

It will be noticed that there are only a few orifices of the sweat glands in this region. (Magnified 8 diameters.)]

The cells on the papillae are the means by which the hairs grow. As these are pushed upwards by new ones formed beneath, they are compressed, and the shape of the follicle determines their cylindrical growth, the shaft of the hair. So closely are these cells welded to form the cylinder, that even under a microscope the hair presents only a fibrous appearance, except in the center, where the cells are larger, forming the medulla, or pith (Fig. 106).

The medulla of the hair contains the pigment granules or coloring matter, which may be of any shade between a light yellow and an intense black. It is this that gives the great variety in color. Generally with old people the pigment is absent, the cells being occupied by air; hence the hair becomes gray or white. The thin, flat scales on the surface of the hair overlap like shingles. Connected with the hair-follicles are small bundles of muscular fibers, which run obliquely in the skin and which, on shortening, may cause the hairs to become more upright, and thus are made to "stand on end." The bristling back of an angry cat furnishes a familiar ill.u.s.tration of this muscular action.

[Ill.u.s.tration: Fig. 100.--Hair and Hair-Follicle.

A, root of hair; B, bulb of the hair; C, internal root sheath; D, external root sheath; E, external membrane of follicle; F, muscular fibers attached to the follicle; H, compound sebaceous gland with its duct; K, L, simple sebaceous gland; M, opening of the hair-follicle.

Opening into each hair-follicle are usually one or more sebaceous, or oil, glands. These consist of groups of minute pouches lined with cells producing an oily material which serves to oil the hair and keep the skin moist and pliant.

238. The Nails. The nails are also formed of epidermis cells which have undergone compression, much like those forming the shaft of a hair. In other words, a nail is simply a thick layer of h.o.r.n.y scales built from the outer part of the scarf skin. The nail lies upon very fine and closely set papillae, forming its matrix, or bed. It is covered at its base with a fold of the true skin, called its root, from beneath which it seems to grow.

The growth of the nail, like that of the hair and the outer skin, is effected by the production of new cells at the root and under surface. The growth of each hair is limited; in time it falls out and is replaced by a new one. But the nail is kept of proper size simply by the removal of its free edge.

239. The Sweat Glands. Deep in the substance of the true skin, or in the fatty tissue beneath it, are the sweat glands. Each gland consists of a single tube with a blind end, coiled in a sort of ball about 1/60 of an inch in diameter. From this coil the tube pa.s.ses upwards through the dermis in a wavy course until it reaches the cuticle, which it penetrates with a number of spiral turns, at last opening on the surface.

The tubes consist of delicate walls of membrane lined with cells. The coil of the gland is enveloped by minute blood-vessels. The cells of the glands are separated from the blood only by a fine part.i.tion, and draw from it whatever supplies they need for their special work.