What other purpose do they serve outside of the body?

10. Why are levers of the second cla.s.s not adapted to the work of the body?

11. Name the cla.s.s of lever used in bending the elbow; in straightening the elbow; in raising the knee; in elevating the toes; and in biting. Why is one able to bite harder with the back teeth than with the front ones when the same muscles are used in both cases?

12. Measure the distance from the middle of the palm of the hand to the center of the elbow joint. Find the attachment of the tendon of the biceps muscle to the radius and measure its distance to the center of the elbow joint. From these distances calculate the force with which the biceps contracts in order to support a weight of ten pounds on the palm of the hand.

13. How does exercise benefit the health? How does a short walk "clear the brain" and enable one to study to better advantage?

14. When exercisers taken for its effects upon the health, what conditions should be observed?

PRACTICAL WORK

The reddish muscle found in a piece of beef is a good example of striated muscle. The clear ring surrounding the intestine of a cat (shown by cross section) and the outer portion of the preparation from the cow's stomach, sold at the butcher shop under the name of _tripe_, are good examples of non-striated muscular tissue. The heart of any animal, of course, shows the heart muscle.

*To show the Structure of Striated Muscle.*-Boil a tough piece of beef, as a cut from the neck, until the connective tissue has thoroughly softened.

Then with some pointed instrument, separate the main piece into its fiber bundles and these in turn into their smallest divisions. The smallest divisions obtainable are the muscle cells or fibers.

*To show Striated Fibers.*-Place a small muscle from the leg of a frog in a fifty-per-cent solution of alcohol and leave it there for half a day or longer. Then cover with water on a gla.s.s slide, and with a couple of fine needles tease out the small muscle threads. Protect with a cover gla.s.s and examine with a microscope, first with a low and then with a high power.

The striations, sarcolemma, and sometimes the nuclei and nerve plates, may be distinguished in such a preparation.

*To show Non-striated Cells.*-Place a clean section of the small intestine of a cat in a mixture of one part of nitric acid and four parts of water and leave for four or five hours. Thoroughly wash out the acid with water and separate the muscular layer from the mucous membrane. Cover a small portion of the muscle with water on a gla.s.s slide and tease out, with needles, until it is as finely divided as possible. Examine with a microscope, first with a low and then with a high power. The cells appear as very fine, spindle-shaped bodies.

*To ill.u.s.trate Muscular Stimulus and Contraction.*-Separate the muscles at the back of the thigh of a frog which has just been killed and draw the large sciatic nerve to the surface. Cut this as high up as possible and, with a sharp knife and a small pair of scissors, dissect it out to the knee. Now cut out entirely the large muscle of the calf of the leg (the gastrocnemius), but leave attached to it the nerve, the lower tendon, and the bones of the knee. Mount on an upright support, as shown in Fig. 120, and fasten the tendon to a lever below by a thread or small wire hook:

[Fig. 120]

Fig. 120-*Apparatus* for demonstrating properties of muscles.

1. Lay the nerve over the ends of the wires from a small battery which are attached to the support at _A_, and arrange a second break in the circuit at _B_. At this place the battery circuit is made and broken either by a telegraph key or by simply touching and separating the wires. Note that the muscle gives a single contraction, or twitch, both when the current is made and when it is broken.

2. Remove the current and pinch the end of the nerve, noting the result.

With very fine wires, connect the battery directly to the ends of the muscle. Stimulate by making and breaking the current as before. In this experiment the muscle cells are stimulated by the direct action of the current and not by the current acting on the nerve.

3. With the wires attached to either the muscle or the nerve, make and break the current in rapid succession. This causes the muscle to enter into a second contraction before it has relaxed from the first, and if the shocks follow in rapid succession, to continue in the contracted state.

This condition, which represents the method of contraction of the muscles in the body, is called _teta.n.u.s_.

NOTE.-In these experiments a twitching of the muscle is frequently observed when no stimulus is being applied. This is due to the drying out of the nerve and is prevented by keeping it wet with a physiological salt solution. (See footnote, page 38.)

*To show the Action of Levers.*-With a light but stiff wooden bar, a spring balance, and a wedge-shaped fulcrum, show:

1. The position of the weight, the fulcrum, and the power in the different cla.s.ses of levers, and also the weight-arm and the power-arm in each case.

2. The direction moved by the power and the weight respectively in the use of the different cla.s.ses of levers.

3. That when the power-arm and weight-arm are equal, the power equals the weight and moves through the same distance.

4. That when the power-arm is longer than the weight-arm, the weight is greater, but moves through a shorter distance than the power.

5. That when the weight-arm is longer than the power-arm, the power is greater and moves through a shorter distance than the weight.

*To show the Loss of Power in the Use of the Body Levers.*-Construct a frame similar to, but larger than, that shown in Fig. 120, (about 12 inches high), and hang a small spring balance (250 grams capacity) at the place where the muscle is attached. Fasten the end of a lever to the upright piece, at a point on a level with the end of the balance hook.

(The nail or screw used for this purpose must pa.s.s loosely through the lever, and serve as a pivot upon which it can turn.) The lever should consist of a light piece of wood, and should have a length at least three times as great as the distance from the hook to the turning point. Connect the balance hook with the lever by a thread or string, and then hang upon it a small body of known weight. Note the amount of force exerted at the balance in order to support the weight at different places on the lever.

At what point is the force just equal to the weight? Where is it twice as great? Where three times? Show that the force required to support the weight increases proportionally as the weight-arm and as the distance through which the weight may be moved by the lever. Apply to the action of the biceps muscle in lifting weights on the forearm.

*A Study of the Action of the Biceps Muscle.*-Place the fingers upon the tendon of the biceps where it connects with the radius of the forearm.

With the forearm resting upon the table, note that the tendon is somewhat loose and flaccid, but that with the slightest effort to raise the forearm it quickly tightens. Now transfer the fingers to the body of the muscle, and sweep the forearm through two or three complete movements, noting the changes in the length and thickness of the muscle. Lay the forearm again on the table, back of hand down, and place a heavy weight (a flatiron or a hammer) upon the hand. Note the effort required to raise the weight, and then shift it along the arm. Observe that the nearer it approaches the elbow the lighter it seems. Account for the difference in the effort required to raise the weight at different places. Does the effort vary as the distance from the tendon?

CHAPTER XVI - THE SKIN

Protective coverings are found at all the exposed surfaces of the body.

These vary considerably at different places, each being adapted to the conditions under which it serves. The most important ones are the _skin_, which covers the entire external surface of the body; the _mucous membrane_, which lines all the cavities that communicate by openings with the external surface; and the _serous membrane_, which, including the synovial membranes, lines all the closed cavities of the body. In addition to the protection which it affords, the skin is one of the means by which the body is brought into proper relations with its surroundings. It is because of this function that we take up the study of the skin at this time.

*The Skin* is one of the most complex structures of the body, and serves several distinct purposes. It is estimated to have an area of from 14 to 16 square feet, and to have a thickness which varies from less than one eighth to more than one fourth of an inch. It is thickest on the palms of the hands and the soles of the feet, the places where it is most subject to wear. It is made up of two distinct layers-an outer layer called the _epidermis_, or cuticle, and an inner layer called the _dermis_, or cutis vera (Fig. 121).

*The Dermis.*-This is the thicker and heavier of the two layers, and is made up chiefly of connective tissue. The network of tough fibers which this tissue supplies, forms the essential body of the dermis and gives to it its power of resistance. It is on account of the connective tissue that the skins of animals can be converted into leather by tanning. A variety of structures, including blood and lymph vessels, oil and perspiratory glands, hair follicles, and nerves, are found embedded in the connective tissue (Fig. 122). These aid in different ways in the work of the skin.

[Fig. 121]

Fig. 121-*Section of skin* magnified, _a, b._ Epidermis, _b._ Pigment layer. _c._ Papillae, _d._ Dermis. _e._ Fatty tissue. _f, g, h._ Sweat gland and duct. _i, k._ Hair and follicle. _l._ Oil gland.

On the outer surface of the dermis are numerous elevations, called _papillae_. These average about one one-hundredth of an inch in height, and one two hundred and fiftieth of an inch in diameter. They are most numerous on the palms of the hands, the soles of the feet, and the under surfaces of the fingers and toes. At these places they are larger than in other parts of the body, and are closely grouped, forming the parallel curved ridges which cover the surfaces. Each papilla contains a loop of capillaries and a small nerve, and many of them are crowned with touch corpuscles (page 342).

[Fig. 122]

Fig. 122-*Diagram* of section of skin showing its different structures.

*The Epidermis* is much thinner than the dermis. It is made up of several layers of cells which are flat and scale-like at the surface, but are rounded in form where the epidermis joins the dermis. The epidermis has the appearance of being _moulded onto_ the dermis, filling up the depressions between the papillae and having corresponding irregularities (Fig. 121). No blood vessels are found in the epidermis, its nourishment being derived from the lymph which reaches it from the dermis. Only the part next to the dermis is made up of _living_ cells. These are active, however, in the formation of new cells, which take the place of those that are worn off at the surface. Some of the cells belonging to the inner layer of epidermis contain _pigment granules_, which give the skin its color (Fig. 121). The epidermis contains no nerves and is therefore non-sensitive. The hair and the nails are important modifications of the epidermis.

*A Hair* is a slender cylinder, formed by the union of epidermal cells, which grows from a kind of pit in the dermis, called the _hair follicle_.

The oval and somewhat enlarged part of the hair within the follicle is called the _root_, or _bulb_, and the uniform cylinder beyond the follicle is called the _shaft_. Connected with the sides of the follicles are the _oil_, or _sebaceous, glands_ (Figs. 121 and 122). These secrete an oily liquid which keeps the hair and cuticle soft and pliable. Attached to the inner ends of the follicles are small, involuntary muscles whose contractions cause the roughened condition of the skin that occurs on exposure to cold.

*A Nail* is a tough and rather h.o.r.n.y plate of epidermal tissue which grows from a depression in the dermis, called the _matrix_. The back part of the nail is known as the _root_, the middle convex portion as the _body_, and the front margin as _the free edge_ (Fig. 123). Material for the growth of the nail is derived from the matrix, which is lined with active epidermal cells and is richly supplied with blood vessels. Cells added to the root cause the nail to grow in length (forward) and cells added to the under surface cause it to grow in thickness. The cuticle adheres to the nail around its entire circ.u.mference so that the covering over the dermis is complete.

[Fig. 123]