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against a fixed point, the fulcrum, in order to raise a certain weight. seesaw is a lever; here the fulcrum is in the middle, the weight is at one end, and the power to lift the weight is applied at the other end. There are three classes of levers, named according to the position of the fulcrum. In the first class, the fulcrum lies between the weight and the power; the seesaw is an example of this. The best example in the human body, of a lever of the first class, is seen when the head nods. Here the fulcrum is the vertebra known as the atlas; the power is the muscles of the neck attached to the back of the skull and to the spine; the weight is the front part of the head. When one keeps the head erect, this lever is used; the nodding head when one is sleeping shows this plainly.

A lever of the second class has the fulcrum at one end, and the weight between it and the power; when we rise on our toes, we use this kind of lever. Try to explain this by referring to a skeleton of a frog or of man. In a lever of the third class, the fulcrum is at one end, with the power between it and the weight. This is the kind of lever seen most frequently in the human body. The flexing (drawing up) of the lower leg or the forearm is an example of the use of this kind of lever. In such a lever, a wide range of movement is obtained.

SPRAINS. A sudden strain or twisting in the region of a joint may result in the pulling out or tearing of the ligaments or tendons of that joint. Such an injury may be recognized by the sudden swelling in that region, followed by great pain. A cure of the sprain is effected only by nature's own remedy, complete rest. For immediate relief hot water applications, followed by arnica or some liniment, are best; a tight bandage should be applied at once and a doctor called as soon as possible.

DISLOCATIONS.—The bones of a joint may be accidentally forced apart. Such a separation is called a dislocation and is known by the intense pain which follows any attempt to move the joint. There is often considerable swelling of the affected part; the bone may even protrude. A physician should be called at once so that the bone may be slipped into place again. Hot or cold water applied to the joint and rest in a comfortable position should be given until the doctor comes.

FRACTURES. A break or fracture usually occurs in one of the long bones of the body. The clavicle or collar bone (easily felt in the front part of the shoulder below the neck), because of its exposed position, is most frequently broken. The immediate treatment for fracture is rest in a comfortable position; cold water applications may relieve the pain. Send for a doctor at once. To heal a fracture, it is necessary to bring the two broken ends of the bone together, and hold them so that they will grow together or knit; for this purpose splints are often employed.

XXXIII. RESPIRATION

Necessity for Respiration. We have seen that plants and animals need oxygen in order that the life processes may go on. Food is oxidized to release energy, just as coal is burned to give heat to run an engine. As a draft of air is required to make a fire under the boiler, so, in the human body, oxygen must be given so that foods or tissues may be oxidized to release energy used in growth. Blood, in its circulation to all parts of the body, is the medium which conveys the oxygen to that place in the body where it will be used. But where does the blood get this supply of oxygen? We have alluded, in Circulation, to the fact that the lungs are the organs which give oxygen to the blood and take from it carbon dioxide. Let us examine the organs used by the frog in breathing, and see if this matter becomes any plainer.

Study of the Organs of Respiration in the Frog; Comparison with Man. Notice the pumping movement in the throat as a frog breathes. The frog swallows air. Does this method differ from breathing in man? How and where are the nostril holes (nares) placed? How is this of advantage to the animal? Do you notice any movement of the nares in breathing? Use a preserved frog for the following exercise: Open the mouth of the frog, and find the openings in the roof of the mouth, leading from the anterior nares. Find a vertical slit in the floor of the throat. This is the glottis or opening to the windpipe. If the muscles be carefully removed from the ventral surface of the body just beneath the arms, and a careful incision made, the windpipe may be seen. Notice that it branches into two smaller tubes, each of which leads to a lung. These tubes are the bronchi. The lungs are two spongy bags, the walls of which are filled with tiny blood vessels. Inflate the lungs by inserting a blowpipe in the glottis and blowing into it. Are the lungs elastic?

The Organs of Respiration in Man.-The course of air passing from the outside of the lungs in man is much the same as in the

frog. Air passes through the nares, the glottis, and into the windpipe. This cartilaginous tube, the top of which may easily be felt as the Adam's apple of the throat, divides into two bronchi. The bronchi within the lungs break up into a great number of smaller tubes, the bronchial tubes, which divide somewhat like the small branches of a tree. This branching increases the surface of the air tubes within the lungs. The bronchial tubes, indeed all the air passages, are lined with ciliated cells. The cilia of these cells are constantly in motion, beat

Air passages in the human lungs; a, larynx; b, trachea (or windpipe); c, d, bronchi; e, bronchial tubes; f, cluster of air cells.

ing with a quick stroke toward the outer end of the tube, that is,

toward the mouth. Hence if any foreign material should get into the windpipe or bronchial tubes, it will be expelled by the action of the cilia. It is by means of cilia that phlegm is raised from the throat. Such action is of great importance as it prevents the filling of the air passages with foreign matter. The bronchi end in very minute air sacs called alveoli; these are little pouches having elastic walls. It is into these pouches that air is taken when we inspire or take a deep breath. Thus we see the lung of man gets a very

great increase in wall area by having a large number of tiny sacs instead of one large one as in the frog. In the walls of the alveoli are numerous capillaries, the ends of arteries which pass from the heart into the lung. It is in the very thin walls of the alveoli that an interchange of gases takes place which results in the blood giving up part of its load of carbon dioxide, and taking up oxygen in its place.

The Pleura. The lungs are covered with a thin elastic membrane, the pleura. This forms a bag in which the lungs are hung. Between the walls of the bag and the lungs is a space filled with lymph. By this means, the lungs are prevented from rubbing against the walls of the chest.

Breathing. In every full breath there are two distinct movements; inspiration (taking air in) and expiration (forcing air out). Thus this action differs considerably from "breathing" movements of the frog. The frog pumps air into its lungs by raising and lowering the floor of the mouth and then shutting the flaps or valves in the anterior nares. It is actually a process of swallowing. In man the act of inspiration is partly under the control of the will. We are able to take a long breath, or a short breath, though we cannot stop breathing. An inspiration is produced by the contraction of the muscles between the ribs together with the contraction of the diaphragm, a muscular wall just below the heart and lungs (not found in the frog); this results in pulling down the diaphragm and pulling upward and outward of the ribs, thus making the space within the chest cavity larger. The lungs, which lie within this cavity, are filled by the air rushing into the larger space thus made. An expiration is simpler than an inspiration for it requires no muscular effort; the muscles relax, the breastbone and ribs sink into place, while the diaphragm returns to its original position.

Apparatus to illustrate the action of the diaphragm in respiration.

A piece of apparatus which illustrates to a degree the mechanics of breathing may be made as follows: Attach a string to the middle of a piece of sheet rubber. Tie the rubber over the large end of

a bell jar. Pass a glass tube through a rubber stopper. Fasten a small toy balloon to the lower end of the tube. Close the small end of the jar with the stopper. Adjust the tube so that the balloon shall hang free in the jar. If now the rubber sheet is pulled down by means of the string, the air pressure in the jar is reduced and the toy balloon within expands, owing to the air pressure down the tube. When the rubber is allowed to go back to its former position, the balloon collapses.

COUGHING, SIghing, and SnEEZING. — Coughing is a sudden strong expiration, with the glottis (or top of the windpipe) closed. A sigh is a quick inspiration followed by a quiet expiration. A sneeze is a sharp expiration, the air passing through the nose because the passage to the mouth is closed by the descent of the soft palate.

RATE OF BREATHING AND AMOUNT OF AIR BREATHED. - During quiet breathing, the rate of inspiration is from fifteen to eighteen times per minute; this rate largely depends on the amount of physical work performed. About thirty cubic inches of air are taken in and expelled during the ordinary quiet respiration. The air so breathed is called tidal air. In a "long breath," we take in about 100 cubic inches in addition to the tidal air. This is called complemental air. By means of a forced expiration, it is possible to expel from 75 to 100 cubic inches more than tidal air; this air is called reserve air. What remains in the lungs, amounting to about 100 cubic inches, is called the residual air. (See diagram, page 384.) The value of deep breathing is seen by a glance at the diagram. It is only by this means that we clear the lungs of the reserve air with its accompanying load of carbon dioxide.

The actual amount of oxygen used in the body during the course of a day is nearly 25 ounces; this being almost entirely used in oxidizing the food materials taken into the body during the 24 hours.

RESPIRATION UNDER NERVOUS CONTROL. The muscular movements which cause an inspiration are partly under the control of the will, but in part the movement is beyond our control. The nerve centers which govern inspiration are part of the sympathetic nervous system of which we shall learn later. That the sympathetic nervous system controls respiratory movements is seen, for example, in the involuntary short breath taken by the bather who plunges into cold water. Anything of an irritating nature in the trachea or larynx will cause a sudden expiration or cough. When a boy runs, the quickened respiration is due to the fact that oxygen is used up rapidly and a larger quantity of carbon dioxide is formed. These facts, together with the presence of certain other poisonous materials in the lung cells, stimulate the nervous center which has control of respiration to greater activity, and quickened inspiration follows.

EXPERIMENTS TO DETERMINE CHANGES UNDERGONE BY AIR IN THE LUNGS.-1. Breathe on the bulb of a thermometer and record any changes. 2. Breathe gently on any polished glass or metal surface. Note what happens.