bone; move the leg in as many different directions as possible; notice that it may be flexed or bent; that it may be extended to its original position; that it may be rotated; that it may be moved to and from the midline of the body; that, with the knee held stiff, the whole limb may be made to describe the arc of a circle.

These same movements are possible in the leg of a man. This movement between bones is obtained by means of joints. If, in the frog, we carefully separate the muscles of the thigh to the bone, we find that they are attached to the bone by white, glistening tendons. Careful examination shows that the bones themselves are held together by very tough white bands or cords; these are the ligaments. We find, too, that one end of the large thigh bone fits into a socket in the hip bone or pelvic arch. It is thus easy to see how such free movement is obtained in the leg.

Ball and Socket Joint. Such a joint as just described is called a ball and socket joint. In

man the movement of the leg is obtained in exactly the way described for the hind leg of the frog. The two best examples of a ball and socket joint are found between the long bone in the arm and the shoulder, and between the bones of the hip and the long bone of the leg.

HINGE JOINTS. The second kind of joint, in the leg of the frog, is found between the thigh and the shank. Notice that movement here occurs freely in only two directions, backward and forward; hence

this is called a hinge joint. In man the best examples of a hinge joint are found in the knee and elbow; others are in the fingers and toes.

GLIDING JOINTS. - Another form of joint, best seen in the skeleton of man, is a sliding or gliding joint. Here the range of movement is slight. Gliding joints are found between the vertebræ or bones of the vertebral column (backbone).

PIVOT JOINTS. - Another rather unusual joint is the pivot joint. This is best seen between the skull of man and the topmost bone of the vertebral column. The skull is held in place by means of two small knobs which project downward and rest in cavities in the bone directly beneath them.

- It is evident that movement

b

Hinge joint, show

ing muscle (a) and its tendon

(b).

LEVERS IN THE BODY.. of a joint is caused by muscles which act in coöperation with the bones to which they are attached; the latter form true levers. A lever is a structure by which either greater work power or greater range of motion is obtained. In this apparatus, the lever works

A

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

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,

Diagram of two air cells, showing the capillary network which covers them, and at a the structures which intervene between the air and the blood are indicated; 1, mucous membrane of the air cell; 2, submucous meshwork; 3. wall of capillary; 4, plasma in capillary; 5, red blood corpuscle.

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