[Ill.u.s.tration: Fig. 22.--Right Femur, or Thigh Bone.]

47. The Thigh. The longest and strongest of all the bones is the femur, or thigh bone. Its upper end has a rounded head which fits into the _acetabulum_, or the deep cup-like cavity of the hip bone, forming a perfect ball-and-socket joint. When covered with cartilage, the ball fits so accurately into its socket that it may be retained by atmospheric pressure alone (sec. 50).

The shaft of the femur is strong, and is ridged and roughened in places for the attachment of the muscles. Its lower end is broad and irregularly shaped, having two prominences called _condyles_, separated by a groove, the whole fitted for forming a hinge joint with the bones of the lower leg and the knee-cap.

48. The Lower Leg. The lower leg, like the forearm, consists of two bones. The tibia, or shin bone, is the long three-sided bone forming the front of the leg. The sharp edge of the bone is easily felt just under the skin. It articulates with the lower end of the thigh bone, forming with it a hinge joint.

The fibula, the companion bone of the tibia, is the long, slender bone on the outer side of the leg. It is firmly fixed to the tibia at each end, and is commonly spoken of as the small bone of the leg. Its lower end forms the outer projection of the ankle. In front of the knee joint, embedded in a thick, strong tendon, is an irregularly disk-shaped bone, the patella, or knee-cap. It increases the leverage of important muscles, and protects the front of the knee joint, which is, from its position, much exposed to injury.

[Ill.u.s.tration: Fig. 23.--Patella, or Knee-Cap.]

49. The Foot. The bones of the foot, 26 in number, consist of the tarsal bones, the metatarsal, and the phalanges. The tarsal bones are the seven small, irregular bones which make up the ankle. These bones, like those of the wrist, are compactly arranged, and are held firmly in place by ligaments which allow a considerable amount of motion.

One of the ankle bones, the _os calcis_, projects prominently backwards, forming the heel. An extensive surface is thus afforded for the attachment of the strong tendon of the calf of the leg, called the tendon of Achilles. The large bone above the heel bone, the _astragalus_, articulates with the tibia, forming a hinge joint, and receives the weight of the body.

The metatarsal bones, corresponding to the metacarpals of the hand, are five in number, and form the lower instep.

The phalanges are the fourteen bones of the toes,--three in each except the great toe, which, like the thumb, has two. They resemble in number and plan the corresponding bones in the hand. The bones of the foot form a double arch,--an arch from before backwards, and an arch from side to side. The former is supported behind by the os calcis, and in front by the ends of the metatarsal bones. The weight of the body falls perpendicularly on the astragalus, which is the key-bone or crown of the arch. The bones of the foot are kept in place by powerful ligaments, combining great strength with elasticity.

[Ill.u.s.tration: Fig. 24.--Right Tibia and Fibula (Anterior surface.)]

[Ill.u.s.tration: Fig. 25.--Bones of Right Foot. (Dorsal surface.)]

The Joints.

50. Formation of Joints. The various bones of the skeleton are connected together at different parts of their surfaces by joints, or articulations. Many different kinds of joints have been described, but the same general plan obtains for nearly all. They vary according to the kind and the amount of motion.

The princ.i.p.al structures which unite in the formation of a joint are: bone, cartilage, synovial membrane, and ligaments. Bones make the chief element of all the joints, and their adjoining surfaces are shaped to meet the special demands of each joint (Fig. 27). The joint-end of bones is coated with a thin layer of tough, elastic cartilage. This is also used at the edge of joint-cavities, forming a ring to deepen them.

The rounded heads of bones which move in them are thus more securely held in their sockets.

Besides these structures, the muscles also help to maintain the joint-surfaces in proper relation. Another essential to the action of the joints is the pressure of the outside air. This may be sufficient to keep the articular surfaces in contact even after all the muscles are removed.

Thus the hip joint is so completely surrounded by ligaments as to be air-tight; and the union is very strong. But if the ligaments be pierced and air allowed to enter the joint, the union at once becomes much less close, and the head of the thigh bone falls away as far as the ligaments will allow it.

51. Synovial Membrane. A very delicate connective tissue, called the synovial membrane, lines the capsules of the joints, and covers the ligaments connected with them. It secretes the _synovia_, or joint oil, a thick and glairy fluid, like the white of a raw egg, which thoroughly lubricates the inner surfaces of the joints. Thus the friction and heat developed by movement are reduced, and every part of a joint is enabled to act smoothly.

52. Ligaments. The bones are fastened together, held in place, and their movements controlled, to a certain extent, by bands of various forms, called ligaments. These are composed mainly of bundles of white fibrous tissue placed parallel to, or closely interlaced with, one another, and present a shining, silvery aspect. They extend from one of the articulating bones to another, strongly supporting the joint, which they sometimes completely envelope with a kind of cap (Fig. 28). This prevents the bones from being easily dislocated. It is difficult, for instance, to separate the two bones in a shoulder or leg of mutton, they are so firmly held together by tough ligaments.

While ligaments are pliable and flexible, permitting free movement, they are also wonderfully strong and inextensible. A bone may be broken, or its end torn off, before its ligaments can be ruptured. The wrist end of the radius, for instance, is often torn off by force exerted on its ligaments without their rupture.

The ligaments are so numerous and various and are in some parts so interwoven with each other, that s.p.a.ce does not allow even mention of those that are important. At the knee joint, for instance, there are no less than fifteen distinct ligaments.

53. Imperfect Joints. It is only perfect joints that are fully equipped with the structures just mentioned. Some joints lack one or more, and are therefore called imperfect joints. Such joints allow little or no motion and have no smooth cartilages at their edges. Thus, the bones of the skull are dovetailed by joints called sutures, which are immovable.

The union between the vertebrae affords a good example of imperfect joints which are partially movable.

[Ill.u.s.tration: Fig. 26.--Elastic Tissue from the Ligaments about Joints.

(Highly magnified.)]

54. Perfect Joints. There are various forms of perfect joints, according to the nature and amount of movement permitted. They an divided into hinge joints, ball-and-socket joints and pivot joints.

The hinge joints allow forward and backward movements like a hinge.

These joints are the most numerous in the body, as the elbow, the ankle, and the knee joints.

In the ball-and-socket joints--a beautiful contrivance--the rounded head of one bone fits into a socket in the other, as the hip joint and shoulder joint. These joints permit free motion in almost every direction.

In the pivot joint a kind of peg in one bone fits into a notch in another. The best example of this is the joint between the first and second vertebrae (see sec. 38). The radius moves around on the ulna by means of a pivot joint. The radius, as well as the bones of the wrist and hand, turns around, thus enabling us to turn the palm of the hand upwards and downwards. In many joints the extent of motion amounts to only a slight gliding between the ends of the bones.

55. Uses of the Bones. The bones serve many important and useful purposes. The skeleton, a general framework, affords protection, support, and leverage to the bodily tissues. Thus, the bones of the skull and of the chest protect the brain, the lungs, and the heart; the bones of the legs support the weight of the body; and the long bones of the limbs are levers to which muscles are attached.

Owing to the various duties they have to perform, the bones are constructed in many different shapes. Some are broad and flat; others, long and cylindrical; and a large number very irregular in form. Each bone is not only different from all the others, but is also curiously adapted to its particular place and use.

[Ill.u.s.tration: Fig. 27.--Showing how the Ends of the Bones are shaped to form the Elbow Joint. (The cut ends of a few ligaments are seen.)]

Nothing could be more admirable than the mechanism by which each one of the bones is enabled to fulfill the manifold purposes for which it was designed. We have seen how the bones of the cranium are united by sutures in a manner the better to allow the delicate brain to grow, and to afford it protection from violence. The arched arrangement of the bones of the foot has several mechanical advantages, the most important being that it gives firmness and elasticity to the foot, which thus serves as a support for the weight of the body, and as the chief instrument of locomotion.

The complicated organ of hearing is protected by a winding series of minute apartments, in the rock-like portion of the temporal bone. The socket for the eye has a jutting ridge of bone all around it, to guard the organ of vision against injury. Grooves and ca.n.a.ls, formed in hard bone, lodge and protect minute nerves and tiny blood-vessels. The surfaces of bones are often provided with grooves, sharp edges, and rough projections, for the origin and insertion of muscles.

[Ill.u.s.tration: Fig. 28.--External Ligaments of the Knee.]

56. The Bones in Infancy and Childhood. The bones of the infant, consisting almost wholly of cartilage, are not stiff and hard as in after life, but flexible and elastic. As the child grows, the bones become more solid and firmer from a gradually increased deposit of lime salts. In time they become capable of supporting the body and sustaining the action of the muscles. The reason is that well-developed bones would be of no use to a child that had not muscular strength to support its body. Again, the numerous falls and tumbles that the child sustains before it is able to walk, would result in broken bones almost every day of its life. As it is, young children meet with a great variety of falls without serious injury.

But this condition of things has its dangers. The fact that a child's bones bend easily, also renders them liable to permanent change of shape.

Thus, children often become bow-legged when allowed to walk too early.

Moderate exercise, however, even in infancy, promotes the health of the bones as well as of the other tissues. Hence a child may be kept too long in its cradle, or wheeled about too much in a carriage, when the full use of its limbs would furnish proper exercise and enable it to walk earlier.

57. Positions at School. Great care must be exercised by teachers that children do not form the habit of taking injurious positions at school. The desks should not be too low, causing a forward stoop; or too high, throwing one shoulder up and giving a twist to the spine. If the seats are too low there will result an undue strain on the shoulder and the backbone; if too high, the feet have no proper support, the thighs may be bent by the weight of the feet and legs, and there is a prolonged strain on the hips and back. Curvature of the spine and round shoulders often result from long-continued positions at school in seats and at desks which are not adapted to the physical build of the occupant.

[Ill.u.s.tration: Fig. 29.--Section of the Knee Joint. (Showing its internal structure)

A, tendon of the semi-membranosus muscle cut across; B, F, tendon of same muscle; C, internal condyle of femur; D, posterior crucial ligament; E, internal interarticular fibro cartilage; G, bursa under knee-cap; H, ligament of knee-cap; K, fatty ma.s.s under knee-cap; L, anterior crucial ligament cut across; P, patella, or knee-cap ]

A few simple rules should guide teachers and school officials in providing proper furniture for pupils. Seats should be regulated according to the size and age of the pupils, and frequent changes of seats should be made.

At least three sizes of desks should be used in every schoolroom, and more in ungraded schools. The feet of each pupil should rest firmly on the floor, and the edge of the desk should be about one inch higher than the level of the elbows. A line dropped from the edge of the desk should strike the front edge of the seat. Sliding down into the seat, bending too much over the desk while writing and studying, sitting on one foot or resting on the small of the back, are all ungraceful and unhealthful positions, and are often taken by pupils old enough to know better. This topic is well worth the vigilance of every thoughtful teacher, especially of one in the lower grades.

58. The Bones in After Life. Popular impression attributes a less share of life, or a lower grade of vitality, to the bones than to any other part of the body. But really they have their own circulation and nutrition, and even nervous relations. Thus, bones are the seat of active vital processes, not only during childhood, but also in adult life, and in fact throughout life, except perhaps in extreme old age. The final knitting together of the ends of some of the bones with their shafts does not occur until somewhat late in life. For example, the upper end of the tibia and its shaft do not unite until the twenty-first year. The separate bones of the sacrum do not fully knit into one solid bone until the twenty-fifth year. Hence, the risk of subjecting the bones of young persons to undue violence from injudicious physical exercise as in rowing, baseball, football, and bicycle-riding.

The bones during life are constantly going through the process of absorption and reconstruction. They are easily modified in their growth.

Thus the continued pressure of some morbid deposit, as a tumor or cancer, or an enlargement of an artery, may cause the absorption or distortion of bones as readily as of one of the softer tissues. The distortion resulting from tight lacing is a familiar ill.u.s.tration of the facility with which the bones may be modified by prolonged pressure.

Some savage races, not content with the natural shape of the head, take special methods to mould it by continued artificial pressure, so that it may conform in its distortion to the fashion of their tribe or race. This custom is one of the most ancient and widespread with which we are acquainted. In some cases the skull is flattened, as seen in certain Indian tribes on our Pacific coast, while with other tribes on the same coast it is compressed into a sort of conical appearance. In such cases the brain is compelled, of course, to accommodate itself to the change in the shape of the head; and this is done, it is said, without any serious result.

59. Sprains and Dislocations. A twist or strain of the ligaments and soft parts about a joint is known as a sprain, and may result from a great variety of accidents. When a person falls, the foot is frequently caught under him, and the twist comes upon the ligaments and tissues of the ankle. The ligaments cannot stretch, and so have to endure the wrench upon the joint. The result is a sprained ankle. Next to the ankle, a sprain of the wrist is most common. A person tries, by throwing out his hand, to save himself from a fall, and the weight of the body brings the strain upon the firmly fixed wrist. As a result of a sprain, the ligaments may be wrenched or torn, and even a piece of an adjacent bone may be torn off; the soft parts about the injured joint are bruised, and the neighboring muscles put to a severe stretch. A sprain may be a slight affair, needing only a brief rest, or it may be severe and painful enough to call for the most skillful treatment by a surgeon. Lack of proper care in severe sprains often results in permanent lameness.

A fall or a blow may bring such a sudden wrench or twist upon the ligaments as to force a bone out of place. This displacement is known as a dislocation. A child may trip or fall during play and put his elbow out of joint. A fall from horseback, a carriage, or a bicycle may result in a dislocation of the shoulder joint. In playing baseball a swift ball often knocks a finger out of joint. A dislocation must be reduced at once.

Any delay or carelessness may make a serious and painful affair of it, as the torn and bruised parts rapidly swell and become extremely sensitive.