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 space 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 vertebræ affords a good example of imperfect joints which are partially movable.
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 vertebræ (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.
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 canals, 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.
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.
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 mass 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