THE SPINAL COLUMN
V, vertebra; C, cartilage protecting spinal cord; A, point of articulation on the right side.
The thickness of the lower jaw and the size and squareness of the angle where it bends upward to be hinged to the skull, below the ear, are what give the appearance of squareness and determination to the faces of strong, vigorous men or women. If we want to imply that a person has a feeble will, or weak character, we say he has a "weak jaw."
The skull rests upon the top of the backbone, or spinal column, which, instead of being one long solid bone, is made up of a number of pieces, or sections, known as vertebræ. Each one of these vertebræ has a ring, or arch, upon its back. These, running one after the other, form a jointed, bony tube to protect the spinal cord, or main nerve-cable of the body, which runs through it.
Although the backbone can bend forward or backward, or twist from side to side a little, by the little pieces of bone of which it is built up gliding and turning upon one another, it is really very stiff and rigid, so as to protect the spinal cord and prevent its being stretched or pinched. Most of the movements which we call bending the spine are really movements of other joints which connect the body or head with it. When we bend our necks, for instance, we hardly bend the backbone at all, as most of the movement is made in the joint at the top of it, between it and the skull. Similarly, when we bend our backs, we really bend our backbones very little; for most of the movement comes at the hip joints, between the thighs and the hip bones.
Each of the limbs has a single, long, rounded bone in the upper part, known in the arm as the humerus, and two bones in the lower part. These last are known as the radius and ulna (the "funny bone") in the forearm, and the tibia and fibula in the leg. The shoulder-joint is made by the rounded head of the humerus fitting into the shallow cup of the scapula, or shoulder-blade. It is shallower than the hip joint to allow it freer movement; but this makes it weaker and much more easily dislocated, or put out of joint,—the most so, in fact, of any joint in the body.
| A BALL-AND-SOCKET JOINT Hip joint. | A HINGE JOINT Knee joint, with the knee cap removed |
The hip joints are deep, strong, cup-shaped sockets upon each side of the hip bones, or pelvis, into which fit the heads of the femurs or thigh bones. When the hip joint does become dislocated, it is very hard to put back again, on account of its depth and the heavy muscles surrounding it. It is quite subject to the attack of tuberculosis, or "hip-joint disease."
LENGTHWISE SECTION OF BONE