英文科學(xué)讀本 第四冊(cè)·Lesson 06 More about the Skeleton

Lesson 06 More about the Skeleton

The human skeleton, and the skeletons of most of the animals we have examined, are built upon one general plan as regards the limbs. There are two pairs of limbs. In birds the upper limbs are modified to form wings, but the structure even here is essentially the same.

Let us examine the human limbs now. The arm has thirty-two distinct bones, and consists of three parts—an upper arm, a forearm, and a hand; the leg also consists of three corresponding parts—an upper leg (thigh), a lower leg, and a foot. It contains thirty separate bones.

The upper arm and the thigh are each formed by a single long bone. The forearm consists of two bones (one larger than the other), jointed at the elbow to the bone of the upper arm. The lower leg also consists of two bones (one larger than the other), jointed at the knee to the thigh-bone. The knee-joint, however, differs from the elbow-joint in having a small bone—the kneecap—placed over the joint. There is no corresponding bone in the arm.

The hand and the foot are built on very much the same plan. The wrist of the one corresponds to the ankle of the other. The wrist has eight small bones; the ankle seven. The palm of the hand corresponds to the sole of the foot, and each of them has five bones running through it, which carry the fingers and toes respectively.

The hand has four fingers and a thumb—each finger being formed of three bones, the thumb of two. The foot, in its turn, has four toes and a great toe; the great toe having two bones, each of the others three.

We have only to consider the purposes for which the hand and the foot are respectively designed, and we must be at once struck with admiration for the manner in which each is adapted to its work. In the hand, flexibility and easy rapid movement, delicate touch, and firm grasping powers have to be secured. Hence the wrist bones are small, those of the fingers very long, while the thumb is placed so as to be able to move in the opposite direction to the fingers. In the foot, we need strength and solidity of support, consequently the whole of the bones are short, thick, heavy, and clumsy in appearance. The great toe has no grasping power, corresponding to that of the thumb.

The bone of the upper arm is joined at the shoulder to a large flat bone—the shoulder-blade, which lies behind the ribs. The shoulder-blade is kept in position in a wonderful way by the collar-bone. This bone stretches from the top of the breast-bone to the shoulder joint, its outer end being firmly fixed to the shoulder-blade. It is this bone which forms the prominent part of the shoulder. It acts as a rigid bar to brace the shoulder-blade up. The two collar-bones in front and the two shoulder-blades behind form what is known as the shoulder-girdle.

The thigh-bone is joined to the great haunch or hip bone.