Читать книгу A Practical Physiology: A Text-Book for Higher Schools - Albert F. Blaisdell - Страница 10

Table of Contents

42. The Upper Limbs. Each of the upper limbs consist of the upper arm, the forearm, and the hand. These bones are classified as follows:

 Upper Arm:Scapula, or shoulder-blade,Clavicle, or collar bone,Humerus, or arm bone,

 Forearm:Ulna,Radius,

 Hand:8 Carpal or wrist bones,5 Metacarpal bones,14 Phalanges, or finger bones,

making 32 bones in all.

43. The Upper Arm. The two bones of the shoulder, the scapula and the clavicle, serve in man to attach the arm to the trunk. The scapula, or shoulder-blade, is a flat, triangular bone, placed point downwards, and lying on the upper and back part of the chest, over the ribs. It consists of a broad, flat portion and a prominent ridge or spine. At its outer angle it has a shallow cup known as the glenoid cavity. Into this socket fits the rounded head of the humerus. The shoulder-blade is attached to the trunk chiefly by muscles, and is capable of extensive motion.

The clavicle, or collar bone, is a slender bone with a double curve like an italic f, and extends from the outer angle of the shoulder-blade to the top of the breastbone. It thus serves like the keystone of an arch to hold the shoulder-blade firmly in its place, but its chief use is to keep the shoulders wide apart, that the arm may enjoy a freer range of motion. This bone is often broken by falls upon the shoulder or arm.

The humerus is the strongest bone of the upper extremity. As already mentioned, its rounded head fits into the socket of the shoulder-blade, forming a ball-and-socket joint, which permits great freedom of motion. The shoulder joint resembles what mechanics call a universal joint, for there is no part of the body which cannot be touched by the hand.

Fig. 18.--Left Scapula, or Shoulder-Blade.

When the shoulder is dislocated the head of the humerus has been forced out of its socket. The lower end of the bone is grooved to help form a hinge joint at the elbow with the bones of the forearm (Fig. 27).

44. The Forearm. The forearm contains two long bones, the ulna and the radius. The ulna, so called because it forms the elbow, is the longer and larger bone of the forearm, and is on the same side as the little finger. It is connected with the humerus by a hinge joint at the elbow. It is prevented from moving too far back by a hook-like projection called the olecranon process, which makes the sharp point of the elbow.

The radius is the shorter of the two bones of the forearm, and is on the same side as the thumb. Its slender, upper end articulates with the ulna and humerus; its lower end is enlarged and gives attachment in part to the bones of the wrist. This bone radiates or turns on the ulna, carrying the hand with it.

Experiment 10. Rest the forearm on a table, with the palm up (an attitude called supination). The radius is on the outer side and parallel with the ulna If now, without moving the elbow, we turn the hand (pronation), as if to pick up something from the table, the radius may be seen and felt crossing over the ulna, while the latter has not moved.

Fig. 19.--Left Clavicle, or Collar Bone. (Anterior surface.)

45. The Hand. The hand is the executive or essential part of the upper limb. Without it the arm would be almost useless. It consists of 27 separate bones, and is divided into three parts, the wrist, the palm, and the fingers.

Fig. 20.--Left Humerus.

Fig. 21.--Left Radius and Ulna.

The carpus, or wrist, includes 8 short bones, arranged in two rows of four each, so as to form a broad support for the hand. These bones are closely packed, and tightly bound with ligaments which admit of ample flexibility. Thus the wrist is much less liable to be broken than if it were to consist of a single bone, while the elasticity from having the eight bones movable on each other, neutralizes, to a great extent, a shock caused by falling on the hands. Although each of the wrist bones has a very limited mobility in relation to its neighbors, their combination gives the hand that freedom of action upon the wrist, which is manifest in countless examples of the most accurate and delicate manipulation.

The metacarpal bones are the five long bones of the back of the hand. They are attached to the wrist and to the finger bones, and may be easily felt by pressing the fingers of one hand over the back of the other. The metacarpal bones of the fingers have little freedom of movement, while the thumb, unlike the others, is freely movable. We are thus enabled to bring the thumb in opposition to each of the fingers, a matter of the highest importance in manipulation. For this reason the loss of the thumb disables the hand far more than the loss of either of the fingers. This very significant opposition of the thumb to the fingers, furnishing the complete grasp by the hand, is characteristic of the human race, and is wanting in the hand of the ape, chimpanzee, and ourang-outang.

The phalanges, or finger bones, are the fourteen small bones arranged in three rows to form the fingers. Each finger has three bones; each thumb, two.

The large number of bones in the hand not only affords every variety of movement, but offers great resistance to blows or shocks. These bones are united by strong but flexible ligaments. The hand is thus given strength and flexibility, and enabled to accomplish the countless movements so necessary to our well-being.

In brief, the hand is a marvel of precise and adapted mechanism, capable not only of performing every variety of work and of expressing many emotions of the mind, but of executing its orders with inconceivable rapidity.