We have already examined the upper ends of the bones of the forearm (radius and ulna) in relation to the elbow-joint. We have still to examine their shafts and lower extremities; and in this chapter we will continue their description, and study the movements of the two bones, and the relation which they bear to the wrist-joint and the hand. In a state of rest the two bones lie parallel, the radius externally, and the ulna internally, separated except at their extremities by a wide interosseous space.

A superficial glance at the bony structure of the forearm (Fig. 21, p. 70) suffices to show that the two bones which compose it offer in many respects a striking contrast to each other. They differ first of all in the relative position which they hold in the forearm. The inner bone is the ulna, which extends upwards beyond the radius by reason of the length of the olecranon process. On the other hand, in the neighbourhood of the wrist, the radius extends beyond the ulna and descends below it. We should also note at the outset that the radius, descending below the ulna, is found to be the only bone of the forearm which articulates directly with the hand and forms the wrist-joint, or radio-carpal articulation. The lower end of the ulna is separated by a wide space from the carpus (cuneiform bone), which is filled up in the living subject by the triangular fibro-cartilage of the wrist-joint. As regards size also these bones present a contrast; the ulna is thick and bulky in its upper part, but becomes thinner as it descends, and its inferior extremity is slender (Fig. 21); the radius, on the other hand, is relatively small at its upper end, and increases in size below, so that its inferior extremity, articulating with the hand, forms a large bony surface.

We need not enter into minute details regarding the shape of the shafts of the two bones.

They are regularly prismatic in form. At the upper end of the shaft of the radius, immediately below the neck, is a tuberosity (13, Fig. 21), directed forwards and inwards, which gives insertion to the tendon of the biceps (bicipital tubercle). From this tuberosity an oblique line passes downwards and outwards, and terminates upon the middle part of the external surface of the bone in a rough space (14, Fig. 21) called the impression for the pronator radii teres, because it gives insertion to the muscle of that name.

With regard to the shaft of the ulna, we need only notice that its posterior border is subcutaneous in its whole extent, and can be felt beneath the skin from the elbow to the wrist.

The lower end of the radius is enlarged and massive. On its inner side it articulates with the ulna. Externally it is prolonged downwards as the styloid process, which forms a well-marked prominence at the outer side of the wrist. The inferior surface is hollowed into a smooth triangular surface for articulation with two of the bones of the wrist (scaphoid and semi-lunar).

The inferior extremity of the ulna is much smaller. It presents a rounded head (8, Fig. 21), which articulates externally with the radius, and below with the triangular fibro-cartilage of the wrist, which intervenes between the ulna and the carpus, and fills the gap between that bone and the cuneiform bone. The ulna is prolonged on its inner side and behind into a styloid process which forms the internal prominence of the wrist. We have already said that the inferior extremity of the ulna does not descend as low as the corresponding portion of the radius; the triangular fibro-cartilage, which lies below the ulna, proceeds from the external border of the inferior extremity of the radius to the base of the styloid process of the ulna; the bones of the wrist articulate with the radius and with this triangular fibro-cartilage, so that the ulna does not take part directly in the articulation of the forearm with the hand (Fig. 28, page 88). The relation of the ulna to the wrist-bones is well shown in a radiograph of the hand, in which the space between the bones at the inner side of the wrist is clearly seen (Fig. 25). This arrangement is of prime importance, as will be seen later, in the movements of the wrist. On account of this space between the ulna and the carpus, the movement of adduction (drawing inwards of the hand) is much freer and more extensive than abduction.

Up to the present we have considered the two bones of the arm as placed parallel side by side, and separated by a comparatively broad interval, called the interosseous space (7, Fig. 21). In fact, they are so placed, when the arm is hanging beside the body, in a state of rest, with the palm of the hand directed forwards (Fig. 26), or placed on a flat surface, palm upwards; the hand is then lying on its back, and this position we call supination (supinus, lying on the back). But the hand may be changed in position, turned round so that its posterior surface is directed forward (Fig. 27), or, if the forearm be placed on a flat surface, the back of the hand is turned upwards. In this new position, when the hand lies on its palmar aspect, we speak of it as being in the position of pronation (pronus, lying on the belly).

This change from supination to pronation is accomplished by a change in the relations of the bones of the forearm to one another. They cease to be parallel when the hand is prone, and cross each other; but the two bones do not move similarly in this action; one of them, the ulna, practically remains fixed; the other, the radius, changes its position so as to cross it. On examining the points of contact between the radius and ulna—namely, the superior and inferior articulations of these two bones—we see that the superior radio-ulnar articulation is formed by the circumference of the head of the radius received within the cavity (lesser sigmoid cavity), situated on the outer surface of the coronoid process of the ulna; whilst the inferior radio-ulnar articulation is formed by a sigmoid cavity situated on the internal surface of the inferior extremity of the radius, which articulates with the circumference of the head of the ulna.

The axis of movement may be represented by a line drawn through the centres of the upper end of the radius and the lower end of the ulna. In the superior radio-ulnar articulation, the head of the radius revolves on its own axis and turns in the sigmoid cavity of the ulna; the superior extremity of the radius itself does not change its position; in the inferior radio-ulnar articulation, on the contrary, the radius moves round the head of the ulna, as a wheel round its axle.

These considerations of articular mechanism may be best understood by examining a portion of the skeleton containing the bones of the forearm, which, as is usual in articulated preparations, are connected together by metallic bands that permit the normal movements. In causing the hand to pass from supination to pronation, we see that it is necessary to bring the shaft of the radius across that of the ulna in such a manner that while the upper end of the radius still remains on the outer side, its lower end is entirely altered in position, and is carried to the inner side of the ulna (Figs. 26 and 27). In accomplishing this movement, we perceive that the hand, which articulates only with the radius, must follow the movement of this bone, so that the thumb or radial border of the hand must change from the outer to the inner side; the palm of the hand, which in supination is directed forwards, is turned backwards in pronation, and it is this movement of the radius on the ulna which constitutes the passage from supination (Fig. 26) to pronation (Fig. 27).

The general form of the forearm, irrespective of the details which we shall explain later, regarding the configuration of the muscles, depends directly on the position of these bones, and is changed according as they are parallel or crossed. When the hand is supinated (Fig. 26), the radius being then placed parallel to the ulna and separated from it by a large interosseous space, the form of the forearm is that of the segment of a limb presenting two borders—an external, or radial, and an internal, or ulnar—and two surfaces, one anterior, the other posterior. The forearm, in a word, is slightly flattened from front to back, because the bones are parallel to each other. But when, from the position of supination, the hand passes to that of pronation, the two bones cross each other and come in contact, and the interosseous space practically disappears (Fig. 27). The radius and ulna, taken together, form a single mass, which may be compared to that which two rods assume, placed at first parallel at a certain distance from each other, and which afterwards cross and come into direct contact. Thus in pronation the shape of the forearm becomes completely changed, especially in its lower two-thirds. Instead of a segment of a limb with two surfaces and two borders, it represents a segment rounded and almost cylindrical in its middle part; in complete supination, only the inferior part (wrist) and the superior part (bend of the elbow) preserve the form flattened from front to back.

Artists are usually not sufficiently imbued with these important facts; they are inclined to believe that if a figure has been represented with the forearm in a position of supination, and that for some reason this attitude has been changed to that of pronation, it is enough to change the hand and wrist alone, without altering the model of the forearm in any other detail. On the other hand, the form of the forearm throughout its entire extent and particularly in the middle part, undergoes a change when the hand passes from supination to pronation, and the reverse; and this fact will be still more evident when, in studying the muscles of the region, we see that their direction is completely altered, and this also helps to modify the shape of the limb.

In the movements of pronation and supination, the forearm changes not only in form, but also in direction. We have previously seen that when the radius and ulna are placed parallel one with the other (supination), the axis of the forearm makes with that of the arm an angle opening outwards. We may again express this fact by saying that if in this case we prolong the axis of the humerus downwards (see the dotted line in Fig. 26), this axis falls internal to the head (inferior extremity) of the ulna, and consequently lies well to the inner side of the radius and interosseous space. But in pronation, the result is that when the radius crosses the ulna at its centre, and is placed internal to it at its lower end, the two crossed bones of the forearm, taken together, make a continuous line with the humerus, the angle at the inner side of the elbow disappears, and the axis of the arm and that of the forearm are almost in the same straight line (Fig. 27).

In order to make the best use of the various details of the bony structures which we have been studying in their application to outward form we will, before commencing the study of the hand, mark once more the prominences which occur at the lower ends of the radius and ulna at the level of the wrist. Of these two projections, which are to the hand what the ankle-bones, or malleoli, are to the feet, one is external, formed by the styloid process of the radius (16, Fig. 21), the other is internal, and is formed by the head of the ulna and its styloid process (9, Fig. 21).

The styloid process of the radius is situated much lower than the styloid process of the ulna.

This position of the bones we may easily verify upon ourselves, without a skeleton, by clasping with the thumb and index-finger of one hand the wrist of the other; we then perceive that the radius descends much lower than the ulna (Fig. 21). Hence the articular line of the forearm with the hand is obliquely directed from above downwards and outwards (the hand being supinated). So that the hand is articulated with the forearm at an angle, due to the downward prolongation of the lower end of the radius: a position which, along with the presence of the articular fibro-cartilage of the wrist, is responsible for the more extensive capacity for adduction than abduction of the hand at the wrist-joint.

In the extended and supine position of the limb the junction of the arm and the forearm forms an angle opening outwards; that of the forearm and hand forms an angle opening inwards.