SKELETON OF THE FOREARM: PRONATION AND SUPINATION.
The bones of the forearm; ulna and radius.--The lower
extremities of these bones; their styloid processes; triangular
fibro-cartilage of the wrist.--_Pronation_ and _supination_:
change of form and direction of the forearm: and position of the
hand.--Prominences of the wrist (styloid processes); the angle
which the axis of the hand makes with that of the forearm.
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_.
[Illustration: FIG. 25.
THE HAND AND WRIST: From a radiograph taken specially for this
work by Dr. Thurstan Holland, Liverpool.]
[Illustration: FIG. 26.
RIGHT FOREARM IN SUPINATION (the radius and radial half of the
hand are shaded by oblique lines): the radius is parallel to the
ulna.]
[Illustration: FIG. 27.
THE RIGHT FOREARM IN PRONATION: the radius (shaded) crosses the
ulna, and the radial half of hand (shaded) is placed on the
inner side.]
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.