THE HUMERUS AND ELBOW-JOINT.
The bony structure of the arm and elbow.--The shaft of the
humerus.--The axis of the arm and forearm.--Spiral groove of the
humerus; the inferior extremity of the humerus; the trochlea
and capitellum; the external and internal supra-condyloid
ridges.--The superior extremities of the two bones of the
forearm; the ulna (sigmoid cavities, olecranon process, coronoid
process); radius (head, neck).--The articulation of the elbow;
transverse hinge joint.--Movements of flexion and extension;
lateral movements.--The external form of the elbow; prominence
of the internal condyle and olecranon process.
The _shaft of the humerus_ is prismatic in its upper and middle parts,
and flattened from front to back at its lower extremity. Covered over
by thick muscles, its outward shape has for us but little interest;
it is necessary only to note with regard to its direction that, when
the arm is hanging down, the humerus is not altogether vertical, but
is tilted slightly downwards and inwards. We shall see that the axis
of the bones of the forearm is oblique in the opposite direction; for
this reason the arm and forearm form at the elbow a very obtuse angle,
looking outwards (Figs. 24, 26).
Among the details to be noted upon the shaft of the humerus, the
_bicipital groove_ is worthy of remark. This vertical groove, which
separates the great from the small tuberosity on the front of the
humerus at its upper end (Fig. 18), is prolonged along the shaft of the
bone, and presents an internal lip slightly prominent, and an external
or anterior lip which is much more marked, which gives insertion to
the broad tendon of the great pectoral muscle. At the line of junction
of the superior with the middle third of the bone, this lip forms the
anterior margin of a rough surface shaped like the letter =V= (7, Fig.
18), of which the angle looks downwards, and which, giving insertion to
the deltoid muscle, has received the name of the _deltoid impression_.
On the back of the humerus is another groove, broad and shallow, called
the _spiral groove_ of the humerus, which passes downwards and outwards
along the shaft of the bone behind and below the deltoid impression.
The inferior extremity of the humerus deserves to be studied in detail,
as its shape gives the key to the movements of the elbow-joint, and
explains at the same time several features of the region to be seen
in the living model. This extremity is flattened from front to back,
and is enlarged into a broad surface which is partly articular, partly
non-articular. In the middle are two important articular prominences,
smooth and covered with cartilage. Of these the internal portion is
called the _trochlea_ (_trochlea_, pulley). It possesses a groove and
two unequal marginal ridges. The internal ridge is more prominent and
descends lower than the external one. The other projection (11, Fig.
18), which is situated on the outer side of the trochlea, is rounded
in shape, and receives the name of the _radial head_, or _capitellum_.
It is only obvious when the humerus is looked at from the front or
from below: in other words, it does not appear (like the trochlear
surface) on the posterior aspect of the lower end of the bone. This
surface articulates with the upper end of the radius. There are three
depressions, or fossæ, to be seen in relation to the articular surfaces
of the lower end of the humerus. On the front of the bone, just above
the trochlea and capitellum, are two, which receive the bones of the
forearm during flexion of the limb: the _coronoid fossa_ (for the
coronoid process of the ulna), above the trochlear surface, and the
_radial depression_ for the head of the radius, much shallower, and
placed above the capitellum. On the back, above the trochlear surface,
is the _olecranon fossa_, into which the olecranon process is received
during extension of the forearm.
The lateral portions of the inferior extremity of the humerus are
formed by rough, non-articular projections which give attachment to
muscles and ligaments, and are known respectively as the _external
condyle_ and the _internal condyle_ of the humerus (14, 14, Fig. 18).
Above each condyle is a well-marked bony ridge, which is called the
_supra-condyloid ridge_ (external or internal) (13, 13, Fig. 18).
The lower end of the humerus articulates with the upper extremities
of the bones of the forearm; and we will next proceed to study the
formation of these extremities in order to understand the movements of
the elbow-joint and the form of the region.
The forearm is formed of two bones (Fig. 21), which, when the upper
limb is hanging beside the body, the palmar surface of the hand being
turned forwards, are placed parallel to each other--one on the outer,
one on the inner side. The inner bone (1, Fig. 21) is called the
_ulna_, or _cubitus_, and it is that which by its upper extremity
(_olecranon_) forms the bony prominence of the elbow; the outer bone
(10, Fig. 21) is called the _radius_ (from the Latin _radius_, a spoke
of a wheel), and it is by this bone chiefly that the bones of the wrist
and hand are carried. For the present we will describe only the upper
extremities of these two bones (Figs. 21 and 22).
[Illustration: FIG. 21.
THE BONES OF THE FOREARM (anterior surface): 1, shaft of
ulna;--2, great sigmoid cavity;--3, lesser sigmoid cavity
with head of radius;--4, olecranon;--5, coronoid process;--7,
interosseous space;--8, inferior extremity of ulna with
styloid process (9);--10, shaft of radius;--11, its head;--12,
neck;--13, tubercle for biceps;--14, impression for pronator
teres muscle;--15, inferior extremity of radius with styloid
process (16).]
The superior extremity of the ulna presents two processes and two
articular cavities. The cavities are the _greater_ and _lesser_ sigmoid
cavities; the processes are the coronoid and olecranon processes. The
bone articulates with the pulley or trochlea of the humerus by means
of the _great sigmoid cavity_, and with the margin of the head of the
radius by the _lesser sigmoid cavity_. The greater sigmoid cavity
(2, Fig. 21) presents in its centre a ridge prominent from before
backwards, which corresponds to the groove in the trochlea of the
humerus. Below and in front, the great sigmoid cavity is formed (5,
Fig. 21) by a bony prominence called the _coronoid process_ (compared
to the beak of a crow: κορώνη, a crow; εἶδος, form), which is lodged
in the coronoid fossa of the humerus in flexion of the forearm (17,
Fig. 18). Above and behind, the great sigmoid cavity is formed by
the _olecranon process_ (ὠλενη, the elbow; κάρηνον, the head), a
large square projection (4, Fig. 21), which constitutes the prominent
point of the elbow, and which accentuates in a high degree the form
of the forearm during flexion. During extension of the forearm the
olecranon process is lodged partly in the _olecranon fossa_ of the
humerus (4, Fig. 23). The lesser sigmoid cavity of the ulna is a small
concave surface placed on the outer side of the coronoid process for
articulation with the margin of the head of the radius.
[Illustration: FIG. 22.
FIGURE OF THE ELBOW JOINT, RIGHT SIDE (anterior view):--1,
inferior portion of shaft of humerus;--2, ulna;--3, radius;--4,
external condyle;--5, internal condyle;--6, capitellum;--7 and
8, trochlea;--9, coronoid fossa;--10, coronoid process;--11,
head of radius.]
The superior extremity of the radius forms a small discoidal head
(11, Fig. 21) attached by a narrow neck to the body of the bone; this
head is flattened at the top and hollowed out to articulate with the
_capitellum_ of the humerus (Fig. 22). The margin of the head of the
radius revolves in the lesser sigmoid cavity of the ulna.
We see, then, that the articular surfaces of the elbow are formed on
the humerus (6, 7, 8, Fig. 22) by a transverse series of projections
(the trochlea and capitellum), and on the bones of the forearm by a
series of depressions moulded on these projections, so that the whole
describes a movement similar to that between two cog-wheels (Fig.
22), a species of transverse hinge. Thus it is easy to understand,
_à priori_, how this disposition of the parts does not permit of
any lateral displacement of the bones, or transverse movement; the
movements forward and backward are, in fact, the only kind possible
in the elbow-joint. The forward movement--that is to say, that by
which the anterior surface of the forearm is brought near the anterior
surface of the arm--constitutes _flexion_ of the forearm. The movement
in the opposite direction constitutes _extension_.
The disposition of the ligaments--that is, of the fibrous bands or
articular capsule which fasten the bones together--modifies very
slightly the mechanism we have just deduced from the shape of the
articular surfaces; in fact, this capsule is formed on the inner and
outer surfaces by ligamentous fibres, very dense and short, called the
lateral ligaments, which prevent all lateral movement. On the other
hand, the anterior and posterior portions of the capsule are loose,
so as not to offer any opposition to the movements of flexion and
extension. The only limit to these movements is that resulting from
the bony projections of the ulna coming in contact with the humerus.
Thus, the movement of flexion can be prolonged until the coronoid
process arrives at the coronoid fossa and touches the bottom of that
cavity; then the fleshy masses of the forearm come into contact with
the anterior surface of the arm, especially if the model is muscular,
and flexion is no longer possible. The movement of extension, on the
contrary, has a limit which it is important to state precisely,
resulting from the beak of the olecranon touching the bottom of the
olecranon fossa (Fig. 23); this is produced when the forearm has
attained, in the movement of extension, that situation which brings its
own axis into direct line with that of the arm. The extension of the
elbow cannot therefore exceed the degree which brings the humerus and
forearm into the same plane; that is to say, the forearm can never make
with the arm an angle facing backwards.
[Illustration: FIG. 23.
ANTERO-POSTERIOR SECTION OF THE ELBOW (through the ulna):--1,
humerus;--2, ulna;--3, coronoid fossa;--4, olecranoid fossa;--5,
olecranon;--6, coronoid process;--7, section of trochlea.]
By comparing the particulars of the mechanism of the elbow with that
which we have previously seen of the mechanism of the scapulo-humeral
articulation, it will be easy to understand how we may, from the
study of the articular surfaces and ligaments, learn the laws of the
mechanism of joints. For example, the head of the humerus received into
a single concavity may allow to the arm every kind of movement; in the
same way the arrangement of a hinge-joint, with a series of projections
and depressions, fitted one to the other in a transverse line, renders
possible in the elbow-joint only the movements of flexion and extension.
In regard to external form, we learn the following facts from a
knowledge of the bones which form the elbow-joint.
1. With respect to the angle which the forearm makes with the arm, if
we examine it either upon the skeleton or upon the living subject, the
upper limb hanging beside the body, with the palm of the hand turned
forward, it is seen that the humerus (page 67) is slightly oblique
from above downwards and inwards, while the two bones of the forearm
are directed obliquely in the opposite direction--that is, from above
downwards and outwards. In other words, the bones of the arm and
forearm form at their point of junction--that is, at the level of the
elbow--an _angle_ the base of which looks outwards and the apex
inwards. This angle only appears in extension of the limb, and is due
to the twisted and tilted form of the trochlear surface of the humerus.
[Illustration: FIG. 24.
OUTLINE OF UPPER LIMB (front view)]
2. Concerning the bony prominences which are seen beneath the skin
at the elbow (Fig. 24), after studying these osseous structures, we
should be able to recognise upon the living model those details of
the figure which correspond to the four bony points--namely, the
coronoid process in front, the olecranon behind, the external condyle
on the outer side, and the internal condyle on the inner side. The
coronoid process, covered by muscles, is so buried in the surrounding
structure that it does not show externally. It is much the same also
with the external condyle, as this projection, not very prominent
in the skeleton, disappears completely in the living subject, since
this external condyle is situated at the bottom of the angle facing
outwards which the forearm makes with the arm, and the mouth of this
angle is filled up by the external muscles of the forearm (especially
the brachio-radialis, or supinator longus), which take their origin
from the external border of the humerus. The external condyle and the
external supra-condyloid ridge can be felt beneath the skin; and the
former occupies a depression at the back of the elbow on its outer side
in extension of the forearm.
On the other hand, the internal condyle, supra-condyloid ridge, and
the olecranon process always show clearly beneath the skin, and the
olecranon forms that projection, commonly called the _point of the
elbow_, which is so prominent behind during flexion of the forearm,
and which follows the movements of the forearm, seeming to rise
towards the arm during the extension of the forearm, and to descend
during flexion. The internal condyle projects as the apex of the angle
formed by the axis of the forearm with that of the arm (Fig. 26, page
81); this is a fixed point placed a little above the line of the
elbow-joint, which is useful in measurements.
There is one detail which we must not forget at this stage; in
comparing the length of the forearm with that of the arm the olecranon
must be excluded, as it projects above the lower end of the humerus; on
the other hand, the internal condyle should rather be chosen as a fixed
point from which measurements may be taken.