THE SHOULDER.
The bones of the shoulder.--The clavicle: its shaft and
extremities, sternal and acromial: its proportions.--The
shoulder-blade: its situation and relations.--The spine
of the scapula: acromion: coracoid process: glenoid
cavity.--Proportions of the scapula: the distance which
separates the right from the left scapula.--The upper portion
of the humerus: the anatomical neck, the surgical neck:
the articular head: the tuberosities.--The shoulder-joint:
its movements: the increase of mobility assured by the
acromio-clavicular and sterno-clavicular articulations the
to-and-fro motion of the scapula: the importance of this
mechanism with regard to external form.
The shoulder is formed of three bones, of which one is situated in
the front, the _collar-bone_, or _clavicle_; a second is behind, the
_shoulder-blade_, or _scapula_; and a third is placed externally, the
upper part of the _arm-bone_, or _humerus_.
The _clavicle_ (_clavicula_, diminutive of _clavis_, a key) is a
long bone, placed horizontally at the junction of neck and chest. It
connects the breast-bone with the shoulder-blade; and the two clavicles
are separated in the middle line by the upper end of the manubrium, or
pre-sternum (episternal or suprasternal notch). Its form is that of
an italic ∫--that is to say, it describes in a horizontal plane two
curves, the internal portion being convex in front, and the outer part
convex behind (Fig. 16). It consists of a shaft and two extremities:
the _shaft_, flattened from above down, presents a smooth _superior
surface_, which in the model shows very clearly beneath the skin, and a
rough _inferior surface_ for the attachment of a muscle (subclavius),
and for the attachment of ligaments connecting it with the first rib
internally, and with the coracoid process of the scapula externally
(see below); it possesses a _posterior_ and an _anterior border_, thick
and curved: an _internal, sternal extremity_, thick and triangular,
which articulates with the corresponding lateral facet on the manubrium
of the sternum: and an _outer, acromial extremity_, flattened and
presenting an oval articular surface for articulation with the acromion
process of the scapula. The clavicle thus serves to connect the scapula
to the trunk, and by the articulations of its two extremities it
increases, as we shall see, the mobility of the shoulder-joint. We find
that the clavicle, which is almost horizontal when the arms are hanging
in an attitude of repose beside the trunk, becomes oblique in direction
when the arms are moved upward and forward, or upward and backward.
The length of the clavicle, as we have already said (p. 46), should be
equal to that of the sternum without the xiphoid appendix.
[Illustration: FIG. 16.
RIGHT CLAVICLE: UPPER SURFACE.--1, body of clavicle;--2, 3,
inner or sternal extremity;--4, 5, outer or acromial extremity.]
The _shoulder-blade_, or _scapula_, is a flat, triangular bone, and
consists of a bony plate very thin at certain points, and thickened
only along its borders. It is placed on the lateral and posterior
aspect of the thorax, and corresponds at its upper border to the second
rib: its lower end reaches to the seventh or eighth rib. It is attached
by a joint called the _acromio-clavicular articulation_, which can be
felt beneath the skin, to the external extremity of the clavicle. As
this joint is the only ligamentous connection of the scapula with the
trunk, the bone is capable of great freedom of movement.
[Illustration: FIG. 17.
SHOULDER-BLADE: POSTERIOR SURFACE.--1, supra-spinous fossa;--2,
infra-spinous fossa;--3, superior border with supra-scapular
notch (4);--5, external or axillary border with impression
for attachment (8) of the long head of the triceps;--6,
glenoid cavity;--7, inferior angle;--9, internal or vertebral
border;--10 and 11, spine of shoulder-blade extending into
acromion process (12);--14, coracoid process.]
The shoulder-blade is described as possessing two surfaces, three
borders, three processes, and three angles. The _posterior surface_,
free and visible throughout its entire extent upon the mounted
skeleton, is divided into two unequal portions--the upper one,
the smaller (_supra-spinous fossa_) and the lower one, the larger
(_infra-spinous fossa_)--by a bony crest, called the _spine of the
scapula_ (10, 11, Fig. 17), which traverses the bone obliquely from
within upwards and outwards. It becomes more and more prominent
externally, and is afterwards prolonged into a free process, broad
and flat, which forms the most elevated and external part of the
skeleton of the shoulder (12, Fig. 17), and is known by the name of the
_acromion process_ (ἃχρος, summit; ὦμος, the shoulder). It is on the
internal or anterior border of this acromion process that the small
oval facet is found, by which the scapula articulates with the outer
extremity of the clavicle by the _acromio-clavicular_ articulation.
The fossa above the spine of the scapula is the _supra-spinous
fossa_ (1, Fig. 17), and the larger space below the spine is called
the _infra-spinous fossa_ (2, Fig. 17). The _anterior surface_ of
the scapula has received the name of the _subscapular fossa_. It is
directed forwards and inwards, and is separated from the ribs and
armpit by large muscles. It is but little visible in the articulated
skeleton.
Of the three borders of the scapula, one is _superior_ and horizontal;
it is the shortest of the three. The second is internal and vertical,
parallel to the series of spinous processes of the vertebræ; to this
is given the name of the _vertebral border_. The third, or outer
border, is oblique, and is directed downwards and backwards; it
corresponds to the region of the armpit, and has received the name
of the _axillary border_. It is particularly to be noticed that the
vertebral border is thin; the axillary border, on the contrary, is
thick, and presents towards its superior limit a rough space, just
below the glenoid fossa, for the attachment of the long head of the
triceps muscle (8, Fig. 17). The superior border is remarkable for the
presence at its outer extremity of a stout process which projects at
first upwards, and is afterwards bent outwards upon itself like a bent
finger. It has been likened to the beak of a crow, and has received
the name of the _coracoid process_ (κὸραξ, a crow; εἷδος, form). The
coracoid process (14, Fig. 17) is placed in front and on the inner
side of the acromion, and the two together form an arch over the
shoulder-joint, called the _coraco-acromial arch_, which is completed
by a fibrous band proceeding from one process to the other, and called
the _coraco-acromial ligament_. The position of the coracoid process
should be particularly noted in relation to the clavicle. It underlies
the outer part of the clavicle, from which two important ligaments
(_conoid and trapezoid ligaments_) descend to be attached to its upper
surface. These ligaments serve the purpose of slinging the scapula
up to the under surface of the clavicle. Of the three angles of the
scapula, one only--the superior external angle, which is situated
beneath the coraco-acromial arch--deserves a particular description; it
is very thick, and becomes enlarged to form an articular surface (6,
Fig. 17), which is directed outwards and forwards, and articulates with
the head of the humerus. This surface is pear-shaped and is slightly
hollowed out, and bears the name of the _glenoid cavity_. It is much
less extensive than the articular head of the humerus, along with which
it forms the shoulder-joint.
In regard to the relative proportions of the scapula, it may be noted
that the length of the vertebral border is equal to the length of the
clavicle, and equal also to the distance which separates the scapulæ
behind, when the vertebral borders are vertical, which is the case when
the arms are hanging beside the body in a position of relaxation.
The movements of the shoulder-blade, and their effects on external
form, will be studied in relation to the articulation of the
scapula with the humerus, the _scapulo-humeral_ articulation, or
shoulder-joint. We must now pass on to describe the upper portion of
the bone of the arm.
[Illustration: FIG. 18.
LEFT HUMERUS (anterior surface): 1, body of the bone;--2,
articular head;--3, anatomical neck;--4, great tuberosity;--5,
lesser tuberosity;--6, bicipital groove;--7, deltoid
impression;--11, capitellum;--12, trochlea;--13, external
supra-condyloid ridge;--14, condyles;--16, internal
supra-condyloid ridge;--17, coronoid fossa.]
The arm bone, or _humerus_, is one of the long bones, and is
composed of a shaft, prismatic or nearly cylindrical in form, and
of two enlarged extremities, one inferior, which takes part in the
articulation of the elbow; the other superior, which takes part in the
articulation of the shoulder. We will only concern ourselves for the
present with the superior extremity.
_The superior extremity_ of the humerus is large and spherical. It is
continuous with the body of the bone by a cylindrical neck, called
the _surgical neck_ of the humerus. This extremity is traversed by
a circular groove, oblique from above downwards, and from without
inwards, called the _anatomical neck_ (3, Fig. 18). This is well
marked, and divides the sphere into two parts; the one, situated above,
and internal to the anatomical neck, is very regularly rounded, smooth,
covered over by a layer of cartilage, and is called the _head_ of the
humerus (2, Fig. 18); it is normally in contact with the glenoid
cavity of the shoulder-blade, in which it glides in the movements of
the arm; the other part of the head, situated below, and external to
the anatomical neck, is rough, and divided into two tuberosities by
a vertical groove, which is prolonged as far as the upper part of
the body of the bone, and which, from its serving to accommodate the
long tendon of the biceps, has received the name of the _bicipital
groove_ (6, Fig. 18). The tuberosity situated on the outer side of the
bicipital groove is large, and is called the _great tuberosity_ (4,
Fig. 18); it presents three small surfaces which receive the insertions
of the deep muscles of the shoulder--supraspinatus, infraspinatus, and
teres minor muscles. The tuberosity situated on the front of the bone,
and internal to the bicipital groove, is smaller. It is called the
_small tuberosity_ of the humerus (5, Fig. 18), and gives attachment to
one muscle only, the subscapularis.
THE SHOULDER-JOINT.--The articulation of the shoulder, or
_scapulo-humeral_ articulation, may serve as a type of articulations
in general. It is necessary in each articulation to consider how the
shapes of the bony surfaces in contact correspond to each other. From
this we should be able to deduce the nature of the movements permitted
by the articulation. It is also necessary to consider the disposition
of the ligaments; that is to say, of the fibrous bands which proceed
from one bone to the other, and from these we can deduce the limits of
the movements of which the joint may be capable.
The scapulo-humeral articulation is a ball-and-socket joint, formed by
the glenoid cavity of the shoulder-blade and the head of the humerus.
[Illustration: FIG. 19.
LIGAMENTS OF SHOULDER, WITH SCAPULA, CLAVICLE, AND HUMERUS.--1,
Acromio-clavicular ligament;--2, coraco-clavicular ligament;--3,
coraco-acromial ligament;--4, supra-scapular ligament;--5,
capsule of shoulder-joint;--6, coraco-humeral ligament;--7,
tendon of biceps (long head).]
Such an arrangement of surfaces in contact with one another permits the
head to glide in all directions within the cavity, and consequently the
upper limb can be moved in all directions: forwards; backwards; inwards
(adduction); outwards (abduction); upwards and downwards. The head of
the humerus can also be rotated on the glenoid cavity, either inwards
or outwards.
The ligamentous covering of the joint is formed by a fibrous capsule,
which is attached on one side to the margin of the glenoid cavity, and
on the other side to the anatomical neck of the humerus (5, Fig. 19).
This ligamentous capsule is sufficiently loose to allow the head of the
humerus great freedom of movement within the glenoid cavity, without
any part of the capsule being put on the stretch, and thus stopping its
movement. Thus the movement of the arm forwards is very extensive, as
are those backwards and directly inwards, this last being checked only
by the meeting of the arm with the lateral surface of the trunk. But
the movement of abduction, or of elevation outwards, becomes difficult
when the arm approaches the horizontal; here an arrangement comes into
play which is of great importance, the study of which shows us that the
upper limb, besides the mobility which belongs to the shoulder-joint
proper (scapulo-humeral articulation), derives an increase of mobility
from the articulations of the shoulder-blade with the clavicle, and of
the clavicle with the sternum.
When we apply a humerus to the glenoid fossa of the scapula in such
a manner as to draw the arm outwards, we observe that at the moment
when the humerus attains the horizontal position, the tuberosities
touch the coraco-acromial arch which surmounts the glenoid cavity, and
which is completed by the coraco-acromial ligament (3, Fig. 19); in
the complete subject, in the living man, when the arm is elevated in
carrying it outwards, the same effect of meeting and, so to speak, of
contact between the tuberosities of the humerus and the acromial arch
is produced, with this result--that the arm, once horizontal, is with
difficulty raised higher by the simple play of the scapulo-humeral
articulation--that is to say, by the gliding of the head of the humerus
in the glenoid cavity. But now a new source of mobility is employed, a
new articulation is brought into play--that of the acromio-clavicular
joint. The entire scapula moves round the extremity of the clavicle;
its inferior angle is carried forward; its external angle--that is
to say, the glenoid cavity with the coraco-acromial arch--is carried
upwards, and the movement of elevation of the arm is then continued by
the play of the shoulder-blade.
[Illustration: FIG. 20.
OUTLINE OF SHOULDER WITH THE ARM RAISED.]
As the result is an important change of shape in the shoulder, we ought
to particularly examine the shoulder in the region of the back, upon
a model in which the arm is raised above the horizontal position. The
point of the shoulder is raised, and as this elevation is accompanied
by a movement of the shoulder-blade, the vertebral border of this bone
does not remain parallel to the spinal column, but while it approaches
closer to it at its upper end, it is drawn further away at its lower
end, and is obliquely directed from above downwards and outwards (Fig.
20). The inferior angle becomes prominent in the lower part of the
armpit, as we may observe in a subject with the arms folded, and shows
in a striking manner on a dead crucified body. If the elevation of the
arm approaches the vertical, the vertebral border of the shoulder-blade
has a tendency to become horizontal, and we observe this change of
movement in the model when the bone is seen along the posterior surface
of the shoulder and the back, and we should scarcely recognise at first
the appearances presented as those we are accustomed to study when it
occupies its ordinary situation in the skeleton.
But when the arm is elevated vertically, so that it comes in contact
with the side of the head, movement takes place not only at the
scapulo-humeral and at the acromio-clavicular joints, but also at
the articulation of the internal extremity of the clavicle with the
sternum. In that case, in fact, the whole of the shoulder is raised
upwards (by the superior fibres of the trapezius muscle), and the
clavicle represents the arm of the lever by which this movement
is accomplished, while the sterno-clavicular articulation is the
hinge. Thus we see the clavicle is changed in its direction, from
the horizontal, to become oblique upwards and outwards--that is, its
external extremity is raised and carried a little backwards.
The clavicle plays a very important part in the movements of the
upper limb. This explains why it is found specially developed
in those animals whose fore-limbs possess the most specialised
movements--flight, digging, burrowing, striking, or hugging--such as
man, the monkey, the bat, the tiger, the mole, etc.; while in those
quadrupedal animals which, like the horse, use their fore-limbs for
progression only, and in which movements occur only in a plane parallel
to that of the body, there is not a trace of clavicle. The part which
the clavicle takes in the movements of the arm explains also why the
bone presents a variable size in different individuals. It is stronger
in the male than in the female; stronger in the working man than in
the student; stronger, finally, on the right side than the left, from
the habit of using the right arm for the most part in those actions
which demand strength and skill. In the left-handed, it is the left
clavicle that is stronger than the right. In a word, this bone is,
like all the other parts of the skeleton, stronger in proportion as
it takes part in active and frequently repeated movements. Thus the
breadth of the shoulders is one of the characteristics of athletes; and
it is to the strength of the bony structure formed by the clavicle and
shoulder-blade, sustained by the first ribs, that the superior portion
of the thorax owes its characteristic aspect.
Owing to the presence of the shoulder girdle (scapula and clavicle)
the thoracic cage does not present much of the form of a cone at its
upper end, or summit; indeed, this region becomes broader in a lateral
direction as the clavicle is more developed. It will be enough to
contrast the region of the thorax in man with that of such animals as
the dog or the horse, which, owing to the absence of a clavicle, have
a thorax transversely flattened in the region of the shoulders, and
consequently the shoulder-blades more closely applied upon the sides of
the thoracic cage.
This is the proper place to discuss the dimensions and proportions of
the shoulder; but, as this study consists entirely of a comparison of
the transverse diameters of the shoulders with the transverse diameters
of the hips, it is better to defer it till after the description of the
pelvis.