EXTREMITY.
159
the humerus to the scapula. The rule seems
to be that where the pectoral extremity is chiefly
a pillar of support, the aspect of the glenoid
cavity is nearly vertically downwards. If free¬
dom and rapidity of motion be required in
addition to strength as a member of support,
the trunk being lighter, the scapula is oblique,
and consequently the glenoid cavity looks
downwards and forwards; or if the limb be
not used to support the trunk, then the aspect
of the glenoid cavity is no longer downwards
but outwards, as in man.
Structure.—The greatest part of the scapula
is composed of very thin almost papyraceous
compact substance ; but its processes, and the
enlargements at its edges and angles, contain
reticular tissue.
Developement.—This bone is developed by
six points of ossification; one for the body,
and five supplementary ones, viz. one for the
coracoid process, two for the acromion, one
for the posterior border of the bone, and one
for its inferior angle. The ossification of the
body commences about the second month, and
the spine appears in the third month as a
growth from the posterior surface of the scapula.
The union of the several epiphyses is not
completed till late, and it is not until after the
fifteenth year that the ossification is finished.
The bones of the upper extremity, properly
so called, are the humerus, radius, ulna, and
bones of the hand.
Humerus, (os brachii; Fr. l'os du bras;
Germ, das Oberarmbein). This is the longest
bone of the upper extremity; it is situated
between the scapula and forearm, being, as it
were, suspended by muscle and ligament from
the former.
Like all long bones, the humerus consists
of a shaft and two extremities. The superior
extremity is formed by a smooth and rounded
convexity, rather less than half a sphere ; a
slight depression in, or constriction of, the
bone, most manifest above, marks the limit of
this articular eminence. The eminence is
called the head of the humerus ; the constric¬
tion indicates what is denominated the anato¬
mical neck of the bone, being that portion
which connects the head to the shaft, and
analogous to the more developed neck of the
thigh-bone. The axis of the neck is but a
continuation of that of the head, and passes in
a direction from within outwards and down¬
wards, forming an obtuse angle with the axis
of the shaft. The head of the humerus is
entirely covered by articular cartilage, and arti¬
culates with the glenoid cavity of the scapula,
to which, however, it obviously does not at all
correspond in dimensions.
The inferior part of the anatomical neck of
the humerus is very slightly marked, and is
continued in a smooth declivity slightly con¬
cave from above downwards, into the shaft of
the bone. Its superior part is more distinct,
and the depth of the groove here seems in a
great degree owing to the prominence of two
bony protuberances, one situated anteriorly,
called the lesser tuberosity, and the other pos¬
teriorly, denominated the greater tuberosity.
The lesser tuberosity of the humerus (tuber-
culum minus) is somewhat conical in shape,
and inferiorly it ends in a smooth, rounded
bony ridge (spina tuberculi minoris), which
extends downwards and inwards, gradually
diminishing in prominence till it is lost in the
shaft of the bone at the inner part of its ante¬
rior surface. The lesser tuberosity gives in¬
sertion to the tendon of the subscapularis
muscle, and the ridge or spine last described
forms the anterior and internal boundary of the
bicipital groove.
The greater tuberosity (tuberculum majus,
externum s. posterius) forms a considerable
prominence on the upper and outer part of
the humerus, being the most external part in
that situation and easily to be felt under the
integuments. Superiorly the constriction cor¬
responding to the anatomical neck separates it
from the head of the humerus ; inferiorly it is
continued into and gradually lost in the shaft
of the bone at its outer part. A very distinct
and prominent ridge (spina tuberculi majoris)
is continued from its anterior extremity down¬
wards and inclining very slightly inwards,
which terminates about the middle of the an¬
terior surface of the bone, just internal to the
deltoid ridge. This ridge is most prominent
but smooth in its upper third, in its inferior
two-thirds it is less prominent but rough; it
forms the posterior boundary of the bicipital
groove. On the greater tuberosity three dis¬
tinct surfaces are marked, to the anterior of
which the supra-spinatus muscle is attached,
to the middle the infra-spinatus, and to the
posterior the teres minor.
The bicipital groove commences above be¬
tween the two tuberosities, and passes down¬
wards and slightly inwards, bounded before
and behind by the spines which proceed from
those tubercles. This groove, very distinct at
its commencement, ceases to be so a little
above the termination of the superior third ;
in the recent state it is lined by the tendinous
expansion of the latissimus dorsi and teres
major muscles, and lodges the tendon of the
biceps muscle, whence its name.
From the anatomical neck the bone grar
dually tapers down and becomes more cylin¬
drical in its form ; this upper portion is, for the
convenience of description, distinguished by
the name of surgical neck of the humerus.
The middle third of the shaft of the bone
is prismatic in form ; the external spine
which commences at the greater tuberosity
is continued down, forming a prominent
ridge all down the front of the bone to the
termination of its flattened inferior third. The
outer part of the middle third of the humerus
is remarkable for the rough surface into which
the deltoid muscle is inserted, the deltoid ridge,
situated nearer the upper than the lower part
of this portion, and directed downwards and
very slightly forwards. The inner part of the
middle third presents a smooth, flattened, and
inclined surface, which is continued down in
this form to within a very short distance of the
inferior extremity of the bone. The posterior
surface is rounded and very smooth.
