The =scapula= is a flat bone, situated on the anterior part of the
lateral wall of the thorax, and extending obliquely from the vertebral
end of the seventh or eighth rib to the sternal end of the first rib. It
is curved slightly and slopes outward in adaptation to the form of the
thoracic wall. It is triangular in outline, and has two surfaces, three
borders, and three angles.
The =external surface= or =dorsum= (Facies lateralis s. dorsalis) is
divided into two fossæ by the =spine= (Spina scapulæ), which extends
from the vertebral border to the neck of the bone, where it gradually
subsides. The free edge of the spine is thick, rough, and in great part
subcutaneous. A little above its middle is a variable prominence, the
=tubercle= of the spine (Tuber spinæ), to which the trapezius muscle is
attached. The =supraspinous fossa= (Fossa supraspinata) is situated in
front of the spine, and the =infraspinous fossa= (Fossa infraspinata)
behind it. The former is much the smaller of the two; it is smooth and
is occupied by the supraspinatus muscle. The infraspinous fossa lodges
the infraspinatus muscle; it is wide and smooth in its upper part,
narrower below, where it is marked by several rough lines for the
attachment of the teres minor muscle; near the neck is the nutrient
foramen, and a little lower is a vascular groove.
[Illustration:
FIG. 41.—LEFT SCAPULA OF HORSE, EXTERNAL VIEW. (After Schmaltz, Atlas
d. Anat. d. Pferdes.)
]
The =costal surface= or =venter= (Facies costalis) is hollowed in its
length by the =subscapular fossa= (Fossa subscapularis); this occupies
nearly the whole of the lower part of the surface, but is pointed above
and separates two rough triangular areas (Facies serrata), to which the
serratus magnus is attached. In the lower third there is a vascular
furrow with several branches.
The =anterior= or =cervical border= (Margo cranialis) is convex and
rough above, concave and smooth below.
The =posterior= or =dorsal border= (Margo caudalis) is slightly concave.
It is thick and rough in its upper third, thin in its middle, and
thickens again below.
The =superior= or =vertebral border= (Margo dorsalis s. basis) carries
the =scapular cartilage= (Cartilago scapulæ). In the young subject the
edge of the bone is thick, and is pitted by impressions into which the
cartilage fits. The cartilage is the unossified part of the fœtal
scapula. Its lower edge fits the depressions and elevations of the bone.
It thins out toward the free edge, which is convex and lies alongside of
the upper parts of the vertebral spines. In front it continues the line
of the scapula, but behind it forms a rounded projection. The lower part
of the cartilage undergoes more or less ossification, so that the
vertebral border of the bone in old subjects is thin, irregular, and
porous.
[Illustration:
FIG. 42.—LEFT SCAPULA OF HORSE, COSTAL SURFACE.—(After Schmaltz, Atlas
d. Anat. d. Pferdes.)
]
The =anterior= or =cervical angle= (Angulus cranialis) is at the
junction of the anterior and vertebral borders and lies opposite to the
second thoracic spine. It is relatively thin and is about a right angle.
The =posterior= or =dorsal angle= (Angulus caudalis) is thick and rough;
its position can be determined readily in the living animal.
The =inferior= or =articular angle= (Angulus glenoidalis) is joined to
the body of the bone by the =neck= of the scapula (Collum scapulæ). It
is enlarged, especially in the sagittal direction. It bears the =glenoid
cavity= (Cavitas glenoidalis) for articulation with the head of the
humerus. The cavity is oval in outline, and its margin is cut into in
front by the =glenoid notch= (Incisura glenoidalis), and is rounded off
externally; just above its postero-external part is a tubercle to which
a tendon of the teres minor is attached. The =bicipital tuberosity= or
=tuber scapulæ= is the large rough prominence in front, to which the
tendon of origin of the biceps brachii is attached; projecting from its
inner side is the small =coracoid process= (Processus coracoideus), from
which the coraco-brachialis muscle arises.
=Development.=—The scapula has four centers of ossification, viz., one
each for the body of the bone, the bicipital and coracoid processes, the
anterior part of the glenoid cavity, and the tuber spinæ. The last
ossifies after birth and fuses with the spine about the third year. The
bicipital tuberosity and coracoid fuse with the body of the bone about
the end of the first year.
In old subjects the spongy substance disappears at the middle part of
the fossæ, so that the bone consists here of a thin layer of compact
substance. Considerable ossification of the cartilage is usual, the
borders become much rougher, the muscular lines are more pronounced,
and a medullary cavity may appear in the neck. Much variation occurs
in dimensions and slope. The average ratio between the length and
breadth (scapular index) is about 1 ∶ 0.5, but in many cases the base
is relatively wider. The inclination on a horizontal plane varies from
50 to 65 degrees. Exceptionally the coracoid process reaches a length
of an inch or more (2½ to 3 cm.), and the chief nutrient foramen may
be on the posterior border or in the subscapular fossa.
[Illustration:
FIG. 43.—DISTAL END OF LEFT SCAPULA OF HORSE, END VIEW. (After
Schmaltz, Atlas d. Anat. d. Pferdes.)
]