The =humerus= is a long bone which extends from the shoulder above,
where it articulates with the scapula, to the elbow below and behind,
where it articulates with the radius and ulna. It is directed obliquely
downward and backward, forming an angle of about 55 degrees with a
horizontal plane. It may be divided into a shaft and two extremities.
[Illustration:
FIG. 44.—PROXIMAL END OF LEFT HUMERUS OF HORSE, END VIEW. (After
Schmaltz, Atlas d. Anat. d. Pferdes.)
]
The =shaft= or =body= (Corpus humeri) is irregularly cylindrical and has
a twisted appearance. It may be regarded as having four surfaces. The
=external surface= is smooth and is spirally curved, forming the
=musculo-spiral groove= (Sulcus musculi brachialis), which contains the
brachialis muscle; the groove is continuous with the posterior surface
above and winds around toward the front below. The =internal surface= is
nearly straight in its length, rounded from side to side, and blends
with the anterior and posterior surfaces. Just above its middle is the
=internal= or =teres tubercle= (Tuberositas teres), to which the tendon
of the latissimus dorsi and teres major muscles is attached. The
=nutrient foramen= is in the lower third of this surface. The =anterior
surface= is triangular, wide and smooth above, narrow and roughened
below. It is separated from the external surface by a distinct border,
which bears on its upper part the =deltoid tuberosity= (Tuberositas
deltoidea). From the latter a rough line curves upward and backward to
the outer surface of the neck, and gives origin to the external head of
the triceps muscle. Below the tuberosity the border inclines forward,
becomes less salient, and ends at the coronoid fossa. The =posterior
surface= is rounded from side to side and smooth.
[Illustration:
FIG. 45.—LEFT HUMERUS OF HORSE, INTERNAL SURFACE. (After Schmaltz,
Atlas d. Anat. d. Pferdes.)
]
[Illustration:
FIG. 46.—LEFT HUMERUS OF HORSE, EXTERNAL SURFACE. (After Schmaltz,
Atlas d. Anat. d. Pferdes.)
]
The =proximal extremity= consists of the head, neck, two tuberosities,
and the bicipital groove. The =head= (Caput humeri) presents an almost
circular convex articular surface, which is about twice as extensive as
the glenoid cavity of the scapula, with which it articulates. In front
of the head is a fossa, in which are several foramina. The neck (Collum
humeri) is well defined behind, but is practically absent elsewhere. The
=external tuberosity= (Tuberculum majus) is placed antero-externally,
and consists of two parts; the anterior part forms the outer boundary of
the bicipital groove and gives attachment to the external branch of the
supraspinatus muscle; the posterior part gives attachment to the short
insertion of the infraspinatus, while its outer surface is coated with
cartilage, over which the chief tendon of the same muscle passes to be
inserted into a triangular facet on the outer aspect of the anterior
part. The =internal tuberosity= (Tuberculum minus) is less salient, and
consists of anterior and posterior parts; the anterior part forms the
inner boundary of the bicipital groove, and furnishes insertion to the
inner branch of the supraspinatus above, and the posterior deep pectoral
muscle below; the posterior part gives attachment to the subscapularis
muscle. The =bicipital= or =intertubercular groove= (Sulcus
intertubercularis) is situated in front; it is bounded by the anterior
parts of the tuberosities, and is subdivided by an intermediate ridge.
The groove is covered in the fresh state by cartilage, and lodges the
tendon of origin of the biceps brachii muscle. Just below the
intermediate ridge is a small fossa in which several foramina open.
[Illustration:
FIG. 47.—LEFT HUMERUS OF HORSE, ANTERIOR VIEW. (After Schmaltz, Atlas
d. Anat. d. Pferdes.)
]
The =distal extremity= has an oblique surface for articulation with the
radius and ulna, which consists of two condyles of very unequal size,
separated by a ridge. The =internal condyle= (Condylus medialis) is much
the larger, and is crossed by a sagittal groove, on the anterior part of
which there is usually a synovial fossa. Posteriorly the groove extends
upward considerably above the rest of the articular surface and reaches
the olecranon fossa, and this part articulates with the semilunar notch
of the ulna. The =external condyle= (Condylus lateralis) is much smaller
and is placed somewhat lower and further back, giving the extremity an
oblique appearance; it is marked by a wide shallow groove. The =coronoid
fossa= (Fossa coronoidea) is situated in front above the groove on the
internal condyle; it furnishes origin to part of the extensor carpi, and
external to it is a rough depression from which the anterior or common
extensor of the digit arises. Behind and above the condyles are two
thick ridges, the epicondyles. The =internal= or =flexor epicondyle=
(Epicondylus medialis s. flexorius) is the more salient; it furnishes
origin to flexor muscles of the carpus and digit, and presents
internally a tubercle for the attachment of the internal lateral
ligament of the elbow joint. The =external= or =extensor epicondyle=
(Epicondylus lateralis s. extensorius) bears externally the =external
supracondyloid crest= (Crista condyloidea lateralis), which forms here
the outer boundary of the musculo-spiral groove, and gives origin to the
extensor carpi. Below this is a rough excavation in which the external
lateral ligament is attached. The lower border of the epicondyle gives
attachment to the flexor carpi externus. Between the epicondyles is the
deep =olecranon fossa= (Fossa olecrani).
=Development.=—The humerus ossifies from six centers, viz., three
primary centers for the shaft and epiphyses, and three secondary centers
for the external tuberosity, the deltoid tuberosity, and the internal
condyle. The proximal end fuses with the shaft at about three and
one-half years, the distal at about one and a half years of age.