STRUCTURE OF THE LIMBS OF THE HORSE
GENERAL DISCUSSION.--Each limb is formed by a column of bones that rest
upon one another, forming more or less open angles. The bones of the column
meet and form articulations that are held together by ligaments, and
attached to their faces, borders and extremities are muscles and tendons.
In the superior portion of the limb the muscles are heavy, tapering
inferiorly, and terminating in the region of the foot in long tendons. Each
limb is divided into four regions. The regions of the _fore-limb_ are the
shoulder, arm, forearm and forefoot. In the _hind limb_ are the regions of
the pelvis, haunch, thigh, leg and hind-foot. The feet in turn are divided
into three sub-regions each. The _forefoot_ is formed by the knee, cannon
and toe, and the _hindfoot_ by the hock, cannon and toe.
THE SHOULDER BONE OR SCAPULA is flat and triangular in shape. It is
attached to the trunk by heavy muscles, one of which, together with its
fellow on the opposite side, may be compared to a great, muscular sling
that supports about two-thirds of the body weight. Attached to the internal
and external faces of the scapula are heavy muscles that pass over the
shoulder-joint, and become attached to the arm bone through the insertion
of their muscular fibres or by a short tendon.
THE ARMBONE OR HUMERUS belongs to the class of long bones. Its superior
extremity forms a flattened head that fits rather imperfectly into a
shallow cavity in the humeral angle of the scapula. The inferior extremity
resembles a portion of a cylinder in shape, and fits into shallow
depressions in the superior extremity of the principal bone of the forearm.
The muscles here are divided into two regions, anterior and posterior
brachial. The most of these muscles originate on the posterior border and
inferior extremity of the shoulder bone, and terminate inferiorly on the
superior extremities of the principal and second or rudimentary bone of the
forearm. The posterior brachial muscles are heavy and powerful. They are
sometimes termed elbow muscles, because they are attached to the point of
the elbow.
THE REGION OF THE FOREARM is formed by two bones, the _radius_ and _ulna_.
The radius is the principal bone and is classed among the long bones. The
ulna is an elongated flat bone. It is attached to the external portion of
the posterior face of the radius and extends above the superior extremity
of this bone to form the point of the elbow. The radius articulates with
the upper row of knee bones. The muscles of this region, the antibrachial,
are divided into two sub-regions, anterior and posterior. They originate
superiorly from the lower extremity of the arm bone and the superior
extremities of the bones of the forearm, and terminate toward the lower
extremity of the region in tendons that become attached to the bones of the
knee, cannon and digit.
THE KNEE OR CARPAL region is formed by seven short bones that are arranged
in two rows. They form a series of articulations. These are the
articulations between the two rows, between the bones of each row, and
between the upper and lower rows and the neighboring regions. Nearly all
the motion takes place in the articulation between the upper row and the
principal bone of the forearm.
THE CANNON OR METACARPAL region is formed by three bones. These are the
principal metacarpal or cannon bone, and the rudimentary metacarpal or
splint bones. The latter are attached to the margins of the posterior face
of the cannon bone. The superior extremities of these bones articulate with
the lower row of carpal bones. The convex extremity of the cannon bone
meets shallow depressions in the superior extremity of the first digital
bone. This is termed the fetlock joint. The anterior and posterior faces of
this region are travelled by the long tendons belonging to the extensor and
flexor muscles of the digit.
THE DIGIT OR TOE is formed by six bones, three of which are termed
accessory or sesamoids. The digital bones may be given numerical names.
THE APPROXIMAL OR THIRD DIGITAL BONE is the shortest long bone in the body.
The two shallow articular cavities belonging to the superior extremity are
completed posteriorly by the two sesamoid bones. The inferior extremity is
smaller than the superior and resembles the inferior extremity of the
cannon bone in shape, excepting that it shows a middle groove. The anterior
and posterior faces are travelled by the tendons of the digital muscles.
THE MIDDLE OR SECOND DIGITAL BONE is quite short. It articulates superiorly
with the first, and inferiorly with the third bone of the digit. The
superior face shows two shallow cavities, and the inferior two convex
surfaces separated by a median groove. The latter face articulates with the
third and navicular bones. The popular name for this articulation is the
coffin joint.
THE THIRD OR DISTAL DIGITAL BONE may be compared to a cone that has been
cut away posteriorly, obliquely downwards and backwards. The superior face
shows two shallow cavities that are completed posteriorly by the superior
face of the coffin or navicular bone. The anterior face is convex and
cribbled by openings, and the inferior face is concave, forming the sole.
Tendons belonging to the digital muscles terminate on the summit and
inferior face of this bone.
THE PELVIS OR HAUNCH is formed by a single bone, the _coxa_ that in the
foetus may be divided into three bones. These are the _ilium, pubis_ and
_ischium_. It belongs to the class of flat bones. Anteriorly it is
flattened from before to behind and directed inward and upward. The
external angle is rugged and is generally termed the angle of the haunch.
The internal face of the opposite angle articulates with the sacrum, to
which it is firmly attached by ligaments. The middle portion is constricted
and forms a neck. The inferior or posterior portion is flattened from above
to below, and directed inward to meet the border of the opposite bone. Just
below the neck and externally, there is a cup-shaped cavity into which the
head of the thigh bone fits. The two coxa, together with the sacral
ligaments (sacrum) and the muscles of the quarter, enclose the pelvic
cavity.
THE REGION OF THE THIGH is formed by the _femur_, the largest long bone in
the body. The superior extremity is formed by a rugged eminence, to which
the heavy muscles of the quarter are attached, and by an articular head.
The inferior extremity is formed by two convex articular surfaces that are
separated by a deep notch, and a third pulley-like articular surface, with
which the patella or knee-cap articulates. The pair of condyles articulates
with the superior extremity of the leg bone. The thigh or femoral region is
heavily muscled.
THE LEG is formed by three bones. The patella, a short bone, has already
been mentioned as articulating with the thigh bone. The tibia and fibula
are the other two bones in the region.
THE TIBIA belongs to the class of long bones and the fibula is quite
rudimentary, being represented by a stylet-shaped bone that lies posterior
to, and along the outer border of the tibia. The superior extremity of the
tibia shows a central spine margined laterally by rather plain articular
faces. It articulates with the thigh bone. The muscles of this region are
divided into two sub-regions, _anterior_ and _posterior_ tibial. The
muscles originate from the lower extremity of the femur and the two bones
in this region, and terminate inferiorly in tendons that are attached to
the bones of the hock, cannon and digit.
THE HOCK OR TARSAL region is formed by six bones. They are described as
forming two rows. In the upper row there are two bones and in the lower
four. They form a series of articulations, the same as the bones of the
knee. Practically all of the movement occurs in the articulation between
one of the large bones in the upper row and the lower extremity of the
tibia. It may be mentioned here that this is the most perfect hinge-joint
in the body. A very large tendon is attached to the summit of the hock.
Other tendons cross and become attached to the hock bones.
The regions of the _hind cannon_ and digit are practically the same as the
corresponding regions of the forefoot.
QUESTIONS
1. Name the different bones of the fore-limb; hind limb.
2. Describe the regions of the shoulder, arm and forearm.
3. Describe the region of the forefoot.
4. Describe the regions of the haunch, thigh and leg.
5. Describe the region of the hindfoot.