BONES OF THE EQUINE DISTAL LIMB
Review Plate 2 and Plate 3 in your text; “Horse Anatomy, A Coloring Atlas”
While this picture is of the entire skeleton of the horse, this lesson will focus on the bones of distal limb
(below the yellow marks) which means it will cover only those structures distal to the carpus (knee) of the
front leg and distal to the tarsus (hock) of the hind
leg.
You should thoroughly study the Glass Horse CD
section on bones and examine all aspects of each
through the 3-d technology the Glass Horse
provides.
COMPARATIVE ANATOMY
Comparative anatomy is the study of similarities and differences in the anatomy of organisms. It is
helpful to understand the similarity of the structures of the equine distal limb to our own body parts with
which we are already very familiar. In this way it allows us to better understand not only how these
structures work, but also to get a feel for their limitations.
The front distal limb of the horse is called the thoracic limb and is similar in structure to the middle finger
of the human hand. The hind distal limb is called the pelvic limb and is similar in structure to the middle
toe of the human foot.
JOINTS
In the horse, what we call the “knee” in the front limb, is more accurately called the “carpus” and it
compares to the human wrist. In the hind limb, what we call the hock is more accurately called the
“tarsus” and it compares to the human ankle and heel.
The fetlock joint in the front limb is called the metacarpophalangeal joint.
The fetlock joint in the hind limb is called the metatarsophalangeal joint.
The pastern joint in the front and hind limb is called the proximal interphalangeal joint and is where the
middle phalanx or short pastern bone, meets the first phalanx, or long pastern bone.
The coffin joint in both the front and hind limb is called the distal interphalangeal joint and is where the
short pastern bone or middle phalanx meets the coffin bone or distal phalanx.
It is important to know that several of the skeletal structures in the distal limb have more than one
common name. You should be familiar with all of them.
Additionally, the structures between the knee and fetlock of the front limb and between the hock and
fetlock of the hind limb are distinguished as front or hind limb by the name which corresponds to the knee
(carpus) in the front limb and the hock (tarsus) in the hind limb.
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Carpus = Knee
Tarsus = Hock
Metacarpophalangeal joint = Fetlock (front limb)
Metatarsophalangeal joint = Fetlock (hind limb)
Proximal interphalangeal joint = Pastern
Distal interphalangeal joint = Coffin joint
Pay close attention to this! The Metacarpal/tarsal bones are numbered 2,3,4…that should be a dead giveaway in telling you the ordering sequence. Ordering moves from proximal to distal and for those
structures that originate in the same horizontal plane, from medial to lateral. Working proximal to distal,
the bones of the equine distal limb are
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2nd metacarpal or metatarsal bones = Medial Splint bone
3rd metacarpal or metatarsal bone = Cannon bone
4th metacarpal or metatarsal bones = Lateral Splint bone
Proximal sesamoids = sesamoid bones (one medial, one lateral)
Proximal phalanx = Long pastern bone = P-1
Middle phalanx = Short pastern bone = P-2
Distal sesamoid bone = Navicular bone
Distal phalanx = Coffin bone = P-3 = Pedal bone
BONE TISSUE
Source: http://www.ivy-rose.co.uk/Topics/Tissue_Bone-Tissue.php
The skeletal system forms the mechanical framework of the body.
The bones themselves are formed from several different tissues, including:
 Bone (called "Osseous") tissue,
 Periosteum,
 Red Bone Marrow,
 Yellow Bone Marrow, and
 Endosteum.
Bone tissue is classified as either "compact bone", or "spongy (or cancellous) bone" depending on how
the bone matrix and cells are organized.
THE STRUCTURE (PHYSICAL DESCRIPTION) OF BONE TISSUE
Structure of Long Bones
There are two main types of bone tissue, compact bone and spongy bone. Individual bones in the body can
be formed from both of these types of bone tissue. The diagram above shows the physical structure of a
typical "long bone".
THE STRUCTURE OF COMPACT BONE
Compact bone forms the outer layer of all bones and most of the structure of "long bones" - see diagram
(right). It contains few spaces and provides protection and support to the bone/s around which it is the
outer-layer, as well as helping to enable the long bones to bear the stress placed on them by the weight of
the body and the use to which the limbs are put, e.g. due any heavy physical work.
The basic unit of Compact Bone is an "osteon", which is also known as a "Haversian System".
Each Haversian System (unit) has a cylindrical structure that consists of four parts:
1. A central tube called a Haversian Canal, which contains blood vessels and nerves.
The Haversian Canal is surrounded by alternate layers of:
2. Lamellae (the word lamellae literally means "little plates") are concentric rings of a strong matrix
formed from mineral salts including calcium and phosphates and collagen fibres. The mineral salts
result in the hardness of the bone structure, while the collagen fibres contribute its strength.
3. Lacunae are the small spaces between the lamellae in which contain the bone cells (called
"osteocytes") are located.
4. The lacunae are linked together by minute channels called canaliculi.
The canaliculi provide routes by which nutrients can reach the osteocytes and waste products can
leave them.
THE STRUCTURE OF SPONGY BONE
Spongy Bone does not include osteons.
Instead, spongy bone consists of an irregular lattice of thin columns of bone called trabeculae (literally
"little beams"), which contain lamellae, osteocytes, lacunae and canaliculi. The spaces between the
trabeculae of some spongy bones are filled with red bone marrow.
Blood vessels from the periosteum penetrate into the trabeculae lattice allowing the osteocytes in the
trabeculae to receive nourishment from the blood passing through the marrow cavities.
FUNCTIONS OF BONE TISSUE
1.
Support
The skeleton is the framework of the body, it supports the softer tissues and provides points of
attachment for most skeletal muscles.
2.
Protection
The skeleton provides mechanical protection for many of the body's internal organs, reducing risk
of injury to them.
For example, cranial bones protect the brain, vertebrae protect the spinal cord, and the ribcage
protects the heart and lungs.
3.
Assisting in Movement
Skeletal muscles are attached to bones, therefore when the associated muscles contract they cause
bones to move.
4.
Storage of Minerals
Bone tissues store several minerals, including calcium (Ca) and phosphorus (P). When required,
bone releases minerals into the blood - facilitating the balance of minerals in the body.
5.
Production of Blood Cells
This process takes place in the red bone marrow inside some larger bones.
6.
Storage of Chemical Energy
With increasing age some bone marrow changes from 'red bone marrow' to 'yellow bone marrow'.
Yellow bone marrow consists mainly of adipose cells, and a few blood cells. It is an important
chemical energy reserve.
CARTILAGE TISSUE
THE STRUCTURE (PHYSICAL DESCRIPTION) OF CARTILAGE TISSUE IN GENERAL
Cartilage is a connective tissue consisting of a dense matrix of collagen fibres and elastic fibres
embedded in a rubbery ground substance. The matrix is produced by cells called chondroblasts, which
become embedded in the matrix as chondrocytes.
That is, mature cartilage cells are called chondrocytes.
They occur, either singly or in groups, within spaces called lacunae (sing. lacuna) in the matrix.
The surface of most of the cartilage in the body is surrounded by a membrane of dense irregular
connective tissue called perichondrium. This is important to remember especially because (unlike other
connective tissues), cartilage contains no blood vessels or nerves - except in the perichondrium.
There are three different types (structures) of cartilage that have slightly different structures and functions.
They are hyaline cartilage, fibrocartilage, and elastic cartilage, described seperately in the sections
below:
HYALINE CARTILAGE
Hyaline cartilage is the most abundant of the three types of cartilage.
It is found in many locations in the body, including:
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Bronchi; Bronchial Tubes; Costal Cartilages; Larynx Nose; Trachea
Covering the surface of bones at joints - especially in areas where damage due to wear may lead to
osteoarthritis incl. e.g. the ends of the long bones, and also the anterior ends of the ribs.
Embryonic skeleton (i.e. in the fetus).
The Structure of hyaline cartilage tissue
Hyaline cartilage consists of a bluish-white, shiny ground elastic material with a matrix of chondroitin
sulphate into which many fine collagen fibrils are embedded. It contains numerous chondrocytes.
The Functions of hyaline cartilage tissue
Hyaline cartilage tissue provides smooth surfaces, enabling tissues to move/slide easily over each other,
e.g. facilitating smooth movements at joints. It is also provides flexibility and support.
FIBROCARTILAGE
Examples include:
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Calli (sing. callus), which is the tissue formed between the ends of the bone at the site of a healing
fracture (bloodclot -> granulation tissue -> cartilage -> bone);
Intevertebral discs (i.e. the discs between the vertebrae of the spine);
Menisci (cartilage pads) of the knee joint.
Pubic symphysis, which is the position at which the hip bones join at the front of the body.
Also in the portions of the tendons that insert into the cartilage tissue, especially at joints.
The Structure of fibrocartilage tissue
Fibrocartilage is a tough form of cartilage that consists of chondrocytes scattered among clearly visible
dense bundles of collagen fibres within the matrix. Fibrocartilage lacks a perichondrium.
The Functions of fibrocartilage tissue
Fibrocartilage tissue provides support and rigidity to attached/surrounding structures and is the strongest
of the three types of cartilage.
ELASTIC CARTILAGE
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Auditory (Eustachian) Tubes;
External Ear (Auricle);
Epiglottis (the lid on the top of the larynx).
The Structure of elastic cartilage tissue
In elastic cartilage, which is yellowish in colour, the cartilage cells (chondrocytes) are located in a
threadlike network of elastic fibres within the matrix of the cartilage. A perichondrium is present..
The Functions of elastic cartilage tissue
Elastic cartilages provides support to surrounding structures and helps the define and maintain the shape
of the area in which it is present, e.g. the external ear.
Diagrams of Cartilage Tissue
There are three different types (structures) of cartilage that have slightly different structures and functions.
They are hyaline cartilage, fibrocartilage, and elastic cartilage:
Hyaline Cartilage
Hyaline cartilage consists of a bluishwhite, shiny ground elastic material within
a matrix of chondroitin sulphate into which
many fine collagen fibrils are embedded.
It contains numerous chondrocytes.
A perichondrium is present, as shown in
the diagram on the right.
Fibrocartilage
Fibrocartilage is a tough form of cartilage
that consists of chondrocytes scattered
among clearly visible dense bundles of
collagen fibres within the matrix.
It contains chondrocytes but lacks a
perichondrium.
Fibrocartilage is the strongest of the three
types of cartilage.
Elastic Cartilage
In elastic cartilage, which is yellowish in
colour, the cartilage cells (chondrocytes)
are located in a threadlike network of
elastic fibres within the matrix of the
cartilage.
A perichondrium is present, as shown in
the diagram on the right.