The Skeletal System

The Skeletal System Bone tissue Bone is a living organ. It is a specialised form of connective tissue, often being called supportive tissue. Bone is rigid and non-elastic. Bone matrix (the matter between the cells), consists of a fibrous tissue framework that contains calcium and phosphorus salts. The bulk of the weight of bone (approx 2/3) is mineral salts (calcium carbonate and calcium phosphate). A single bone is made up of a number of different types of tissues:

Corticol (cortex) or compact bone

This is the dense, solid outer layer of all bone, and makes up the shaft of long bones. Even though it appears quite smooth, when we look at it under the microscope, it is seen to consist of many close- packed cylindrical units called Haversian systems. Each Haversian system consists of a central canal, called the Haversian canal, through which runs an artery, vein and nerve. This is surrounded by concentric rings of bony matrix, called lamellae. Within this, are little cavities (called lacunae) for the bone cells to reside in, and little canals (called canaliculi), that carry nutrients to the bone cells. This can be viewed in the article on tissue types under connective tissue.

Cancellous or spongy bone

This type of bone consists of a fine network of bone tissue with spaces in between, making it look a bit like honeycomb. The visible spaces contain a meshwork of intercommunicating spaces (called trabecular ribs), that are filled with bone marrow. All bones contain both corticol and cancellous bone. Spongy bone tissue makes up most of the bone tissue of short, flat and irregular bones.

Bone marrow

This is a soft form of connective tissue, which is found in the hollow centre (called the medullary cavity) of long bones, and also in the spaces in spongy bone. There are two types of bone marrow, red and yellow. Red bone marrow is produced in the spongy bone and this is very important for making red blood cells (erythrocytes). Yellow bone marrow is produced in the centre cavity of the long bones, which is a fatty type of bone marrow which consists of some blood cells and tissue. Yellow bone marrow stores energy in the form of lipids (fat). This type of marrow is present in mature adult animals.

Periosteum

Is a tough two layered, white, fibrous membrane that surrounds bone, except at the ends. It is (peri = attached to the cortex by fibres, and consists of around, osteo dense connective tissue, blood vessels, and nerves = bone) that pass into the bone. Periosteum is responsible for protection, nutrition, for laying down new bone on the outside surface of

Classification of bones Bones are classified according to their gross appearance (shape), into the following types:  Long bones  Short bones  Flat bones  Irregular bones  Sesamoid bones  Pneumatic bones

Long bones Long bones are typically longer than they are wide. They usually consist of a more or less cylindrical shaft called a diaphysis, which has a larger head at both ends called the epiphysis. There is a bony outgrowth on the head of these bones and it is called the apophysis. Through the centre of the shaft is a cavity where the fatty yellow bone marrow is in adult animals, this is called the medullary cavity. This cavity is lined with fibrous tissue called endosteum. In mature bone, the region where the diaphysis joins the epiphysis is called the metaphysis. In growing bones, this region contains a layer of Hyaline cartilage called the epiphyseal growth plate. This growth plate is where bones grow in length until animals reach physical maturity. The end of long bones is covered with articular cartilage. This is a

thin layer of Hyaline cartilage where the bone forms an articulation (joint) with another bone. The cartilage reduces friction and absorbs shock at freely movable joints.

Short bones Short bones are more or less cuboidal in shape. They do not have a medullary cavity, but consist of an outer shell of cortical bone, with a spongy bone interior. The most common examples are the carpal and tarsal bones

Flat bones The diagram below will help you to understand more about flat bones and skull sutures, (see fibrous joints for more on the skull). Flat bones are a sandwich of cancellous (spongy) bone between layers of compact bone. They are reasonably thin. They do not have a shaft or a growth plate. Flat bones afford considerable protection, as well as providing extensive areas for muscle attachment. Examples of flat bones are bones of the skull, like the flat plates of the cranium, which are mostly joined by sutures, and the bones of the pelvis.

Irregular bones Irregular bones have more complex shapes, and are classified as irregular because they do not fit into any other categories. They are neither flat, nor do they have a shaft, they also vary in the amount of spongy and compact bone that they have. The most common example of irregular bones are bones of the vertebral column.

Sesamoid bones Sesamoid bones are bones shaped a bit like sesame seeds. They are small bones found along the course of tendons where considerable pressure develops. Sesamoid bones do not join with other bones. They are contained within ligaments. Examples of where we can find sesamoid bones are at the back of the knee in some animals, the patella, the navicular bone in the horse and the sesamoids associated with the phalanges (toes) of some animals.

Pneumatic bones Pneumatic bones are bones that have air spaces in them. An example of a pneumatic bone is the sternum or the humerus in birds.

Functions of the skeleton  Support of the body- bones are like the "steel girders" of the body, forming an internal framework that supports all the internal organs. Without this framework we would all creep along the ground like slugs!  Leverage for movement - skeletal muscles are attached to bones by tendons. They use the bones as levers to move the body and its parts.  Protect viscera - bones protect soft body organs (viscera), which would easily be injured otherwise. For example, the fused bones of the skull provide an excellent enclosure for the brain. The vertebrae surround and protect the spinal cord and the rib cage helps to protect the vital organs of the thorax.  Store minerals - especially Calcium and Phosphorus. These minerals are deposited and then withdrawn as needed by the body. Also, some fat is stored in the internal cavities of bones.  Provide a site for blood formation, in the red bone marrow located in flat bones and in the ends of long bones - this process of blood cell production is called haemopoiesis.

Classification of the skeleton The skeleton is divided into: 1. The axial skeleton = skull, vertebrae and ribs 2. The appendicular skeleton = pectoral and pelvic girdles, hind and fore limbs

The axial skeleton The skull is made up of the cranium, facial bones and the mandible (lower jaw) attaches to it. The cranium and facial bones are composed from plates of bones, most of which are fused together by immovable joints called sutures (suture = seam). The mandible connects to the skull by a freely movable joint.

The vertebrae column consists of the following types and numbers of bones, starting from the head to the tail:  the cervical (or neck) vertebrae - there are seven bones here in most mammals  the thoracic (or chest) vertebrae - with variable numbers between 12 and 18 bones, depending on the species of animal  the lumbar (or loin) vertebrae - with five to seven bones depending on the species  the sacral (or pelvic) vertebrae - usually four to five bones here, that are actually fused together giving great strength  the coccygeal (or tail) vertebrae - with the greatest variation of up to 23 bones, depending on the species. The ribs attach to the sternum, which is also part of the axial skeleton. The sternum is commonly called the breast bone, and consists of a number of segments (usually six to eight, depending on species), to which the ribs attach.

The appendicular skeleton The bones of the limbs are often referred to as the appendages. The forelimb is referred to as the thoracic limb. The hind limb is referred to as the pelvic limb. The pectoral (or shoulder) girdle attaches the bones of the forelimb to the Axial skeleton. There are two pectoral girdles in the body, one on either side of the body, and consists of the scapula and the clavicle (which is the collar bone in humans and often does not exist in animals). The pectoral girdle does not articulate via a joint with the vertebrae column, but the scapula unites via a joint with the humerus, which is the first bone of the thoracic limb. The pelvic girdle consists of two hip bones (pelvis), which is made up of three bones, the Ilium, the ischium, and the pubis, on either side of the body. The pelvic girdle articulates with the femur, the first bone of the pelvic limb.

Ligaments Ligaments are strong bands of white fibrous connective tissue that unites bones. They function to keep joint surfaces in apposition, while still allowing movement. They serve to support and strengthen joints. They are usually inelastic, and help stop a joint (with the help of muscles and tendons from bending too far or the wrong way. If a joint is wrenched and stretched too far it results in stretching

(called a sprain) or tearing of the ligaments and can result in joint instability. Ligaments are not designed to stretch and when damaged have a slow capacity for repair. Back to top of page

Skull The shape of the skull varies between different species of animals, but its actual function is basically the same. In this module we are going to use the dog skull as our example. The skull rests on top of the vertebral column and is composed of two sets of bones: 1. the cranial bones -which house and protect the brain 2. the facial bones - which covers the area we call the face, it supports many of the sense organs, and forms passages for the beginning of the digestive and respiratory tract. Cranial bones consist of:

The occipital bone - is at the back of the head, joining the neck. The parietal bone - is in the front of the occipital bone and covers the top of the brain, towards the back, it is like the roof of the cranial cavity. The frontal bone (forehead) -is front part of the top of the head and between the eyes, it forms the front part of the roof of the cranial cavity. The temporal bone -is the sides of the skull, housing the ears.

Facial bones consist of:

The lacrimal bone (lacrima = tear) - is a small delicate bone, forming part of the inside wall of the eye orbit, . It also contains the first part of the tear duct. The nasal bone - is a long bone in most dogs, that joins the frontal bone, and starts just in front of the eyes and goes down to the dogs nose, it houses the nasal cavities (airways). The maxillary bone (or maxilla) - is at the sides of the face and part of the hard palate (across the roof of the mouth) It is classed as the upper jaw bone, running from one side of the face to the other, underneath the nasal bone. It houses the cheek teeth (Canines, premolars and molars). The incisive bone - paired bones at the front of the

upper jaw that houses the incisor teeth. The zygomatic bone - is the bone forming the hard part of the cheek, and the lower outside portion of the orbit. The bone that forms the bottom jaw, is called the mandible. It supports all the lower teeth in the mouth. It articulates with the temporal bones, on both sides of the face, by the only free moving joint in the skull (the temporomandibular joint).

Vertebrae The vertebrae is a collection of bones that together make up the vertebrae column. This is often referred to as the spinal column or the backbone. The intervertebral discs are fibrocartilage discs, that form strong joints, permit the various movements of the vertebral column, absorb concussion and reduce shock. Each group of vertebrae is represented by its first letter followed by the number representing how many are in each section. For example C1 is the first cervical vertebrae and C7 is the last, T1 is the first thoracic vertebrae, and so on.

Common features of a typical vertebra are:

The body: the thick, spool-shaped ventral (towards the ground), part, that articulates with adjacent vertebrae. The vertebral arch: the part that arises from the body, then ends with the spinous process (this is the part you can feel along the back of a "bony" animal, like a dog or a horse). The vertebral foramen: (foramen = hole or opening), this forms the vertebral canal, which houses the spinal cord. The facet joint surface: (facet = little face), are the articular surfaces. The transverse process: these are the lateral extensions of the vertebral arch.

Cervical vertebrae Cervical vertebrae are the seven vertebrae of the neck of all

mammals. It supports and articulates with the head, and controls and supports the neck. The first cervical vertebrae is called the atlas (C1), it has a synovial articulation, with the occipital bone of the head, allowing it to flex and extend. It is often called the "yes" joint, because it allows you to nod. The atlas has two large lateral processes called wings; these are usually able to be felt in the conscious animal. The second cervical vertebrae is called the axis (C2), it is a longer vertebrae, and forms a pivot articulation with the atlas. This allows pivotal motion, and is often called the "no" joint as it allows you to move your head in from side to side. The atlas and axis vertebrae are different to the other five cervical vertebrae, because they perform these functions, (the huge range of head movements seen in normal animals) that are not shared by the other vertebrae.

Thoracic vertebrae The thoracic vertebrae form the back and chest. This can be a little confusing, as when we think of chest we often think of the area below the neck, in front of the forelimbs. This is the breast, or brisket area, with the chest being the area where the thorax is (heart and ribs). Thoracic vertebrae, articulate with the ribs. If you ride a horse, then the saddle sits on the thoracic vertebrae

Lumbar vertebrae The lumbar vertebrae are the loins of all animals (it is referred to as the lower back in humans). Lumbar vertebrae give mobility to the pelvic girdle to assist locomotion. These are probably the largest and definitely the strongest vertebrae in the body. This is because they serve as the site of attachment for the large sling muscles that support the abdomen. They are characterized by their large size and long plate-like transverse processes, which is where the muscles attach to. Lumbar vertebrae also help to protect the kidneys, which lie underneath. Dogs normally have seven lumbar vertebrae.

Sacral vertebrae The sacral vertebrae are usually referred to as the sacrum, because the bones are actually fused together into a solid structure. Sacral vertebrae articulate with the pelvis (the ilium), by a cartilaginous joint called the sacroiliac joint. This articulation allows mechanical forces to be efficiently transferred from the hind leg to the vertebral column. The dog normally has three sacral vertebrae fused together to form the sacrum.

Coccygeal vertebrae Coccygeal vertebrae form the tail, and are sometimes referred to as the caudal vertebrae. The further they move down the tail, the smaller they become. The coccygeal vertebrae help to give the animal balance, and have signalling functions. With some animals they are used to swat flies and other insects, for example the horse. With most animals they protect the perineum (the area containing the anus and the vulva in females). The number of coccygeal vertebrae in the dog may vary between breeds. Examples of signalling functions are:  wagging in the happy dog  erect in the aggressive dog  swishing in the mischievous cat  jammed tight in the nervous horse  up high and to one side in the female horse or dog signalling her desire to mate.

Ribs Ribs are arranged in pairs on both sides of the body. The ribs, which are attached to the thoracic vertebrae, form a bony cavity called the thorax. The thorax encloses and protects the thoracic organs (heart and lungs). With the exception of fish, ribs are essential for respiration., by providing a mechanical framework for the expansion of the thorax during breathing.

True Ribs: these articulate with the sternum through their costal cartilages (a cartilaginous joint). False Ribs: have costal cartilages (hyaline) which attach to the costal cartilages of bottom true ribs. This forms the costal arch, indirectly joining them to the sternum. Floating Ribs: (in dogs and humans): do not join to the sternum, or other costal cartilages. The intercostal space (spaces between the ribs), is for the intercostal muscles to attach. Their major importance is for breathing. The xiphoid process of the sternum is the cranial attachment point for the linea alba, which is the central junction of the abdominal muscles.

Pectoral girdle The pectoral girdle is the foundation of the forelimb. It is anchored by muscular attachments to the ribs and spine and from it originates the muscle which operate the forelimb.

The scapula (shoulder blade) in all animals is a relatively flat bone, triangular shaped. The bottom of the scapula articulates with the humerus of the forelimb, forming the only true joint between the scapula and another bone in most animals. Humans, birds and cats have a clavicle (collar bone), which forms a joint with part of the scapula. In the cat it is a small bone which lies in the muscles just cranial (closer to the head) of the shoulder joint. The lateral (outside) face of the scapula has a ridge along it called the spine. This is often palpable on its lateral surface and extends down almost to the shoulder joint. The scapula attaches via strong muscles to the spine, allowing it to have exceptionally free movement, which it does by sliding smoothly back and forth. In the dog there is a band of cartilage along the top of the scapula where it attaches to the vertebrae and ribs.

Fore limb The forelimb is also called the thoracic limb. In many of our four legged domestic animals, the thoracic limb carries up to 60% of the body weight. The horse is a classic example of this. It carries 60% of its weight on its forelimbs, then we add a rider onto it's back, adding more weight to the forehand. This explains why most lamenessrelated problems in horses are in the forelegs. The thoracic limb consists The humerus - sometimes called arm of the following from top bone, articulates with the scapula. to bottom: The radius and ulna - these bones make up the forearm in most animals. The carpus - consists of two rows of bones called the carpal bones, these bones make up the knee. The metacarpal bones - there is a great variation here among species, in the horse we call them the cannon bone and two splint bones, (so they have three bones), pigs have four bones, and in the dog and cat there are five metacarpal bones

The phalanges - these are the toes in most animal, they correspond to the fingers and toes in man. The number here varies between species, usually between one and five.

Pelvic girdle The pelvic girdle provides a solid foundation for the hind limbs, protects the viscera of the pelvic cavity and provides the birth passage in females. It is composed of three bones, the ilium, ischium, and pubis, fused together. The pelvic girdle or bony pelvis as it is often called, encloses the pelvic cavity. The "pelvic inlet" is the cranial (towards the head) opening, into the pelvic cavity. The "pelvic outlet" is the caudal (towards the tail) opening out of the pelvic cavity. The widest horizontal distance of the pelvic cavity is the transverse diameter (this is the opening where the arrows are in the diagram), this is where the foetus must pass during birth. The bones of the pelvic girdle are on each side of the body, and together they form the basin like structure that we call the pelvis. Each of the three fused bones on each side are called the hemipelvis, and both sides meet at the pubic (or pelvic) symphysis, which is a fibrocartilaginous joint. The acetabulum is the socket that articulates with the femoral head of the femur, which is the first bone of the hind limb. The ilium is the largest bone, which articulates with the sacrum. The iliac crest, is the bony part often called the point of the hip and can be easily felt and seen in thin animals (these are the bony points that we can see jutting out on a skinny cow or horse). The ischium has a long bony part that runs back and forms the "pin bone", which are the two pointed bones on either side of the tail head in cattle. This can be felt in the dog and can be seen in a very poor conditioned dog. This area is the site of attachment of the thigh muscles. The pubis runs along the floor of the pelvis that joins at the symphysis. The pubis and ischium together surround the obturator foramen, which is a large opening in the floor of the pelvis, that allows blood vessels and nerves to pass through.

Hind limb The hind limbs form the back legs in all four legged animals.

They are often referred to as the pelvic limb, and consists of the following, starting from the articulation with the pelvis: The femur -which is the heaviest bone in the body.  The patella - a small plate of bone that is a large sesamoid bone, that sits in front of the lower end of the femur.  The tibia and fibula -two long bones that are joined by an interosseous space (like the radius and ulna in the foreleg), which articulates with the tarsus.  The tarsus - called the "hock" or tarsal bones, are two or three rows of bones, with the number of bones varying with different species.  The metatarsal bones - which are similar to the carpal bones of the thoracic limb.  The phalanges -the toes which have the same pattern as in the thoracic limb.

Hyoid apparatus The hyoid apparatus consists of a collection of small bones that are suspended from the base of the skull. They run from the temporal bone to the larynx, forming the shape of a cradle. Its function is to hold the larynx (voice box) in place, and serve as a major attachment for the tongue. The movement of the hyoid apparatus and so of the larynx, is like a swing. It moves back and forth in an arc.

Os penis The os penis is a bone in the penis of carnivores (mainly the dog).

Joints Introduction Joints are unions or junctions between two or more bones. They are classified according to their degree of movement, and their type. Joints are also called articulations. The surface of the bones that meet to form a joint is called the articular surface. So, when we say that one bone articulates with another bone, we simply mean that one bone forms a joint with another. Joint structure determines how it functions, they can be fixed, slightly moveable or freely moveable. It seems that the closer the fit of a joint at the point of contact, the stronger the joint. However, at tightly fitting joints the amount of movement is restricted. The looser the fit, the more movement, but unfortunately loose fitting joints are more prone to dislocation.

Classification of joints We can classify joints into three main types: 1. Fibrous joints Fibrous joints are united by fibrous tissue. They exhibit little or no movement. Probably the best example of a fibrous joint is the sutures of the skull. The irregular edges of these bones interlock and are bound tightly together by connective tissue fibres. Sutures are temporary joints, that later ossify, for example skull sutures which allow the skull to deform in the birth passage, making birth easier, then they fuse with age. Syndesmoses is another type of fibrous joint. Here tight bone junctions are stitched up with fibrous tissue. The connecting fibres are longer than those of sutures, therefore the joint has more "give". An example of this is type of joint is the lower junction between the tibia and fibula. 2. Cartilaginous joints Cartilaginous joints are slightly movable. They do not have any joint cavities, and are united by cartilage. There are two types of cartilage in this group: a. Fibrocartilage - an example of this is the Intervertebral joints of the spine. Here the articulating bone surfaces are connected by discs of fibrocartilage. The pubic symphysis of the pelvis is another example of this. b. Hyaline cartilage - an example of this is the cartilaginous joints between ribs and sternum, called the

the costal cartilages: see the dark parts between vertebrae. 3. Synovial joints Synovial joints are freely movable. They are united by a synovial joint capsule, containing a lubricating fluid, called synovial fluid, that allows movement, by allowing one surface to glide over another.

There are different types of synovial joints, which are classified by:  the shape of articulating surfaces  the motion of the joint  the mobility of the joint. Because of this, synovial joints are further classified into the following:  Hinge joint - with this type of joint the convex surface of one bone fits into the concave surface of another bone. Movement

is limited to one plane only, similar to that of a hinged door, which is extension and flexion movement. Examples of this is the elbow and the fetlock in the horse.  Plane or gliding joint - here the articulating surfaces are flat, and capable of only limited movement of a simple gliding or sliding motion. Examples of these are the carpal and small tarsal bones.  Ball and socket joint - here the head of one joint fits into a pit or socket allowing movement in almost all directions. An example of this is the hip joint.  Pivot joint - in this type of joint, a rounded or pointed surface of one bone articulates within a ring formed partly by bone and partly by a ligament. The primary movement is rotation. An example of this is the articulation of the axis and atlas cervical vertebrae in the neck. Synovial joint capsule: this is a two-layered structure surrounding the joint. Synovial fluid is contained within the joint capsule. Fibrous layer: (capsular ligament):is the white and yellow elastic outer fibrous part of the Synovial joint capsule. This attaches to the periosteum (see long bone), near the articular cartilage. The thickness varies from joint to joint and within each joint. Synovial membrane: this is the inner layer of the fibrous layer of the joint capsule. It is highly vascular, nerve rich, and produces synovial fluid. Synovial fluid : is a viscous (like raw egg white) fluid produced by the synovial membrane, a joint lubricant that supplies nutrients and removes wastes from the hyaline cartilage and fills the joint cavity . Articular Cartilage: translucent, with a bluish tinge and is usually hyaline cartilage which covers articular ends of bones. It reduces the effects of concussion and friction by its compressibility, elasticity and smoothness, has no blood or nerve supply. It relies on synovial fluid to supply nutrients and remove waste. Intracapsular ligament: ligaments located within the joint capsule. They are not within the joint space because the synovial membrane reflects over them. A common example of this type of ligament is the cruciate ligament in the stifle of the hind leg of dogs. Extracapsular ligament: are located outside of, or as part of the joint capsule and are responsible for thickening of the joint capsule to increase its strength. Joint Cavity: a cavity filled with synovial fluid Meniscus: a plate of fibrocartilage, that partially, or completely,

divides a joint cavity. It’s function is to allow for a greater range of movement and alleviate concussion (eg stifle and temporomandibular joint between skull and lower jaw.)e.g. stifle & temporomandibular joint between skull and lower jaw . Bursa: a sac like a synovial joint capsule which can be found between:  skin and bone  tendon and bone  muscle and bone  ligament and bone It functions to reduce friction and stress. Synovial sheath: like a bursa which wraps around a tendon. It functions to reduce friction between tendon and bone.