Anatomy

Body Systems

Anatomical Terminology

To describe the general anatomical position, the following is what we need to expect:

  • Standing straight up
  • Legs close together
  • Feet are parallel
  • Toes pointing you
  • Arms are down at sides
  • Palms are facing towards you
  • Head is up
  • Gazing straight forward

Planes and Sections

Median plane: goes through the midline of the body, intersecting through the midline of the nose and the belly button. This separates the body into left and right.

Sagittal plane: parallel to the median plane but can be anywhere. This divides the body into uneven left and right parts. The paramedian plane is the sagittal plane which is near the median plane.

Coronal Plane: This is a frontal plane which is perpendicular to the median and sagittal planes. It seperates the body into a front portion and a back portion.

Transverse plane: this is a type of horizontal plane which is perpendiular to the median plane. It divides the body into an upper and lower portion. Sometimes, in radiology, the transverse plan can also be called the axial plane. This is actually called the transaxial plane.

Sections help to provide a view of depth. There are three types of sections: transverse sections, longitudinal sections and oblique sections. Longitudinal sections cut the body the same way as a median plane would – this is known as the longitudinal axis of structure. The transverse section is the same as the transverse plane; it is also referred to as the cross section of structures. It helps to view the relative depths of certain structures and their positions in relationship to each other. The transverse section is perpendicular to the longitudinal section. Finally, oblique sections are actually a type of diagonal cut and not a straight plane. Rather, they can be described as in between the transverse and longitudinal planes.

Terminology of Relationship and Comparison

In order to describe position in terms of up and down, the terms used are superior and inferior. Superior refers to towards the head, and inferior refers to the position towards the feet. These terms can interlink with cranial and caudal. Cranial is used to define a structure being towards the cranium (head) whereas caudal refers the structure to be towards the cauda, which in Latin actually means tail. Cauda can refer to 2 different things: it can refer to the feet or the finishing bone of the vertebral column – the coccyx. Cranial and caudal are commonly used to describe embryological structures.

Anterior structure is a structure which is towards the front of the body. This is also referred to as the term ventral. Rostral is a uncommon term which refers to the anatomical position of a structure in relation to the nose. For example, the forebrain is rostral to the brain stem. In order to describe the structure being towards the back, the term posterior can be used. This can also be referred to as dorsal.

Dorsum or dorsal surface can also be referred to as a superior part of a structure that branches off anteriorly. For example, the nose, the tip of the toe and the finger. The palmar surface of the hand is the anterior surface of the hand, and the plantar surface of the foot is the sole (the inferior aspect of the foot).

The terms medial and lateral are used to describe relative proximity to the medial plane. If something is medial it is towards the midline of the body. However if something is lateral it is away from the midline of the body.

The terms external and internal are used to describe the relative proximity in relation to the surface of an organ or cavity. If something is more inner and deeper it is internal and closer to the centre, however if a structure is further away from an organ or cavity then it is external – outwards and further away from the centre. Similar terms are superficial and deep. These terms are used to describe structure in relation to the surface of the body. Superficial is the outward surface of the body, for example skin is superficial and muscle is deep (inner surfaces). Intermediate can be described as the structure in the middle – this is the position in between. For example, if fat layers are between skin and muscle, the fat can be described as being the intermediate layer.

Proximal and distal are used for structures that have a beginning and an ending, foe example limbs and blood vessels. This describes relative proximity (closeness) in relation to the trunk (i.e. the pelvis). A leg would be proximal to the trunk whereas the toe is more distal (further away), relatively. If we take the example of the humurus bone, the proximal portion helps to form the shoulder joint and the distal portion helps to form the elbow joint.

Terms of laterality

Unilateral is used to describe when a structure is on one side of the body, such as the spleen. Bilateral structures are found on both sides of the body, such as the kidneys and the lungs.

Ipsilateral structures are those that are on the same side of the body, such as the left arm and the left leg. Contralateral, however, means when the structures are on the opposite sides of the body, such as the right and left hand.

Terms of Movement

These movements usually occur at joints which connect 2 or more bones. Flexion and extension refer to movement in the sagittal plane. Flexion is the process of bending, where 2 parts are brought closer together and the angle between them decreases. Usually this sort of movement occurs anteriorly, such as the arms bending. However, the knees are the exception as they bend posteriorly. The opposite of flexion is extension, which brings 2 parts of the body further away from each other (straightening), This increases the angle between them. Examples of extension includes when straightening an arm or leg, or moving your whole body in a backwards position (like when a person makes the bridge position). Dorsiflexion is when flexion occurs towards the dorsal surface, for example when the toes bend towards the dorsal surface of the foot. Plantarflexion is when the toes move towards the plantar surface of the feet.

Abduction and adduction are used to describe movement occurring in the coronal plane. Abduction are used to describe movement away from the midline of the body. Adduction is movement towards the midline of the body. Lateral flexion or lateral bending is bending sideways.

Circumduction is the the circular movement of a body limb. This is the processes of flexion, abduction, extension and adduction together. Usually this causes the proximal end to stay the same but the distal end to move.

Rotation

Rotation usually tends to occur in the longitudinal axis. An example is the shaking of the head and neck to say no. Rotation that causes movement of the anterior surface away from the medial plane is called lateral (external) rotation. Movement of the anterior surface towards the medial plane is called medial rotation.

Pronation and supination are used to describe rotation of the wrist through the radius bone. During pronation, the lateral radius rotates medially, causing the crossing over of the radius and ulna. During supination the radius and ulna are no longer crossing together and are lateral to each other. Here the palm of the hand faces anteriorly.

Supine position is the position of facing upwards (anterior part facing top). The prone position is when the posterior side is facing the top – the back is facing upwards.

Eversion is the movement of the sole of the foot away from the medial plane. Inversion is the movement of the sole of the foot towards the medial plane. Complete eversion is called dorsiflexion and complete inversion is called plantarflexion.

In terms of thumb movement, when the thumb moves towards the medial plane this is called opposition. This allows the thumb to touch one of the fingers. Reposition is when the thumb moves away from the medial plane, back to its position.

Protusion is the anterior movement of a body part such as the tongue or the jaw. The opposite of this is retrusion.

Retraction is when shoulders move backwards and scapulae move posteromedially.  Protraction is when you move the shoulders forward and the scapulae move anterolaterally along the chest wall.

The final two terms ae elevation and depression. Elevation is the movement of a body part superiorly- such as moving eyelids up and shrugging shoulders. Depression is the process of moving a body part inferiorly , such as closing upper eyelids or moving ur shoulders back down.

Skeletal System

The skeleton is divided into 2 subgroups: the axial skeleton and the appendicular skeleton. The axial skeleton contains the bones of the skull (cranium), the vertebral levels and the ribs. The appendicular skeleton consists of bones of the upper and lower limbs.

Cartilage

Cartilage is avascular. It is a form of extracellular tissue which is embedded in a matrix that contains cells localised in small cavities. The amount and kind of extracellular matrix depends on the type of cartilage.

In heavy weightbearing areas or areas prone to pulling forces, the amount of collagen is greatly increased, and the cartilage is almost inextensible. However, in areas which don’t need to support as much weight, cartilage contains elastic fibres and fewer collagen fibres is common.

Functions of cartilage:

  • Support Soft Tissues
  • Provide a smooth, gliding surface for bone articulations (movements)
  • Enable the growth and development of long bones

Types of cartilage:

  • Hyaline Cartilage: this type is the most common type of cartilage. It contains a moderate amount of collagen fibres (present in articulating surfaces of bone)
  • Elastic Cartilage: contains collagen fibres alongside a large proportion of elastic fibres. (present on the external surface of the ear)
  • Fibrocartilage: Limited number of cells and ground substance with a large amount of collagen fibres. (present in intervertebral discs)

Cartilage is avascular – there is no blood, lymphatics or nerves to transport substances. This is why cartilage relies on diffusion for nourishment.

Bone

Bone is a calcified living connective tissue that forms the majority of the skeleton. It consists of an intracellular calcified matrix with several different types of cells, which also consists of collagen fibres.

Function of bone:

  • Supportive structures for the body
  • Protectors of Vital Organs
  • Reservoirs of calcium and phosphorus
  • Levers on which muscles act to produce movement
  • Contains blood producing cells

There are two main types of bone: compact bone and spongy bone. Compact bone is also known as trabecular bone, and it is the dense bone that forms the outer shell of all bones and surrounds spongy bone. Spongy bone consists of spicules of bone enclosing cavities containing blood forming cells (i.e. bone marrow).

Classification of bones:

  • Long bones – tubular. Examples include the femur (of the lower limb) and the humerus (of the upper limb)
  • Short bones – tend to be cuboidal (e.g. bones of the wrist and ankle)
  • Flat bone: 2 compact bone plates separated by spongy bone (e.g. the skull)
  • Irregular bones are bones with various shape (e.g. bones of the face)
  • Sesamoid Bones are round or oval that develop into tendons

Clinicl Reference

Accessory and sesamoid bones

These are extra bones not found usually in the normal skeleton, but exists as a normal variant in many people. They are usually located in the hands and wrists or ankles and feet. They look very similar to fractures and cracks on imaging.

Sesamoid bones are embedded within tendons, the largest being the patella. There are many other sesamoid bones embedded to tendons within the hands and the feet, most frequently being in the flexor tendons of the thumb and the big toe.

Some degenerative and inflammatory alterations can occur on such bones, as well as mechanical stresses, where physiotherapy and targeted steroid injections are required as a part of treatment. In some severe cases removal of these bones may be required via surgery.

Bone is vascularised. Generally an adjacent artery will branch off into a nutrient artery, usually one per bone, and will end up in the internal cavity of the bone. Here it supplies the bone marrow, spongy bone, and the inner layers of compact bone. Bones are all covered externally except for a joint where articulating cartilage is present.

Determination of skeletal age

Throughout life skeletal bones develop in a way that they mature at the end of puberty. This surprisingly differs in different regions. In the western region, usually bone maturation occurs at an age of 20-25 years. Before bone maturity is reached, development typically undergoes a sequence of events. This can be measured through ultrasound, MRI scanning and plain radiographs. This is how bone aging can be detected.

Factors that affect bone maturity:

  • Geographical Location
  • Socioeconomic factors
  • Genetic Factors
  • Disease States

Bone Marrow Transplants

There are 2 main types of bone marrow present in the blood. Red bone marrow is the bone marrow usually present for the production of red blood cells and white blood cells. In the yellow bone marrow fewer white blood cells are produced however there is a high content of fat globules (which are yellow in appearance). From birth most bone marrow is red. As development occurs more bone marrow turns yellow within the medulla of the long flat bones.

2 types of stem cells found in bone marrow:

  • Hemopoietic Stem Cells: give rise to white blood cells, red blood cells, platelets
  • Mesenchymal Cells: structures that form bone, muscle and cartilage

There are many types of issues that can arise with disfunctional bone marrow. It can allow the spread of an infection or malignancy. A malignancy in the bone marrow (leukaemia) can be replaced by non-malignant bone marrow cells of a different person, which can act as a cure. In this case the patient’s own bone marrow is destroyed using chemotherapy/ radiotherapy. The treatment is done through bone marrow transplantation.

Bone Fractures

This occurs due an abnormal amount of load or stress put onto a bone. This can also occur due to osteoporosis where the bone is weak due to disease. During osteoporosis normal stress/load on a bone will cause fractures on a bone as it is incapable of withstanding such pressure. In children where bone is still developing fractures may still occur on the growth plate or the shaft.

After a fracture has occured the natural response is to heal the fracture. Between the fracture margins a blood clot is formed into which new blood vessels grow. A matrix is formed via osteoblasts (which lay down insoluble molecules), which is somewhat jelly-like: here collagen producing cells appear. This is the process of bone reformation, and something known as a callus can appear on the fracture as it heals.

Treatments of Bone Fractures:

  • Fraction line reduction, maintained within a line of plaster of Paris Cast
  • If this can’t be maintained internal or external fixation is required via screws and metal rods.

Avascular necrosis

This is the cellular death of bone resulting from a temporary or permanent loss of blood supply to the bone. A typical site of avascular necrosis is a fracture along the femoral neck of an elderly patient. Here there is a loss of cortical medullary blood flow deep to the retinacula fibres. The femoral head becomes bloodless, undergoing necrosis and collapsing. Here it is necessary to replace the femoral head via prosthesis.

Epiphyseal Fractures

During bone development there are stages of extreme growth during ages of 7-10, and during puberty. These changes are associated with an increased cellular activity around the growth plate between the shaft and the head of a bone. The increase in activity makes these bones more susceptible to fractures and injuries. These could be in the forms of dislocations or general injuries. Sometimes this may cause growth plate compression where the bone at that place destroys the region of the growth plate. This may cause asymmetrical growth across the region.

Joints

The site where 2 skeletal elements come and join together are called joints.

  • The skeletal elements are separated by a cavity: this is a synovial joint.
  • When there is no cavity between the components and they are held together through connective tissue this is a solid, fixed joint.

Synovial Joints:

Connections between the skeletal components where articulating bones involved are separated by a narrow and articular cavity. Usually a layer of hyaline cartilage tends to cover the articulating surface of skeletal elements. When viewed in X-Rays these bones seem to have a big gap that separates the articulating bone since the hyaline cartilage is a lot more transparent than bone itself is.

  • Synovial joints attach to the margins of the joint surfaces at the interface between the cartilage and the bone. It encloses the articular cavity. The synovial membrane is highly vascular and secretes synovial fluid. This is present within the articulating cavity and lubricates the articulating joints. Sometimes closed synovial membranes outside of articulating joints are also present, which form tendon sheaths or ‘synovial bursae’. Bursae are usually present between tendon and joints, tendon and bones or skin and bones. The help to reduce friction. Tendon sheaths usually surround tendons and also help to reduce friction.
  • Fibrous joints are formed by dense connective tissue which surround and stabilise joints. Parts of fibrous joints can be used to form ligaments which further stabilise joints. Ligaments that are present outside of fibrous capsule which have a purpose of reinforcement.
  • Additional components of joints include articular discs, fat pads and tendons. Articular discs help absorb compressive forces. They also adjust to changes in joint contour during movement and allow extensive movement. Fat pads usually occur at the synovial membrane and the fibrous membrane. This allows for extensive movement to occur.

Types of synovial joints

  • Plane (flat) joints: allow sliding and gliding movements when one bone moves across the surface of another. Example: acromioclavicular joint
  • Hinge joints: allow movement around one axis perpendicular through a joint. It allows flexibility and extension. Example: the Humero-ulnar joint
  • Pivot joint: allows movement around an axis longitudinally across the shaft of the bone. This allows rotation. Example: Atlanto-axial joint
  • Bicondylar Joints: Allows movement mainly in 1 axis without much rotation. Usually between bones with flat or concave surfaces.
  • Condylar Joints: Allow movement around two axes that are perpendicular to each other; permit flexion, extension, abduction, adduction, and limited circumduction Example: Wrist joint
  • Saddle joints: allow movement around two axes that are perpendicular to each other, however the articular surfaces are ‘saddle shaped’. Permit flexion, extension, abduction, adduction, and circumduction Example: carpometacarpal joint of the thumb
  • Ball and socket joints: Allow movement around multiple axes. Permit flexion, extension, abduction, adduction, circumduction, and rotation. Example: Hip Joint

Axial terms

  • Uniaxial: Movement in one plane
  • Biaxial: Movement in two planes
  • Multiaxial: Movement in three planes

Solid Joints

Connections between skeletal elements where adjacent surfaces are linked together by fibrous connective tissue or cartilage (e.g. fibrocartilage). Movements at these joints are a lot more restricted than movement at synovial joints.

Fibrous joints include:

  • Sutures: Occurs in the skull where adjacent bones are linked by a thin layer of connective tissue known as a sutural ligament.
  • Gomphoses: Occur between teeth and adjacent bone. Short collagen fibres in the periodontal ligament run between the root of the tooth and the bony socket.
  • Syndesmoses: Joints between adjacent bones joint by a ligament. Ligamentum flavum connects adjacent vertebral laminae. An interosseous membrane links (for example the radius and ulna in the forearm).

Cartilaginous joints:

  • Symphyses occur where two separate bones are interconnected by cartilage. Most of these types of joints occur in the midline. Example: pubic symphysis between the two pelvic bones, and intervertebral discs between adjacent vertebrae.
  • Synchondroses occur where two ossification (bone growth) centres in a developing bone are divided by a layer of cartilage. An example is the growth plate that occurs between the head and shaft of developing long bones. These joints allow bone growth and eventually become completely ossified.

Clinical Application:

Degenerative Joint Disease

This is more commonly known as osteoarthritis or osteoarthrosis. This disease is more prominent as a person ages. There is typical decreasing in proteoglycan and water content within cartilage. The cartilage becomes more fragile and susceptible to normal disruption (mechanically). As the cartilage decreases underlying bone becomes fissured and thickened. Synovial fluid sometimes appears within the fissures causing large cysts. Reactive juxta-articular bony nodules are formed (osteophytes). Slight deformations start appearing, creating abnormal stresses, which further disrupt the joint.

Osteoarthritis may occur secondary to other joint diseases, such as rheumatoid arthritis and infection. Overuse of joints and abnormal strains, such as those experienced by people who play sports, often cause one to be more susceptible to chronic joint osteoarthritis. Treatments include weight reduction, proper exercise, anti-inflammatory drug treatment, and joint replacement.

Arthroscopy

Arthroscopy is a technique of seeing the inside of a joint using a small telescope placed through a tiny incision in the skin. This is usually performed in most joints. However, it is most commonly performed in the knee, shoulder, ankle, and hip joints.

This technique allows the surgeon to view the inside of the joint and its contents. The knee, the menisci and the ligaments are easily seen, and it is possible using separate puncture sites and specific instruments to remove the menisci and replace the cruciate ligaments. The advantages of arthroscopy are that it is performed through small incisions, hence allowing patients to recover fast and return to normal activity, and it only requires either a light anaesthetic or regional anaesthesia during the procedure.

Peritoneum

  • The embryonic body cavity develops into the peritoneal abdominal cavity which turns into the primordial abdominal cavity.
  • The mesoderm helps develop the parietal peritoneum, a transparent serous membrane, helps to form a closed sac- this is the peritoneal cavity.
  • The peritoneal cavity is the fluid filled space in between the parietal and visceral serous membrane filled with interstitial fluid. This space is not filled with organs. The interstitial fluid contains water, electrolytes and other substances.
  • The peritoneal fluid helps keep the viscera moving without much friction, allowing for peristalsis. It also contains white blood cells and antibodies to help resist infection.
  • The abdominal organs push against the internal layer of the peritoneal cavity- known as the visceral peritoneum. This forms a boundary between the abdominal organs and the serous membrane. The outer layer of the serous membrane is the parietal peritoneum. This is interior to the abdominopelvic wall, but exterior to the visceral membrane underneath which there are abdominal organs.
  • The visceral and parietal membrane are continuous with each other.
  • The parietal peritoneum has the same blood supply, lymphatics and nerve supply as the region it is present in (hence the external layer of the abdominal wall). However, the visceral peritoneum has the same blood supply, nerve supply and lymphatics as the insides of the viscera. The visceral peritoneum is sensitive to chemical irritation and stretch. The parietal peritoneum is sensitive to pain, pressure, heat and cold.

Specification of peritoneal organs

  • Intraperitoneal Organs: Almost completely covered with visceral peritoneum but not inside the peritoneal cavity. First part of the duodenum, jejunum, ileum, transverse colon, sigmoid colon, spleen and liver
  • Retroperitoneal Organs: organs that are posterior to the peritoneal cavity. They are outside of the peritoneum. These include the kidneys, ureters, suprarenal glands and the rectum.
  • Secondary retroperitoneal organs are organs that initially begin as being intraperitoneal and later on develop to attach posteriorly to the peritoneal cavity (posterior abdominal wall). These are the second, third and fourth parts of the duodenum, pancreas, and the ascending and descending colon.
  • Subperitoneal organs such as the urinary bladder are similar to retroperitoneal organs where hey aren’t part of the peritoneum but are inferior to it.

The omentum

  • The omentum is a type of folding of the peritoneum. There are two parts: the greater omentum and the lesser omentum.
  • The greater omentum is a 4 layered structure that hangs from the stomach to the proximal point of the duodenum. After descending it also attaches to the anterior surface of the transverse colon and its mesentery. (The mesentery fold is a fold that attaches the intestines to the abdominal walls). The 4 layers end up fusing with one another.
  • The lesser omentum is a double layered fold that connects the lesser fold of the stomach to the proximal point of the duodenum and the liver.
  • Posterior to the lesser omentum is a empty space called the lesser sac/ omental bursa. The rest of the peritoneal cavity is called the greater sac.
  • There is a communication between the lesser sac and the greater sac through the omental foramen. This is also known as the epiploic foramen.
Relative PositionStructures and vessels
AnteriorHepatoduodenal Ligament. It is considered a free region in the lesser omentum where there is a portal triad (portal vein, hepatic artery and bile duct, alongside lymphatic vessels
Posterior The inferior vena cava, and the right elongated side (crus) of the diaphragm
SuperiorThe liver
Inferior First portion of the duodenum
Positions of structures surrounding the omental foramen
  • The peritoneum is divided into the supracolic and infracolic compartment through the transverse mesocolon. The supracolic compartment contains the stomach, liver and spleen. The infracolic compartment is situated posterior to the greater omentum, containing the small intesine and the ascending and descending colon.
  • Communication between the compartments occurs through the paracolic gutters, which are grooves between the lateral ascending or descendin colon and the posterior abdominal wall.

Mesentary

  • A double layer of peritoneum that occurs as a result of developing abdominal organs
  • Continuous with the parietal and visceral layers of the peritoneum.
  • The aorta, inferior vena cava and nerve and lymphatic nerves are all posterior to the peritoneum, hence all the mesenteries provides a means of communication between the body wall and abdominal organs.
  • Largest mesentery is the mesentery of the small intestine, often just called the mesentery. This connects the posterior abdominal wall to the jejunum and the ileum.
Other mesenteries and what they attach

The embryological development of the peritoneum

  1. Initially the GI tract is a gut tube. It is very close to the posterior abdominal wall, however as it develops, it becomes further away from the posterior wall.
  2. The gut tube is suspended from the dorsal wall through connective tissue called the dorsal mesentery and from the ventral wall through the ventral mesentery.
  3. The ventral mesentery is derived from a plate of mesoderm called the septum transversum. This extends from the proximal part of the duodenum to the developing liver.
  4. The dorsal mesentary develops from the lower portion of the oesophagus to the rectum, attaching it to the posterior abdominal wall. This allows a passage for blood, nerve and lymphatic vessels.
  5. The gut tube rotates as the abdominal organs develop, until they reach their final destinations.
  6. The liver slowly protrudes the ventral mesentery. The anterior ligament is called the falciform ligament, whilst the posterior portion forms the lesser omentum.
  7. The falciform ligament is continuous with the umblical vein, which closes off after birth and remains as the round ligament of the liver.
  8. The posterior connection to the liver is the lesser omentum. This is actually made up of two ligaments: the hepatoduodenal ligament and the hepatogastric ligament. The hepatoduodenal ligament contains the portal triad which contains the vessels that come to and from the liver. This extends from the proximal part of the duodenum to the liver. The hepatogastric ligament is the rest of the lesser omentum, from the liver to the lesser curvature of the stomach.
  9. The superior part of the liver is called the bare area of the liver, as it is in direct contact with the inferior surface of the diaphragm. This area contains coronary ligaments. The bare area doesn’t attach to ventral mesentery.
  10. As the gut rotates, so does the stomach and the developing pancreas, causing the duodenum to move from the midline to the right side of the peritoneal cavity. These organs end up pushing into the dorsal body wall. This is where the dorsal aspect of the mesoduodenum connects to the posterior abdominal wall. This is why these organs are retroperitoneal. However, the first part of the duodenum remains intraperitoneal, which is the section attached to the stomach.
  11. The ascending and descending colon will also push against the posterior wall, hence their mesenteries will attach with the posterior abdominal wall. The appendix and cecum, however, contain free mesenteries.
  12. The dorsal mesentray also remains attached to the stomach, known as the dorsal mesogastrium. This extends and bulges out to form the 4 folded greater omentum.
  13. The dorsal mesentery of the jejunum and illeum is called the mesentery proper.
  14. Similar to the liver, the spleen is posterior to the primitive gut tube and protudes the dorsal mesentary. A gastrosplenic ligament forms between the spleen and the stomach, whereas the spenlorenal ligament forms between the spleen and the left kidney. This allows for a passage of blood, nerve and lymphatic vessels.
  • There is also the gastrophrenic ligament, which connects the stomach to the diaphragm. The phrenicocolic ligament is located at the left colic flexure and attaches to the diaphragm. The colic flexure is the bend in the large intestine in the left upper quadrant of the abdomen.

Barriers and Compartments

Connective Tissue

  • Tissues are a collection of tissue that all work together to perform a specific function. These can be the same type or different type of cells.
  • They all consist of 3 main types of components: fibrous, ground substance and cells.

Loose Connective Tissue

  • Collagen and elastin fibres embedded into an amorphous, hydrated ground substance
  • Amorphous means it has no specific shape
  • Ground substance is the substance that fills around and between the cells and fibres of the connective tissue
  • Composed of fibroblasts and other cell types that is involved in allergic, inflammatory and immune response.

Dense

  • Composed mainly of collagen fibres with very little hydrated ground substance and fibroblasts
  • Dense Regular – all the fibres line up and are highly organised such as tendons
  • Dense Irregular – the fibres are less organised, such as the dermis.

Mediastinum

  • The mediastinum is the central component of the thoracic cavity.
  • It is surrounded by the two pleural sacs.
  • The mediastinum is divided into 2 parts anatomically through the sternal angle. This is the junction at T4 between the manubrium and the sternal body.
  • The superior mediastinum extends upwards, terminating at the superior thoracic aperture.
  • The inferior mediastinum, extending downwards and terminating at the diaphragm. This is further divided into the anterior mediastinum, middle mediastinum and posterior mediastinum.

Borders of the Superior Mediastinum

  • Superior: the thoracic inlet
  • Inferior: continuous with the inferior mediastinum at the T4 vertebral level.
  • Anterior: the manubrium of the sternum
  • Posterior: Vertebral bodies T1-T4
  • Lateral: Pleura of the lungs

Contents of the superior mediastinum

VesselsNervesStructures
Arch of the Aorta:
1. The brachiocephalic artery
2. Left Common Carotid Artery
3. The Left Subclavian Artery
Vagus Nerve:
1. Right Vagus Nerve
2. Left Vagus Nerve
Thymus
Superior Vena Cava:
1. Brachiocephalic Vein
2. Left superior intercostal vein
3. Supreme Intercostal Space
4. Azygos Vein
Phrenic NerveTrachea
Cardiac Nerves Thoracic Duct
Sympathetic TrunkOesophagus
Muscles:
1. Sternohyoid
2. Sternothyroid
3. Logus colli

The anterior mediastinum

Borders:

  • Lateral Borders: mediastinal Pleura
  • Anterior Border: Sternal Body and thoracic muscles
  • Posterior border: pericardium
  • Superior Border: Continuous with the superior mediastinum at the sternal angle
  • Inferior Border: Diaphragm

Contents:

  • No main contents in the anterior mediastinum.
  • Mainly composed of loose connective tissue (e.g. the sternopericardial ligaments)
  • Lymphatic vessels
  • Lymph nodes
  • Internal thoracic vessels
  • Fat
  • In children the thymus extends inferiorly into the anterior mediastinum but as the child grows this is usually replaced by adipose tissue.

Middle Mediastinum

Borders:

  • Anterior: Anterior margin of the pericardium
  • Posterior: Posterior border of the pericardium
  • Lateral: Mediastinal pleura of the lungs
  • Superior: Sternal angle and the T4 vertebrae
  • Inferior: Superior Surface of the diaphragm

Contents:

OrgansVesselsNervesLymphatics
HeartAscending Aorta (superiorly continuous with the aortic arch, branches into left and right coronary arteries)Cardiac PlexusTracheobronchial lymph nodes (associated with the trachea and respiratory bronchi). Enlarged during lung pathologies. Formed from bronchial nodes and hila of the lungs. Nodes are connected by lymphatic channels.
Pericardium (protective sheath of the heart)Pulmonary Trunk (Arises the left and right pulmonary arteries. Thick to allow blood to pass through). Phrenic Left and Right Nerves
Tracheal BifurcationSuperior Vena Cava (Returns deoxygenated blood from the upper half of the body. Formed by the right and left brachiocephalic veins)
Left and right main bronchi

Posterior Mediastinum

Borders:

  • Anterior Border: Pericardium
  • Posterior Border: T5-T12 Vertebrae
  • Lateral Border: Mediastinal Pleura
  • Superior Border: Continuous with superior mediastinum
  • Inferior Border: Diaphragm

Contents:

  • Thoracic Aorta: The thoracic aorta is continuous with the arch of aorta, beginning at T4. It descends through the posterior mediastinum, becoming more medial as it moves. At T12 the thoracic aorta becomes the abdominal aorta, passing through the aortic hiatus of the diaphragm.
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