Locomotor System: Skeleton

Skeleton

Introduction

The skeleton is the main system to support the body. It forms an internal hard structure that supports other organs, protecting them. Some delicate organs are enclosed in a sort of armour made of bones. The brain, for instance, is enclosed in the cranium and the lungs and heart are protected by the rib cage.

The skeletal muscles are attached to bones and the contraction of different muscles move the bones they are joined to, providing movement to the body.

The bones are, besides, the main reservoir for calcium in human body. When the calcium is required in the blood, it is extracted from the bones.

Finally, in the bone marrow, which can be found in the interior of large bones. The haematopoiesis process is carried out to produce blood cells.

The bones have three parts:

• Periosteum: it is the outermost layer that surrounds the bone. It is made up of connective tissue and it is related to the growth the bone thickness.
• Compact bone: it is the hard part of the bone and its main structural component. It is made up of a hard extracellular matrix, rich in collagen and calcium, and cells that maintain this matrix that are called osteocytes. Another less abundant type of cells are the osteoclasts, located in the interior part of the compact bone and responsible for destroying extracellular matrix in order to release calcium into the blood. The extracellular matrix is extremely ordered, forming cylindrical structures called osteons (also known as harversian systems). In the centre of the osteons there is canal where the blood vessels and nerves can be found. Each bone is, in its compact part, made up of thousands of parallel osteons.
• Spongy bone: it is in the interior of large bones. It is a complex network of tissue that forms a trabecular system, similar to a sponge. The interior of this trabecular system is the place where the stem cells responsible for producing blood cells are nested. Due to this, it is the place where the haematopoietic process occurs.

According to its shape, there are four types of bones:

• Short bones: they are usually small and they have similar a size in the three dimensions (similar length, width and thickness). They tend to be cubical.
• Flat bones: they are flat, so two dimensions are more relevant than the third one (they are long and wide, but they are thin).
• Long bones: one dimension (length) is much more relevant than the other two (they are long, but not wide or thick). They have three parts:
• Epiphysis: it is the end part of the bone, that is usually enlarged.
• Diaphysis: it is the central part of the bone, long and hollow.
• Metaphysis: it is the transition between epiphysis and diaphysis. It is the place from which the bone grows longitudinally.
• Irregular bones: they have complex shapes, that can not be described as long, short or flat. Sphenoid (in the cranium) and vertebrae are two examples. 

Skeleton: Bones

Cranium

These group of bones enclose and protect the brain.

• Frontal bone: anterior part of the head above the orbits(forehead) and orbit ceiling.
• Parietal bone (2): lateral part and ceiling of the cranium.
• Temporal bone (2): inferior lateral par of the cranium (in the temples).
• Occipital bone: posterior part and base of the cranium
• Sphenoid bone: irregular bone in the centre of the cranium, articulated with the rest of the cranial bones.
• Ethmoid bone: anterior par of the cranium, between the orbits, in front of the sphenoid and behind the nasal bones.

Facial bones.

They form the face.

• Superior nasal bone (2): they form the nasal bridge.
• Inferior nasal concha (2): small flat bones located in the interior of the nasal cavity.
• Maxilla (maxillary bones) (2): upper part of the mouth. They support the upper teeth.
• Lacrimal bone (2): small bones located in the interior of the ocular orbit.
• Mandible: inferior part of the face. It supports the lower teeth. It is the only movable bone in the head.
• Palatine bones (2): anterior part of the palate, called bony palate.
• Vomer: central part of the nasal cavity. It is a part of the the nasal septum.
• Zygomatic (2): they form the prominence of the cheek. It is articulated with the frontal, maxilla, sphenoid and temporal.  

Neck

• Hyoid bone: it is in the anterior part of the neck, in front of the cervical vertebrae. It is the only bone that does not articulate with any other, but is supported by ligaments.

Spine (vertebral column)

The spine, the sternum and the ribs are the central bones of the trunk. The spine is made up of irregular bones called vertebrae. There are 26 vertebrae, classified according to their position:

• Cervical vertebrae (7): the neck vertebrae. They are numbered from C1 to C7. C1 and C2 are called atlas and axis respectively.
• Dorsal vertebrae (12): bones of the upper (or dorsal) part of the back. They are articulated with the ribs. They are numbered from D1 to D12.
• Lumbar vertebrae (5): vertebrae from the lumbar part of the back. The are numbered from L1 to L5.
• Sacrum: lower part of the hip. It is made from the fission of five bones.
• Tail bone: lowest bone of the spine. It is made from the fusion of four bones.

Thorax

Bones that form the rib cage. They protect some vital organs (heart and lungs) and support the upper limbs.

• Sternum: flat bone located in the middle of the thorax. It is articulated with the cartilaginous part of the ribs.
• Ribs (24): flat curved bones, articulated with the dorsal vertebrae on one side and with the sternum on the other side. There are twelve pairs of ribs that can be classified as:
• True ribs (14): they are numbered from 1 to 7. They are directly articulated with the sternum by a cartilaginous bridge. 
• False ribs (6): they are numbered from 8 to 10. They are indirectly articulated with the sternum, via a common cartilaginous piece.
• Floating ribs (4): they are numbered from 11 to 12. They are not attached to the sternum.

Shoulder articulation

The shoulder articulation connects the upper limps to the trunk.

• Clavicle (2): it is articulated with the sternum and the scapula (shoulder blade). It lays on he second rib.
• Scapula (shoulder blade) (2): these are big flat bones, slightly triangular, located in the back, between the second and the seventh ribs.

Upper limb

These bones form the arms.

• Humerus (2): long bone in the upper part of the arm, from the shoulder to the elbow. It is the biggest bone in the arm.
• Ulna (2): one of the long bones in the forearm. It runs parallel to the radius, in the internal part of the forearm, or in other words, in the side of the little finger. It is the main bone of the forearm in the elbow articulation.
• Radius (2): long bone in the outer part of the external part of the forearm or, in other words, in the side of the thumb. It is the main bone in the wrist articulation.
• Carpal bones (16): these are the bones that make up the wrist. There are eight bones per wrist. Four of them form the proximal row and are articulated with the radius. The other four form the posterior row and are articulated with the metacarpal bones:
• Proximal row:
• Scaphoid (2).
• Lunate (2).
• Triquetrum (2).
• Pisiform (2).
• Distal row:
• Trapezium (2).
• Trapezoid (2).
• Capitate (2).
• Hamate (2).
• Metacarpal bones (10): these bones make up the palm of the hand. Each one is articulated with a carpal bone on one side and with the proximal phalange on one bone on the other side. They are numbered from one to five, starting on the thumb side.  
• Phalanges (28): these are the bones that make up the digits on the hands. Each finger, apart from the thumb, has three phalanges, called proximal, intermediate and distal. The thumb only has two phalanges, called proximal and distal. These digits are numbered from one to five, starting with the thumb (each bone can be named as a number followed by its position). 

Hip articulation

• Pelvis (2): there are two pelvic bones joined to both sides of the sacral bone.  Each pelvic bone articulates with the femur. The pelvic  bones have three parts:
• Illium: it is the uppermost pelvic region. It has a flat and semicircular part called the iliac crest.
• Ischium: posterior part of the pelvis. It is horseshoe-shaped.
• Pubis: anterior part of the pelvis.

Lower limb

These are the leg bones.

• Femur (2): it is the thigh bone and is articulated with the pelvis on one side and with the tibial bone on the other side. It is the longest and heaviest bone in the human body.
• Patella (2): this bone is inserted in the knee articulation in order to support and strengthen the lever formed by the bones, muscles and tendons of this articulation.
• Tibia (2): anterior part of the leg below the knee. It also called shinbone.
• Fibula (2): posterior part of the knee below the knee.
• Tarsal bones (14): they make up the ankle. There are seven tarsal bones in each foot.
• Calcaneus (2): this is the heel bone.
• Talus bone (2): this is the upper bone of the ankle. It is articulated with the tibia.
• Navicular (scaphoid) bone (2): it is in the medial side of the foot.
• Cuboidal bone (2): it is adjacent to the navicular.
• Cuneiform bones (6): there are three cuneiform bones in each foot, numbered from first to third. The first one is articulated with the first metatarsal bone, on the big toe side.
• Metatarsal bones (10): these bones form the sole. There are five metatarsal bones in each foot, articulated with the proximal phalanges. They are numbered from 1 to five, starting with the big toe.
• Phalanges (28): each toe has three phalanges, called proximal, intermediate and distal, apart from the big toe which only has two phalanges (called proximal and distal).  They are numbered from 1 to 5 starting with the big toe (just like in the hands, each bone can be named as a number followed by its position).