16. Skeleton and Movements - part 04 - Skeletal System

 

16. Skeleton and Movements - part 04 - Skeletal System

Skeletal System :

• Multicellular animals need support to maintain body structure.  
• Skeletal structures When present on outer surface of the body are called exoskeleton.
• Skeletal structures When present on inside  the body are called endoskeleton .
• Bones and cartilage form major endoskeletal components. 
• Exoskeletal components include chitinous structures, nails, horns, hooves, scales, hair,etc;
• Our skeletal system is made up of cartilage and bone; which together form the framework of the body. 
• Cartilage is slightly pliable while bones have hard matrix. 
• Bones form the framework of our body and thus provide shape which giveus our identity. 
• They protect delicate organs.
  
• Joints between bones help in movement and locomotion. 
• Bones provide firm surface for attachment of muscles. 
• They are reservoirs of calcium and one important site for haemopoiesis.
• Endoskeleton of an adult human consists of 206 bones which can be grouped into two principle divisions; axial and appendicular skeleton.
• Bones of axial skeleton lie along the longitudinal axis of human body.
• Bones of appendicular skeleton include bones of fore limb, hind limb and girdles.
• Girdles are the bones that connect the limbs to the axial skeleton.

Group of skeleton :
A. Axial Skeleton :
  Skull :

• Made up of 22 bones
  
• Skull is located at superior end of vertebral column. 
• Consists of two main sets of bones, cranium or brain box and facial bones. 
• Bones are joined by fixed or immovable joints except for lower jaw.

Cranium :

• It is made up of four median and two paired bones.

Frontal bone : Median bone (Unpaired)

• forms forehead, roof of orbit (eye socket) and most of the anterior part of cranium. 
• It is connected to two parietals, sphenoid and ethmoid bone.

Parietal bones :

• Paired bones
• form roof of the cranium and greater portion of sides of the cranium

Temporal bones : 

• Paired bones situated laterally just above the ear on either side. 
• Each temporal bone gives out zygomatic process that joins zygomatic bone to form zygomatic arch. 
• Just at the base of zygomatic process is mandibular fossa, a depression for madibles (lower jaw bone) that forms only movable joint of skull. 
• This bone harbours ear canal that directs sound waves into the ear.
• Processes of temporal bones provide points for attachment for various muscles of neck and tongue.

Occipital bone : 

• Present at the back of the head.
• This single bone forms posterior part and most of the base of cranium. 
• Inferior part of this bone shows ‘Foramen magnum’ the opening through which medulla oblongata connects with spinal cord. 
• On either side of foramen magnum are two prominent protuberances called ‘Occipital condyles’. 
• These fit into the corresponding depressions present in 1st vertebra.

Sphenoid bone : 

• Median bone present at the base of the skull that articulates with all other cranial bones and holds them together.
• This butterfly shaped bone has a saddle shaped region called sella turcica. 
• In this hypophyseal fossa, the pituitary gland is lodged.

Ethmoid bone : 

• This median bone is spongy in appearance. 
• It is located anterior to sphenoid and posterior to nasal bones. 
• It contributes to formation of nasal septum and is major supporting structure of nasal cavity.

Sutures, a type of immovable joints:

• In skull there are many sutures present; four prominent ones are;

1. Coronal suture : Joins frontal bone with parietals.
2. Sagittal suture : Joins two parietal bones.
3. Lambdiodal suture : Joins two parietal bones with occipital
bone.
4. Lateral/squamous sutures : Joins parietal and temporal bones on
lateral side.

Facial Bones :

•  Fourteen facial bones give characteristic shape to face. 
  
• Growth of face stops of the age of 16. 
• Facial bones are as under :

Nasals : Paired bones form bridge of nose.
Maxillae :

• Upper jaw bones, paired bones that join with all facial bones except mandible.
• Upper row of teeth are lodged in these.

Palatines : Paired bones, form roof of buccal cavity or floor of nasal cavity.
Zygomatic bones : Commonly called cheek bones.
Lacrimal bones : 

• Smallest of the facial bones. 
•  Form medial wall of each orbit.
• Have lacrimal fossa that houses lacrimal sacs. 
• Sacs gather tears and send them to nasal cavity.

Inferior nasal conchae :

• Form part of lateral wall of nasal cavity. 
• Help swirl and filter air before it passes to lungs.

Vomer : Median, roughly triangular bone that forms inferior portion of nasal septum.
Mandible :

• Median bone that forms lower jaw. 
• Largest and strongest facial bone. 
• Only movable bone of skull. 
• It has curved horizontal body and two perpendicular branches i.e. rami. 
• Help in attachment of muscles.
• It has lower row of teeth lodged in it.
  

Hyoid bone : 

• It is a ‘U’ shaped bone that does not articulate with any other bone. 
• It is suspended from temporal bone by lingaments and muscles. 
• It is located between mandible and larynx. 
• It has horizontal body and paired projections called horns. 
• It provides site for attachment of some tongue muscles and muscles of neck and pharynx.

Something interesting :

• If Police suspect strangulation, they carefully inspect hyoid bone and cartilage of larynx. 
• These get fractured during strangulation. 
• Various such investigations are done in case of suspicious death of an individual where ossification of sutures in skull, width of pelvic girdle, etc. are examined to find out approximate age of victim or gender of victim, etc.

Ear ossicles : 

• Three tiny bones namely malleus, incus and stapes together called ‘ear ossicles’
• Present in each middle ear.

Vertebral Column :

• Human backbone or vertebral column is made up of a chain of irregular bones called vertebrae.
• It consists of 33 vertebrae during childhood. 
• In adults, five sacral vertebraen fuse to form one sacrum and four coccygeal vertebrae fuse to form single coccyx.
• Total number of bones are 26.
  
• There are four curvatures in human spine, cervical and lumbar curves are secondary and convex whereas thoracic and sacral curvatures are primary and concave. 
• Curvatures help in balancing in upright position, absorb shocks while walking and also protect vertebrae from fracture.

Five types of vertebrae in human spine :

• 7 cervical (neck)
• 12 Thoracic (chest) 
• 5 lumbar (abdominal)
• 5 sacral (hip region, fused in adults forming 1 sacrum) and 4 coccygeal (fused to form vestigial tail bone called coccyx).

Typical Vertebra :

• Each vertebra has prominent central body called centrum. 
• Centra of human vertebrae are flat in anterio-posterior aspect. Hence human vertebrae are amphiplatyan. 
• From either side of centrum are given out two short, thick processes which unite to form an arch like structure called neural arch, posterior to centrum. 
• Neural arch forms vertebral foramen which surrounds the spinal cord.
• Vertebral foramina of all vertebrae form a continuous 'neural canal'. 
• Spinal cord along with blood vessels and protective fatty covering passes through neural canal. Point where two processes of centrum meet, neural arch is drawn into a spinous process called neural spine. 
• From the base of neural arch, two articulating processes called Zygapophyses are given out on either side. 
• The anterior are called superior and posterior called inferior zygapophyses. 
• In a stack of vertebrae, inferior zygaphyses of one vertebra articulates with superior zygapophyses of next vertebra.
• This allows slight movement of vertebrae without allowing them to slip off. 
• At the junction of zygapophyses, a small opening is formed on either side of vertebra called intervertebral foramen that allows passage of spinal nerve. 
• From the base of neural arch, lateral processes are given out called transverse processes.
• Neural arch, neural spine and transverse processes are meant for attachment of muscles.

Atlas vertebra : 

• This is a ring like 1st cervical vertebra. 
• It consists of anterior and posterior arches. 
• It does not have centrum and spinous process.
• Transverse processes and transverse foramina are large. 
• Vertebral foramen is large and divided into two parts by transverse ligament. 
• Spinal cord passes through anterior compartment. 
• Anterior zygapophyses, are replaced by facets for attachment with occipital condyle of skull that forms ‘Yes Joint’.

Axis vertebra : 

• This is the second cervical vertebra. 
• Centrum of this vertebra gives out tooth-like ‘Odontoid Process’. 
• This process fits into the anterior portion of vertebral foramen of Atlas vertebra forming pivot joint, also called‘No joint’.

Typical cervical vertebrae : 

• Vertebrae number 3 to 6 are called typical cervical vertebrae. 
• They show short centrum and bifid spinous process.
• Transverse processes of these vertebrae are reduced; each having large vertebrarterial canal at it’s base for passage of vertebral artery.

7th cervical vertebra ( Vertebra prominens) :

• It is the largest cervical vertebra where neural spine straight.

Thoracic vertebra : 

• These are twelve in number and found in chest region. 
• Centrum of thoracic vertebrae is heart shaped and all processes are well developed. 
• Except for vertebrae number11, 12; transeverse process of other thoracic vertebrae show facets for attachment with ribs.

Lumbar vertebra : 

• There are five lumbar vertebrae. 
  
• These are well developed vertebrae that exhibit all characters of a typical vertebra.
• Centrum is kidney shaped.

Sacrum :

• Sacrum is a triangular bone formed by fusion of five sacral vertebrae. 
• It is located in pelvic cavity between two hip bones. 
• Anterior end of sacrum is broad and posterior end is narrow. 
• Vertebral foramina that are formed by fusion of vertebrae can be seen. 
• Reduced neural spines can be observed projecting from dorsal aspect of sacrum. 
• It gives strength to pelvic girdle.

Coccyx : 

• Coccyx is formed by fusion of four coccygeal vertebrae. 
• It is reduced and does not show vertebral foramina and spinous processes. 
• Transverse processes of coccygeal vertebrae are reduced. 
• It is a triangular bone.

Thoracic cage :

• It consists of twelve thoracic vertebrae
• twelve pairs of ribs and breast bone, the sternum.

Sternum : 

• It is a flat, narrow bone, around 15 cms in length. 
• It is placed medially in anterior thoracic wall (chest region). 
• It consists of three distinct parts-manubrium, body and xiphoid processes.
• Manubrium shows two notches on anterio-lateral side for attachment with clavicle of each side. 
• It also shows two notches on each of the lateral side for attachment of first two pairs of ribs.
• Body of sternum is a flat bone thatshows five notches on lateral aspect which are meant for direct or indirect attachment of ribs. 
• Ribs are attached to sternum by means of cartilaginous extensions called coastal cartilages. 
• Xiphoid process is lowermost part of sternum which is cartilaginous initially and gets ossified in adults. 
• It provides space for attachment of diaphragm and abdominal muscles.

Rib : 

• A rib is a 'C' shaped bone that is attached to respective thoracic vertebrae on dorsal side. 
  
• Twelve pairs of ribs are attached to twelve thoracic vertebrae. 
• For this attachment, posterior ends of ribs have two protuberances namely the head and tubercle. 
• The head of rib attaches to facet formed by demifacets of adjacent thoracic vertebrae at the base of transverse processes. 
• Tip of transverse processes of these vertebrae also have facets for attachment of ribs where tubercles of ribs are attached On the ventral side.
• the ribs may or may not attach to the sternum.

Types of ribs :

i. True ribs : 

• First seven pairs of ribs are attached directly to the sternum by means of their coastal cartilages.

ii. False ribs : 

• Coastal cartilages of ribs no. 8, 9 and 10 are attached to rib number 7 on either side and not directly to the sternum.
• These are called false ribs.

iii. Floating ribs : 

• Last two pairs of ribs have no ventral connection. Hence are called floating ribs. 
• Space between ribs is called intercoastal space. 
• Ribs provide space for attachment of intercoastal muscles.

Something interesting :

• Approximately 8 % of humans have an extra pair of ribs attached to the lumbar vertebra. 
• Such a rib is found in some types of gorillas. 
• Hence 13th pair of ribs is called gorilla rib.

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