Skeletal System

 Skeletal System

Skeletal system
The Skeletal system in an adult body is made up of 206 individual bones.
 These bones are arranged into two major divisions : the axial skeleton and the appendicular  skeleton..
 The axial skeleton runs along the body's  midline  axis and is made up of 80 bones in the following  regions:
 Skull
Hyoid
Auditory ossicles
 Ribs
Sternum
Vertebral column

 The appendicular  skeleton is made up of 126 bones in the following regions:
 Upper limbs
 lower limbs
Pelvic girdle
 pectoral[ shoulder] girdle .
 
 Skull
 The  skull is composed of 22 bones those  are fused  together except for the mandible . These 21 fused bones are separate in children to allow the skull and  brain to grow, but   fuse to give added strength  and protection  as an adult .  The mandible  remains as a movable  jaw bone and forms the only movable joint in the skull with the temporal bone . The bones of the superior portion of the  skull are known as the  cranium  and protect the brain  from damage . The bones  of the inferior  and anterior  portion of the skull are known as  facial  bones and support the eyes, nose, and mouth . Hyoid  and Auditory Ossicles.
 The  hyoid  is a small , U-shaped bone  found just inferior to  the mandible .The hyoid is the only bone in the body that does not form a joint  with any other bone -it is a floating  bone.The hyoid's function  is to help hold the trachea open and to form a bony connection  for the tongue muscles .
 The  malleus, incus, and stapes -known collectively as the auditory  ossicles - the  smallest bones  in the body .Found in a small cavity  inside of the temporal bone, they serve to transmit and amplify sound from the eardrum to the inner   ear.
 Vertebrae
  Twenty -six vertebrae  form the vertebral column  of the human body. They are named by region :
 Cervical{neck}-7  vertebrae
Thoracic{chest} _12 vertebrae
Lumbar {lower back}-5 vertebrae
 Sacrum-1 vertebra
 Coccyx {tailbone}-1 vertebra
 with the  exception of the singular sacrum and coccyx , each vertebra is named for the first letter of its region and its position along the superior - inferior  axis .
 For example , the most superior thoracic  vertebra is called T1 and the most inferior is called T12.
  Ribs and Sternum
  The sternum, or breastbone  ,is a thin, knife -shaped bone  located along the midline  of the anterior side of the thoracic region of skeleton .The sternum connects to the ribs by thin bands of cartilage  called the costal cartilage . 
 There are 12 pairs of ribs that together with the sternum form the ribcage  of the  thoracic region .The first seven ribsare known as ''true ribs'' because they connect the thoracic vertebrae  directly   to the sternum through their own band of costal cartilage . Ribs 8,9 and 10 all connect to the sternum through cartilage that is connected to the cartilage of the seventh rib ,so we consider these to be ''false  ribs. Ribs 11 and 12 are also false ribs , but are also considered to be ''floating ribs'' because they  do not have any  cartilage  attachment to the  sternum at all . 
 Pectoral Girdle and Upper Limb
The pectoral girdle  connects the upper limb[arm] bones  to the axial skeleton and consists of the left and right  clavicles and left and right scapulae. The humerus  is the bone of the upper arm.It forms  the ball and socket joint of  the shoulder  with the scapula and forms the elbow joint with the lower arm bones .The radius  and ulna are the  two bones of the forearm .The ulna  is on  the  medial side of the forearm  and forms a hinge  joint  with the  humerus  at the  elbow . The  radius  allows the forearm  and hand to turn over at the wrist joint .
 The  lower arm  bones  form the wrist joint with the  carpals, a group of eight small bones that  give added flexibility  to the wrist .
 The carpals are connected to the five  metacarpals  that form the bones of the  hand and connect to each of the fingers . Each finger has three bones known as phalanges , except for the thumb , which  only  has  two phalanges.

 Pelvic Girdle   and  lower limb
 Formed by  the left  and  right  hip   bones , the pelvic girdle  connects  the lower limb{leg} bones  to the axial skeleton.

 The femur   is  the largest  bone in the  body  and the only bone  of thigh { femoral} region . The femur  forms  the  ball and socket  hip joint  with  the hip bone  and the forms the  knee joint with the tibia  and patella. Commonly  called  the  kneecap , the patella is  special  because it is one of the  few bones  those are not  present  at birth . The  patella  forms  in early  childhood  to support  the  knee  for walking  and  crawling .
 The tibia  and  fibula  are  the bones  of the  lower leg . The  tibia is much larger  than the  fibula  and  bears  almost all of the  boy's weight . The  fibula  is  mainly  a muscle  attachment  point  and is   used to help maintain  balance . The  tibia  and  fibula  form the  ankle  joint  with the  talus , one of the  seven tarsal bones in the foot .
 The  tarsals  are  a group of seven small bones  that form the  posterior  end  of the foot  and heel . The tarsals  form joints  with the  five long metatarsals  of the foot . The each of the metatarsals  forms  a joint with one of the  set of phalanges , except  for the big toe, which  only has two phalanges .
 Microscopic  Structure of  Bones 
  The skeleton makes up about 30-40% of  an adult's  body mass .The skeleton's  mass is made up of nonliving  bone matrix  and many tiny  bone cells . Roughly  half of the  bone   matrix's mass is water , while  the  other half  is collagen protein and solid  crystals  of calcium carbonate  and calcium  phosphate .
 Living bone  cells  
 are found  on the edges of  bones and in small cavities  in side of  the bone  matrix .   even though  these cells  make up  very little  of the total bone  mass , they have  several  very important  roles  in the functions of the skeletal system. The bone  cells allow  bones to grow  and develop
 Be  repaired following  an injury or daily wear
 Be  broken down to release  their  stored  minerals.
 Types of bones 
 All of the bones of the body  can be broken down into five types : long , short ,flat , irregular , and  sesamoid .
 Long = long bones are  longer than they  are wide and  are the major  bones  of the limbs . Long bones  grow more  than the other classes  of bone throughout  childhood  and so are responsible  for the hulk  of our  height  as adults. A hollow  medullary  cavity  is found in the center  of long bones  and serves as a storage area for bone  marrow . Examples of long  bones include  the  femur, tibia , fibula , metatarsals , and phalanges .
 Short . Short bones are  about  as long as  they are wide  and often cubed  or round  in shape . The carpal bones  of the wrist and the tarsal  bones of the foot  are examples  of short bones .
 Flat . Flat bones  vary  greatly  in size  and shape ,but have the common feature  of being very thin  in one direction . Because  they are thin , flat  bones do not  have a  medullary  cavity  like the long bones. The frontal, parietal, and occipital  bones of the cranium - along with the ribs  and the hip bones - are all examples of flat bones 
 Irregular . Irregular bones  have a  shape  that does not fit  the pattern  of the long , short , or flat bones , The vertebrae sacrum , and coccyx  of the  spine- as well as  the sphenoid ,  ethmoid  , and  zygomatic  bones  of the skull - are all irregular  bones  .
 Sesamoid .  The sesamoid  bones  are formed  after birth inside of  tenbons  that  run across joints . Sesamoid  bones  grow to protect  the tenbons  from  stresses  and strains  at the joint  and can help  to give a mechanical  advantage  to muscles  pulling  on the tenbons . The  patella  and the pisiform bone  of the carpals  are the only   sesamoid bones  that are counted  as part  of the 206 bones  of the body . Other sesamoid bones  can form in the joints  of the hands , feet,but  are not  present in all people .
 Parts of Bones
 The long bones  of the body  contain  many  distinct region  due to the way  in which  they  develop  . At birth , each long bone is  made of three  individual bones  separated  by hyaline cartilage . Each end bone  is called  an epiphysis  { epi=on;physis=to grow } while the middle bone is called  diaphysis {dia=passing through }. The epiphyses  and diaphysis  grow towards  one another  and eventually fuse into one bone . The region  of growth and  eventual  fusion  in between the  epiphysis  and disphysis is called the metaphysis {meta= after} . Once the long bone  parts  have fused together , the on;y  hyaline  cartilage  left in the bone  is  found as articular  cartilage  on the ends of the bone that  form joints  with other bones . The  articular  cartilage  surface   acts  as a shock  absorber  and gliding  surface  between the bones  to  facilitate  movement at the joint . Looking at a bone  in cross section, there are several distinct layered regions  that make up a one  .The outside of a bone is covered  in a thin layer of dense  irregular  connective tissue  called  the  periosteum . The periosteum  contains  many strong collagen fibers  that are used to firmly anchor tenbons and muscles  to the bone for movement . Stem cells  and osteoblast  cells  in the periosteum  are involved in the growth  and the  repair  of the outside  of the bone  due to stress  and  injury . Blood  vessels present  in the  periosteum  provide  energy  to the cells  inside of the bone  . The periosteum also contains  nervous  tissue  and  many nerve  endings  to give bone  its  sensitivity   to pain when injured . 
 Deep to the compact bone   layer is  a  region  of  spongy  bone  where the bone  tissue  grows  in thin columns  called   trabeculae  with spaces foe red  bone  marrow  in between . The  trabedulae  grow  in a  specific  pattern  to resist outside  stresses  with the lest amount  of mass  possible , keeping  bones   light  but strong . Long bones have  a spongy  bone  on their  ends but  have a hollow  medullary  cavity  in the middle m of the  diaphysis . The medullary  cavity  contains  red bone  marrow  during  childhood , eventually  turning  into yellow  bone  marrow  after puberty . Articulation
 An articulation , joint , is  a point  of  contact  between  bones , between  a bone and  cartilage  ,or  between  a bone  and a tooth . Synovial joints  are the most  common type  of articulation  and feature  a small  gap  between the bones . This gap  allows  a free range  of motion  and space  for synovial  fluid  to lubricate  the joint .  Fibrous  joints  exist  where  bones  are very  tightly  joined  and offer  little  to no movement  between the bones . Fibrous  joints also hold  teeth in their  bony sockets . Finally  cartilaginous  joints  are  formed  where bone meets cartilage  or where  there  is a layer of  cartilage  between two bones . these joints  provide  a small  amount  of  flexibility in the joint  due to the  gel-like  consistency   of cartilage .