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Human Skeleton and Locomotion

Skeletal System

Joints

Joints are the points or places in the body where two or more bones attach with each other, giving a framework to the skeleton. The study of joints is known as arthrology.

The bones attach with each other with the help of the bands of tough and flexible connective tissues called ligaments.

Significance of Joints

  • They help in the voluntary movements of different body parts.
  • They bring elasticity and flexibility to the rigid skeleton.
  • They are protective and act as shock absorbers.

Joints can be classified on the basis of their degree of movement and structure.




Synarthrosis/Fibrous/Fixed Joints

Bones are articulated at these joints by a thin layer of white fibrous connective tissue.
As the name reflects, fixed joints do not allow the movement of articulating bones in any direction.
These are meant for the growth and moulding of bones during the birth of a child.
The line of fusion of two bones at these joints is called suture. 

I. Sutures of Skull

Sutures are found in skull bones. The major function of the skull is to protect the brain; this is ensured by the repeated interlocking of the skull bones at sutures. This repeated interlocking makes the the margins of the articulated bones serrated, due to which these joints are also called serrate joints. 
Some prominent sutures between the cranial bones of the skull are:
Coronal suture - It is present between the frontal bone and the parietal bone.
Sagittal bone - It is present between two parietal bones.
Lambdoidal suture - It is present between parietal and occipital bones
Lateral suture - It is present between temporal and parietal bones



II. Syndesmoses  
 
Syndesmosis is a kind of joint containing a fibrous connective tissue connecting two bones such as tibia and fibula and radius and ulna.



III. Gomphosis

Many short periodontal ligaments connect thecodont teeth to the sockets of the jaw bones. Such joints are called gomphosis joints.



Amphiarthrosis/Partially Movable/Cartilaginous Joints

These joints are neither fixed nor freely movable, thus allowing only a partial movement of bones. The line of fusion between articulating bones is known as symphysis or synchondrosis.

I. Symphysis

In symphysis, the connecting material is fibrocartilage. The presence of numerous white fibres of collagen makes it strong but flexible. The two pubic bones of the pelvic girdle are joined to each other with fibrocartilage, which provides the joint flexibility, allowing it to perform slight movement in response to twist, compression or bending. This joint in the pelvic girdle helps in increasing the size of the birth canal during parturition.
 


II. Synchondrosis

Hyaline cartilage, which is very soft and elastic,  acts as a connecting material for synchondrosis joints.
The epiphyseal plate, which is found between the epiphysis and diaphysis of long bones, is an example of synchondrosis joint . The main function of this joint is to provide space to the long bones to grow. This joint is present only in children and gets ossified in adults.



III. Intervertebral Joints

They are found between adjacent vertebrae. The major functions of these fibrous, cartilaginous joints are shock absorption and spinal cord protection.



Diarthrosis (Synovial Joint)

Synovial joints are the most evolved joints; they provide free movement.

Structure of Synovial Joint

It consists of ligaments, synovial fluid, synovial cavity, synovial membrane and capsule. The articulating surfaces of joined bones are covered by hyaline cartilage.

(i)  Ligaments: Synovial joints have capsular ligaments along with many other accessory ligaments. They make joints stronger and protect the bones from dislocation.
(ii)  Hyaline cartilage: Its main function is to avoid friction between joined bones.
(iii) Synovial fluid: The fluid reduces friction a…

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