A group of cells that perform a specific function is called tissue.
Animal tissues:- Tissues found in animals are called animal tissues. Important type of animal tissues are epithelial tissue, connective tissue, muscle tissue, and nervous tissue.
1) EPITHELIAL TISSUE
Epithelial tissue covers the outside of body and lines organs and cavities. The cells in this tissue are very closely packed together. This tissue has many types on the basis of the shape of cells as well as the number of cell layers. Some types include:
Squamous epithelium consists of a single layer of flat cells. It is found in lungs, heart and blood vessels. Here, it allows the movement of materials across it.
Cuboidal epithelium consists of a single layer of cube-shaped cells. It is found in kidney tubes, small glands etc. where it makes secretions.
Columnar epithelium has elongated cells. It is found in alimentary canal, gall bladder etc. where it makes secretions.
Ciliated columnar epithelium has elongated cells with cilia. It is present in trachea and bronchi and propels mucous.
Stratified squamous epithelium has many layers of flat cells. It is present in the lining of oesophagus and mouth and also over the skin. It protects the inner parts.
- connective tissue:- These serves as connecting function. These are major supportive tissues. There is two types of connective tissues,. A) soft connective tissues like fatty tissues. B) Hard connective tissues like bones and cartilages. Blood is also connective tissue. The adipose tissue (found around kidneys, under skin, in abdomen etc.) is also a type of connective tissue. It provides energy and support organs.
Muscle tissue consists of bundles of long cells called muscle fibers. It is the most abundant tissue in an animal. The cells of this tissue have ability to contract. There are three kinds of muscle tissue.
Skeletal muscles or striated muscles are attached to bones. Their cells are striated (striped) and contain many nuclei. They are responsible for the movements of bones.
Smooth muscles are found in the walls of alimentary canal, urinary bladder, blood vessels etc. They contain smooth (non-striated) cells, each with a single nucleus. They are responsible for the movement of substances.
Cardiac muscles are present in the wall of heart. Their cells are also striated but there is a single nucleus in each cell. They produce heartbeat.
Skeletal muscles are voluntary in action i.e. their contraction is under the control of our will. Smooth and cardiac muscles are involuntary in action i.e. their contraction is not under the control of our will.
4) NERVOUS TISSUE:- Nervous tissue is specialized to react to stimuli and to conduct impulses to various organs in the body which bring about a response to the stimulus. It forms communication system. These are mainly composed of neurons and found in brain spinal cord and nerves.