The cells of multicellular organisms - plants, fungi, and animals are organized into multicellular tissues. A tissue is a group of similar cells working together to perform a common function. An organ consists of multiple tissue types. An organ system is composed of multiple organs working together. Animals contain 4 principal tissue types:
Muscle tissue can contract which leads to movement. Voluntary muscles are attached to the skeleton and are under conscious control. Involuntary muscles include heart muscle and muscles of the stomach and other organs. Muscles are responsible for
movements such as running and facial expression, and for less obvious functions such as blood vessel diameter. Muscle tissue composes the extrinsic eye muscles that move the eye. These muscles are located external to the eyeball.
Nervous tissue conducts electrical impulses within the brain, spinal cord, and nerves. Nervous tissue is composed of neurons, cells that have the ability to receive, process, and send impulses to all regions of the body. The optic nerve is located in the back of the eyeball and carries impulses of vision from the retina to the visual cortex of the brain. The retina contains receptors that convert light waves to electrical impulses. A damaged optic nerve or retina can result in blindness.
Epithelial tissue lines surfaces of the body and composes glands. Epithelial tissue lines the inside of the mouth and other entrances into the body. The lining of the stomach, blood vessels, and heart composed of epithelial tissue. Epithelial tissue covers the cornea of the eye.
Connective tissue includes blood, fat, bone, cartilage, ligaments, tendons, the dermis of skin, and other structures. Adipose tissue (fat) is found external to the eyeball to cushion the eye. The sclera (white) of the eye is composed of fibrous connective tissue.