The backbone, also known as the spinal column or vertebral column, is a bony structure extending along the dorsal side of the body. It provides a central axis for the skeleton, supporting the organism, allowing movement, and protecting the spinal cord. Its presence classifies a large group of animals.
Anatomy of the Backbone
The backbone is not a single bone but a series of individual bones called vertebrae. In humans, there are 33 vertebrae: the upper 24 are articulating and separated by intervertebral discs, while the lower nine are fused to form the sacrum and coccyx. These vertebrae are organized into five regions: seven cervical (neck), twelve thoracic (upper back), five lumbar (lower back), five sacral (fused), and four coccygeal (fused). Each vertebra consists of an anterior vertebral body and a posterior vertebral (neural) arch, forming a central opening known as the vertebral foramen.
When these foramina align, they create a continuous tube, the spinal canal, which protects the spinal cord. Between each movable vertebra lies an intervertebral disc, a soft cushion that absorbs shock. Each disc has a strong outer ring of fibers, the annulus fibrosus, and a soft, jelly-like center, the nucleus pulposus. Ligaments and tendons connect and support the vertebrae, providing stability and allowing for controlled movement. The spine also exhibits natural curves—a lordotic curve in the neck and lower back, and a kyphotic curve in the upper back—which help in bearing weight and maintaining balance.
Functions of the Backbone
The backbone serves multiple functions, providing structural support for the body. It forms the main axis of the skeleton, allowing animals to maintain an upright posture and resist gravitational forces. This is particularly evident in the lumbar region, which bears most of the body’s weight and the stress of daily activities like lifting. The interconnected vertebrae, along with surrounding muscles and ligaments, create a stable framework for the torso, head, and limbs.
Another function of the backbone is the protection of the spinal cord. The spinal cord, a column of nerve fibers, runs through the spinal canal formed by the stacked vertebrae. This bony enclosure shields the nerve tissue from external damage, as the spinal cord transmits messages between the brain and the rest of the body. Spinal nerves branch out from the cord through openings between the vertebrae, facilitating communication throughout the organism.
Beyond support and protection, the backbone enables movement and flexibility. While movement at a single vertebral level is limited, the collective motion of multiple intervertebral discs and facet joints allows for bending, twisting, and turning. The cervical spine, for instance, permits head tilting and rotation, while the lumbar spine facilitates bending and lifting. This segmented design, combined with the cushioning effect of the intervertebral discs, also helps the spine act as a shock absorber, reducing the impact of movements like walking and running.
Animals with Backbones
Animals are categorized into two groups based on the presence or absence of a backbone. Vertebrates are animals that possess a backbone, a defining characteristic that gives the subphylum Vertebrata its name. This group includes many species, ranging from tiny fish to large mammals. Vertebrates inhabit diverse environments, including aquatic, terrestrial, and aerial habitats.
The classification of vertebrates includes several classes. Mammals, such as humans, bats, and whales, are warm-blooded and give birth to live young, nourishing them with milk. Birds, including eagles and owls, are characterized by feathers and wings, with many species capable of flight, and they lay hard-shelled eggs. Fish, encompassing bony fish like salmon and cartilaginous fish like sharks, are cold-blooded, breathe with gills, and live entirely underwater.
Reptiles, such as snakes, turtles, and crocodiles, are cold-blooded, possess dry, scaly skin, and lay leathery eggs. Amphibians, including frogs, toads, and salamanders, are small vertebrates that require moist environments and lay eggs in water. In contrast, animals without backbones are known as invertebrates, a group that includes insects, worms, and jellyfish.