The spinal cord is the medium by which motor and sensory impressions are conducted to and from the brain. It receives impressions from all parts of the body by means of the sensory nerves, and conveys them to the brain, where they produce sensation. It conducts the command of the brain to the voluntary muscles by the motor nerves, and thus causes movement. In all this the brain is the power, and the cord the conductor ; but, as well as this, the cord has a special function —that of reflex action. If the connection of the cord with the brain be severed, and the skin supplied by afferent nerves below the injured part be irritated, movement will be produced in the part supplied by efferent nerves from the same part of the cord. This is reflex action.

The irritation of an afferent nerve connected with the spinal cord sends an impulse to a nerve centre (gray matter) in the cord, and without communication with the brain this nerve centre has the power of sending back an impulse by an efferent nerve (or nerves), producing contraction of the muscle or muscles in which the efferent fibres terminate. A person paralyzed from spinal injury will thus draw up his legs and kick out when the soles of his feet are tickled. Both in the brain and cord active power is confined to the gray matter.

The brain itself gives rise to reflex actions—actions which take place without the will or consciousness of the individual, such as instinctive shrinking from a threatened blow, or blinking from a flash of light.