Spinal cord.The spinal nerve divides into two roots.All the sensory fibres enter through the dorsal root and the motor fibres all leave through the ventral root.In the simplest reflex action,many nerve fibres,synapses and muscles are involved.All the cell bodies are concentrated in the grey matter.The withe matter consists of nerve fibres. Reflexes:bright light stimulates the light-sensitive cells of the retina.The nerve impulses in the sensory fibres from these receptors travel through the optic nerve to the brain. The brain:the medulla is concerned with regulation of the heart beat,breathing rate and temp.The cerebellum controls posture,balance and co-ordinated movement.The mid-brain deals with reflexes involving the eye.The cerebellum(the largest part)is made up of two cerebral hemispheres. The endocrine system.This system depends on chemicals called hormones wich are released from special glands,called endocrine glands,into the bloodstream.The hormones circulate round the body to the target organs.Responses of the body to hormones are much slower than responses to nerve impulses. Thyroid gland is situated in the front of the windpipe it produces thyroxine wich is formed from an amino acid and iodine.It has a stimulatory effect on the metabolic rate of all the body cells. Adrenal glands are attached to the back of the abdominal cavity.There is an outer layer called the adrenal cortex and an inner zone called the adrenal medulla it produces adrenaline.The cortex produces corticosteroids that help to control the metabolismof carbs,fats,proteins,salts and water.