The nerves that are distributed to the viscera, heart, blood vessels and glands are called visceral nerves, which are further divided into visceral motor nerves and visceral sensory nerves. Of these, the visceromotor nerves, in turn, are divided into the sympathetic and parasympathetic nervous systems. The nerves that generally increase the excitability of organs are called the sympathetic nervous system, and the low-level centers are located in the gray matter lateral horns of the thoracic 1 to lumbar 3 spinal cord ganglia, which means that the sympathetic nerves throughout the body all emanate from the gray matter measurement horns of the thoracic 1 to lumbar 3 spinal cord ganglia. The emitted fibers are called preganglionic fibers and are replaced by neurons in 22-24 pairs of paravertebral and prevertebral ganglia before emitting postganglionic fibers. The postganglionic fibers have three routes to the innervated organs: 1) they return to the spinal nerves to distribute to the blood vessels, sweat glands, and erector spinae of the trunk and extremities; 2) they wrap around the arteries to form plexuses and follow them to the innervated organs; 3) they travel independently and directly to the innervated organs.