According to acupuncture theory, the sympathetic nervous system is the complex of nerves that control unconscious activity. The lungs breathe, the heart sends blood by regular beats into the arteries, the stomach digests, the transformation of food into energy is controlled so the body temperature remains constant, the glands increase or decrease their output. This autonomic nervous system is distinguished from the central nervous system, which is controlled consciously. But the borderline between the two systems is not clearly defined, A certain number of bodily functions that require sustained attention and will power in early infancy, such as walking, can be performed in maturity without the permanent control of the higher nerve centers.
Besides, there are certain diseases in which the patients organism can perform habitual acts without his being aware of it. The sympathetic nervous system itself comprises two systems whose modes of action oppose and complement one another. The first is the orthosympathetic system, more briefly known as the sympathetic system. It simultaneously alerts all the structures when a danger presents itself; the heart beats faster, arterial pressure mounts, blood flows to the muscles, the sugar level rises in the blood, the pupils dilate and the eyelids open wider. The organism is preparing itself for struggle.
The second is the parasympathetic system, which confines its activity to discrete, local effects. It restores the organism from the process of deterioration. The heart slows down its pulse, arterial pressure drops, intestinal absorption increases, the retina is protected against an excess of light, the intestine and bladder are relaxed. These two antagonistic systems seem to watch over one another. Health depends on maintaining the balance between their vital energies. In acupuncture theory, when one of these systems appears to predominate over the other, that system is said to be in hypertonia and the other in hypotonia.
The coexistence of these two simultaneously opposed systems, and complementary, is a verified anatomical fact, and shows affnity with a philosophical notion called the association of contraries. Chinese thought, by strict application of this principle to the analysis of the human body and its functions, conceived of acupuncture.