The doctrine of viscera is a study of the physiological activities, pathological changes and their interrelationships of the body's viscera and organs. Ancient people called it “hidden image” (see “Suwen - six sections of the theory of dirty images”). “Hidden”, that is, viscera, refers to the internal organs hidden in the body; ‘elephant’, that is, the image or signs, so that the ‘hidden image’ refers to the physiological activities and pathological changes in the internal organs are reflected in the external (the surface and the five senses and nine orifices) of the signs. Five senses and nine orifices) of the signs. As can be seen, the doctrine of viscera is mainly through the observation of changes in the external signs of the animal body, to determine whether the physiological function of the internal organs is normal, that is, “look outside and know their internal”. The doctrine of internal organs is actually the physiological pathology of Chinese veterinary medicine, which is the core of the theoretical system of Chinese veterinary medicine.