The small intestine lives in the abdomen and distributes up and down the umbilicus. The function of small intestine is to clear turbidity separately. The hand and temple belong to the small intestine and the collaterals are in the heart. So the small intestine and the heart are both inside and outside.

The small intestine is located in the lower part of the stomach, its upper end is connected with the pylorus of the stomach, and its lower end is connected with the large intestine at the gate. It is the longest part of the digestive tract and its shape is flat. The upper part of the small intestine is jejunum and the lower part is ileum. Jejunum and ileum are fixed on the posterior wall of abdominal cavity through lipid membrane. The description of the anatomical position, shape and length of small intestine in TCM is basically consistent with the understanding of modern anatomy.

The physiological function of small intestine is mainly clear and turbid, which is an important place for Shui Gu to digest and absorb nutrition.

The main substances are clear and turbid respectively. "Shousheng" means "Shousheng". The small intestine is the official of receiving excess, which describes Shui Gu's digestive function, and its small intestine can bear the digestion of the stomach. After the small intestine receives Shui Gu which has been digested by the stomach, after further digestion and separation, Shui Gu's subtle qi is biochemically transformed into substances easily absorbed by the human body. The main substance of small intestine refers to the function of small intestine to digest Shui Gu. This physiological function is characterized by turbidity. "Qing" refers to substances containing nutrients. "Turbid" refers to substances without nutrients. The function of clearing turbidity alone can be roughly divided into two processes. First, the upper segment of the small intestine: Shui Gu, which goes down from the stomach to the small intestine, is further digested and separated by the small intestine, that is, Shui Gu's essence is absorbed and transmitted to the five internal organs through the clearing function of the spleen. Turbidity, that is, the residue of water grains and water, continues to move down and spread to the lower part of the small intestine under the action of reducing stomach qi turbidity. Secondly, it is the lower segment of the small intestine: the dregs of Shui Gu are conducted to the lower segment of the small intestine. At the diaphragmatic gate where the small intestine meets the large intestine, the small intestine has the function of clearing turbidity alone, so that the water in the dregs seeps into the bladder and is discharged through the front of the vagina, and the dregs in the dregs are discharged through the large intestine and the rear of the vagina. To sum up, the main substances in the small intestine, which can clear turbidity respectively, mainly mean that the nutrients in the dregs of Shui Gu and Shui Gu are differentiated from the small intestine, so the small intestine has the functions of digestion, absorption and excretion. If the small intestine is sick, it will not only affect the function of digestion and absorption, but also cause abnormal conditions such as diarrhea or dry stool. Secondly, the small intestine is the main fluid. Small intestinal fluid mainly refers to the small intestine controlling urination. Small intestine plays a certain role in the metabolism of water and liquid in the process of clearing turbidity. Its function is mainly manifested in the formation of urine and the amount of water in stool. Therefore, the small intestine is the main fluid.

Because the small intestine and the heart are both external and internal, excessive heart fire will transfer heat to the small intestine and cause oliguria, hematuria and even burning urine. The compounds in the small intestine are related to the rise and fall of the spleen and the evaporation of triple energizer, so when the spleen yin is insufficient or the triple Jiao Yang is insufficient, it will affect the main liquid function of the small intestine, resulting in too much or too little water in the stool, which will lead to diarrhea or dry stool. The function of small intestine depends on the health of Yang Qi and the turbidity of heart fire.

(1) Hand Taiyang Meridian: The small intestine meridian is called Hand Taiyang Meridian. The sun of the hand passes from Shaoze point at the lateral end of the little finger of the hand, along the lateral side of the back of the hand to the wrist, out of the styloid process of the ulna, straight along the rear edge of the forearm, between the olecranon of the ulna and the medial epicondyle of the humerus, along the rear edge of the lateral side of the upper arm, around the scapula from the brow joint, then meets the spine on the shoulder, enters the pelvic cavity downwards, touches the heart, passes through the esophagus and passes through the diaphragm. A branch of it, from the lack of basin up along the neck, through the cheek, to the outer corner of the eye, and finally into the ear, to the Tinggong point. Its other branch goes up from the cheek to the zygomatic skull, then reaches the nose, reaches the mystery point of the inner corner of the eye, and meets the bladder meridian of the foot sun. (see photo)

(2) Physiology of the Hand Sun Meridian: The physiological function of the Hand Sun Meridian mainly includes two paintings. On the one hand, it plays the role of running large intestine blood gas. Because the hand sun originates from the lateral end of the little finger and belongs to the small intestine when it runs in the abdomen, it mainly runs the blood gas of the small intestine. Therefore, there is a saying in Lingshu Jing that "the small intestine holds the pulse of the sun". On the other hand, it links and nourishes the small intestine and its closely related tissues and organs. Because the hand sun passes around the outer back side of the upper limb, around the shoulder blade and around the heart from the lack of basin, it belongs to the small intestine. One of its branches enters the ear, and the other branch enters the cheeks, nose, eyes, zygomatic parts and other parts, so the small intestine is connected with the tissues and organs of the ear, cheeks, eyes and other body surface parts through the sun meridian of the hand. At the same time, these tissues and organs need to be moistened and nourished by the qi and blood running through the meridians of the hand-sun, so as to maintain their normal physiological functions. If the meridian qi of the hand sun meridian is abnormal, it will affect the small intestine and its related tissues. For example, the small intestine and the heart are in the exterior and interior, and the meridians of the hand and the sun are full of heat, so the heat will move to the heart, and the heart will begin to understand on the back, leading to sores on the mouth, lips and tongue. Obstruction of the movement of the sun meridian of the hand will lead to shoulder, arm and upper limb side back pain.

In a word, the Hand Taiyang Meridian belongs to the small intestine, which mainly runs the blood gas of the small intestine. It is the way of communication and nutrition supply between the small intestine and other tissues and organs. However, it must also rely on the blood gas nourishment of the small intestine to play its normal physiological function.