Defecation is a reflex activity. When stool enters the rectum, it stimulates receptors in the rectal wall and impulses are transmitted along afferent fibers in the pelvic and inferior ventral nerves to the primary defecation center in the lumbosacral region of the spinal cord. The afferent impulses are also transmitted to the cerebral cortex, causing the urge to defecate. If conditions permit, the impulses are transmitted through the efferent fibers (parasympathetic fibers) of the pelvic nerve, causing contraction of the descending colon, sigmoid colon and rectum and diastole of the internal anal sphincter, while the efferent impulses of the pubic nerve are reduced and the external anal sphincter is diaped and the stool is expelled. In addition, the nerves innervating the abdominal muscles and diaphragm become excited, the abdominal muscles and diaphragm contract, and intra-abdominal pressure increases to facilitate defecation. If conditions do not permit, the cerebral cortex sends impulses to inhibit the activity of the primary lumbosacral centers of the spinal cord, and the inhibitory impulses are transmitted along the efferent fibers of the inferior abdominal nerve (sympathetic fibers), which increases the tension of the anal sphincter, and the sigmoid colon dilates, and the defecation reflex is inhibited. If the defecation reflex is frequently suppressed, the rectum gradually loses its normal sensitivity to the pressure stimulation of feces. When stool stays in the colon for too long, it becomes dry and hard due to excessive water absorption, and as a result, it is not easily expelled, which is one of the most common causes of constipation. Another abnormal phenomenon of defecation is that when the sensitivity of rectal mucosa is increased due to inflammation, only a small amount of feces and mucus in the intestine can cause bowel movement and defecation reflex, and there is always a feeling of incompletion after defecation, which is clinically known as “urgency and heaviness” and is commonly seen in dysentery or enteritis.