The large intestine belongs to the lower part of the digestive tract and receives food sent from the small intestine. Since most of the nutrients of this food have been absorbed in the small intestine, the large intestine mainly absorbs water and minerals, and the remaining residue is continued down into the rectum as feces, which is excreted through the anus. The large intestine is mainly a tube composed of muscle, the lumen of which is covered with a layer of epithelium, under which blood vessels and lymphatic vessels are distributed to transport the absorbed substances. The colon is divided into four segments, the first being the ascending colon, which is on the right side of the abdomen, with intestinal peristalsis directed upward, hence the name ascending colon. The second segment is the transverse colon, which is above the abdomen and continues from the right side of the ascending colon to the left transversely, hence the name. The third segment is the descending colon, which descends from above on the left side of the abdomen and continues to extend into the fourth segment, which is in the shape of a “sigmoid” and is called the sigmoid colon. Below the sigmoid colon, there is a vertical descending section of the intestine called the rectum. The exit of the rectum is the anus. The large intestine is a cavity that is divided from the inside to the outside: the inner sleeve is usually called the mucosal layer, under the mucosa there is a very thin layer of circular muscle and connective tissue, called submucosal tissue, followed by a layer called the intrinsic muscle layer. Some large intestines also have a set of outer coat, called the plasma layer. I. Pathogenesis The rectum and colon have many things in common, and are usually referred to as rectum and colon or large intestine, and the cancer that occurs in the mucosal epithelium of this part is called rectal and colon cancer or colorectal cancer. Colorectal cancer develops slowly. It takes several years for a colorectal cancer visible to the naked eye to go through precancerous lesions and then gradually grow into cancer. Precancerous lesions can be atypical hyperplasia or adenomatous polyps, polyps are the growth of intestinal epithelial tissue protruding into the intestinal lumen. Colorectal cancer can grow toward the lumen, a type that can be seen through colonoscopy, or it can grow outside the wall of the tube. It tends to spread through the lymphatic tract and bloodstream, and the most common site of metastasis is the liver. Colorectal cancer is one of the most common malignant tumors and one of the fastest rising malignant tumors in China. It ranks fourth after stomach cancer, esophageal cancer and lung cancer. The high incidence area is the lower reaches of Yangtze River and southeast coastal areas. In recent years, with the improvement of people’s living standard, the incidence of this disease is increasing and the age of incidence has a tendency to be younger. Colorectal cancer is more likely to occur between 30 and 50 years old, with 45 years old being the peak incidence and more men than women. Colorectal cancer has the best treatment effect among all digestive tract tumors. If detected early, most colorectal cancers can be cured, but sadly, most of the colorectal cancers found so far are already in advanced stage and have lost the chance to be cured. High-fat diet can increase the concentration of bile acids and neutral cholesterol in the colon, both of which can form carcinogenic substances through the action of bacteria; food fiber has the function of absorbing water, increasing the amount of stool, diluting the concentration of intestinal residues, and shortening the time of stool passing through the colon, thus reducing the chance of contact between carcinogenic substances and the mucosa of the colon. Therefore, insufficient dietary fiber is a high risk factor for colorectal cancer. 2, fried food: the burnt part of fried food, especially the burnt part of meat, contains carcinogens that can act on the colon. Some data show that the excess risk of colon cancer for those who consume fried food more than 3 times a week is 2.3 times of those who consume less than 1 time, and 2.6 times for rectal cancer. 3, high salt diet and pickled food: the relative risk of colorectal cancer is increased for those with high salt intake. It has been reported that the excess risk of colon cancer for those who consume pickled food more than 3 times a week is 2.2 times of those who consume it less than once, and 2.3 times for rectal cancer, which may be related to the carcinogenic substances produced in the process of food pickling. 4, trace elements and minerals: lack of molybdenum and selenium in the soil is a factor in the development of colorectal cancer. Animal experiments show that selenium supplementation can inhibit the growth of colorectal cancer. 5.Occupational factors and physical activities: Those who are engaged in asbestos production, metal industry, cotton textile industry and leather manufacturing industry have higher mortality rate of colorectal cancer. The risk of colon cancer is 1.4 times higher in occupations with long-term or frequent sitting than in occupations with greater physical activity. The reason is mainly because: reduced physical activity can make the excrement through the intestinal tract for a longer time, increasing the chance of carcinogens contacting with intestinal mucosa. According to statistics, the incidence of colorectal cancer in people with colon polyps is 5 times higher than the normal population. Among them, the incidence of familial polyposis is higher, and 80% to 100% of patients may develop into malignant tumors after the age of 50. 7, chronic colitis such as ulcerative colitis patients colorectal cancer incidence is higher than the general population, in the process of inflammatory proliferative lesion development, can often form inflammatory polyps, and then develop into cancer. After pelvic radiation therapy, it often causes radioactive colitis, and a few of them can become cancerous. 8, genetic factors colorectal cancer positive family history, the incidence of the disease is 4 times higher than the general population, about 20% of colorectal cancer is related to genetics. The most common early symptoms are changes in bowel habits and stool characteristics, which are manifested in the early stage as increased number of stools, ranging from 3 to 5 times a day, thin stools, which may be accompanied by pus and blood or mucus, and constipation, or alternating constipation and diarrhea when the intestinal canal becomes narrow and obstructed. Patients with rectal cancer may also experience anal cramping, urgency, incomplete bowel movements, and deformation and thinning of stool. 2.Abdominal pain can be hidden pain, distension, colic, etc., which is also one of the early symptoms. It is generally manifested as limited pain along the colon or lower abdominal distension. 80%-90% of colorectal cancer patients complain of abdominal pain when they visit the doctor. When the tumor invades one circle of the intestinal canal and causes narrowing of the intestinal lumen, abdominal cramping pain and abdominal distension may occur. When retroperitoneal metastasis or invasion of deep rectal tissues occurs, severe anal pain and persistent pain in lumbosacral area may appear. 3.Blood in stool is the first symptom of about half of the patients. Right hemicolectomy is often difficult to be detected by the naked eye because the stool and blood are mixed together. When the left hemicolectomy bleeds, the stool is often dark brown or jam-colored, and when the lesion is close to the anus or bleeds a lot, the stool often carries fresh blood or drips blood during stool. 4.The location of the abdominal mass depends on the location of the tumor. The masses of rectal, ascending colon and hepatic flexure of colon are located in the lower right, middle right and upper right abdomen respectively, while the masses of transverse colon cancer can be found around the umbilicus. Rectal masses are hard, nodular on the surface and bleed easily when touched. 6. Systemic symptoms such as emaciation, weakness, anemia, intestinal obstruction, ascites and cachexia.