Preoperative injection of nano-carbon suspension combined with surgical technique of lymph node dissection around the inferior mesenteric artery with preservation of the left colic artery in patients with colorectal cancer undergoing NOSES( natural orifice specimen extraction)

Interventions: early group:Preserve blood vessels in the left colon;medium group:Preserve blood vessels in the left colon;late group:Preserve blood vessels in the left colon
Primary outcome(s): the number of lymph nodes
Study Design: Non randomized control

干预措施：早期组、中期组、晚期组均保留左结肠血管；主要结局指标：淋巴结数量；研究设计：非随机对照研究。