During embryologic development, the colon starts off as a midline structure. As the embryo develops, the colon rotates laterally and fuses with the retroperitoneum. The white line of Toldt represents the lateral fusion line between the colon and the retroperitoneum (Fig. 3.9a, b ). Regardless of type of dissection approach (medial to lat­eral vs. lateral to medial) one uses during a laparoscopic colectomy, the ultimate goal is to separate the colon and its mesentery away from the retroperitoneal structures and develop the colon as a midline structure. When performing the lateral-to-medial approach, the dissection must be started along or just medial to the white line of Toldt (Fig. 3.10a, b ). Dissection in this area will allow an entry into the appropriate plane between the colon mesentery and the retroperitoneum. On the other hand, dissecting lat­eral to the white line will likely lead directly into the retro­peritoneal space and will increase the likelihood of causing unwanted bleeding and injury to the retroperitoneal struc­tures. When performing medial-to-lateral dissection, the mesenteric vessels are isolated and ligated before gaining an access into the retroperitoneum (Fig. 3.10a, b ). Because there is no fusion plane between the colon mesentery and the retroperitoneum in the midline, closest to the named vessels, there is a tendency to veer off from the proper dis­section plane (Fig. 3.11a ). The surgeon has to make a con- scious effort to stay within the appropriate surgical plane (Fig. 3.11b ). The mantra “purple goes down” is useful to remind ourselves from getting too deep into the retroperi­toneal space.

When performing a laparoscopic left colectomy or pelvic dissection, the ureters may be encountered in two locations: (1) where they cross over the common iliac vessels and (2) the lateral walls of the pouch of Douglas as they course beneath either the vas deferens or the uterine artery (Fig. 3.20 ). When dissecting behind the IMA into the retro- peritoneal space, the left ureter is located medial to the gonadal vessels (Fig. 3.21 ). When developing the plane anteriorly, theoretically the ureters should not be seen at this level [ 4 , 5 ].