The SMV and PV are not surrounded by nerve tissue. In contrast, the visceral arteries including the celiac axis, hepatic artery, and SMA are surrounded by the autonomic nerves which innervate the gastrointestinal tract. Pancreatic cancers are frequently characterized by extrapancreatic infiltrative growth, perineural invasion, and extension along the autonomic nerves. One can often see tumor extension along the SMA or celiac axis extending posteriorly to the aorta and this neural invasion is thought responsible for the pain associated with this disease. In such cases, resection of the involved artery, if performed, may not clear the posterior extent of disease, which may involve the para-aortic autonomic ganglia. The visceral veins such as the SMV or PV do not have an investing sheath of autonomic neural tissue.