This section is from the book "Anatomy Of The Arteries Of The Human Body", by John Hatch Power. Also available from Amazon: Anatomy of the Arteries of the Human Body, with the Descriptive Anatomy of the Heart.
The Superior Mesenteric Artery, nearly as large as the cceliac axis, and sometimes even larger, arises about a quarter or half an inch lower down than that vessel, from the aorta; it first descends a little to the left, behind the splenic vein and pancreas, and on the front of the abdominal aorta; having reached the lower margin of the pancreas, it becomes separated from the aorta by the third portion of the duodenum, and the left renal vein. In the next part of its course it descends behind the transverse mesocolon, and then between the laminae of the mesentery, to arrive at the ileum near its termination; it then ascends along this intestine towards the caecum : in this course it has its vein to its right side, and it describes a curvature, the convexity of which looks downwards and to the left side. It first gives small branches to the pancreas and duodenum, which anastomose with the pancreatico-duodenalis artery; lower down it gives off two sets of branches, viz., one from its convexity, or left side; the other from its concavity, or right side.