General Comparative Anatomy of the Small Intestines
The small intestine begins at the pylorus and terminates into the cecum near or at the cecocolic junction. It consists of duodenum, jejunum, and ileum.
Duodenum:
The duodenum is the first part of the small intestine that begins at the pylorus and extends to the beginning of the jejunum. It is divided into three parts due to the presence of two flexures. The cranial part passes to the right and ends at the cranial flexure. It is attached to the liver by the hepatoduodenal ligaments and forms a sigmoid loop (s-shaped flexure) in the horse, ruminants and pigs. The cranial part also receives the bile duct and the pancreatic ducts. The descending duodenum passes caudally from the cranial flexure on the right side. Caudal to the right kidney it turns from right to left around the caudal aspect of the root of the mesentery, and then cranially forming the caudal flexure. The ascending duodenum continues cranially and ventrally from the caudal flexure to join the jejunum at the duodenojejunal flexure. Most of the duodenum is suspended by its mesentery, the mesoduodenum, however, at the duodenojejunal flexure it is attached to the descending colon by the duodenocolic fold.
Jejunum:
The jejunum is the longest part of the small intestine, between the duodenum and ileum. The jejunal mesentery (mesojejunum) is long in dog and horse. Due to the great length of the mesentery of the horse, the mass of the jejunal loops are not confined to a particular area of the abdominal cavity. However, they are found in the left dorsal part of the abdominal cavity. In carnivores, the jejunal loops occupy the ventral part of the abdominal cavity. In ruminants, the large stomach displaces the whole intestine to the right side of the abdominal cavity.
Ileum:
The ileum is the terminal part of the small intestine. It terminates at the ileal orifice at the junction between the cecum and the ascending colon. The ileum is suspended by the mesoileum and ileocecal fold that attaches the antimesenteric surface of the ileum to the cecum