cecum. 1. the first or proximal part of the large intestine, forming a dilated pouch distal to the ileum and proximal to the colon. There is a great deal of variation in the relative size between species.The dog's cecum is a small, coiled organ.ecum. [sÄâ²kÉm]. Etymology: L, caecus, blind, blind gut. a pouchlike structure or cul-de-sac constituting the first part of the large intestine. It is inferior to the junction of the ascending colon and joins the ileum, the last segment of the small intestine, at the ileocecal valve.