Complex sentence: Under the epithelium is the lamina propria of connective tissue, secondly a layer of smooth muscle with bundles of circular fibers internal to longitudinal fibers, and thirdly an outer layer of connective tissue called the adventitia.
Split into simpler sentences: Under the epithelium is the lamina propria of connective tissue, then a layer of smooth muscle with bundles of circular fibers internal to longitudinal fibers. Under this lies an outer layer of connective tissue called the adventitia.