The walls of the GI tract have four layers of tissue, called mucosa, submucosa, muscularis externa, and serosa.
Connective tissue and muscle separate the muscosa from the second layer, the submucosa, which contains blood vessels, lymph vessels, nerves, and glands.
Next to the submucosa is the muscularis externa, consisting of two layers of muscle fibers, one that runs lengthwise and one that encircles the bowel.