The wall of digestive tube was composed of mucous membrane, thin submucosa, tunica muscularis and tunica adventitia, but消化壁一般分为4层，由内向外为粘膜、粘膜下层、肌层和外膜，其中粘膜下层很薄，十二指肠粘膜下层无十二指肠腺。