part_of part of part of located in in taxon Sphenodon respiratory tube bronchus http://upload.wikimedia.org/wikipedia/commons/1/18/Respiratory_system_complete_numbered.svg bronchial In humans, the main bronchus is histologically identical to trachea; 2ary and 3ary bronchi are not; epithelium becomes simple columnar, goblet cell number decreases, elastic fibers in lamina propria increases, distribution more uniform. Muscular layer between mucosa and submucosa appears. cartilage rings become discontinuous plates connected by fibrous connective tissue thoracic cavity tracheobronchial tree