Supernumerary tracheal bronchus.

Accessory bronchi arising from the trachea 'are rare in man (Boyden, 1955) but common in lower animals (McEwen, 1937). When present they commonly arise from the right side of the trachea, approximately 2 cm. above the origin of the right main bronchus (Brock, 1954). Accessory bronchi may be displaced normal branches or true supernumerary bronchi (FosterCarter, 1946). In the former the accessory bronchus takes the place of the normal bronchus (the apical branch of the right upper lobe) arising from the trachea rather than from the right main bronchus. The right main bronchus therefore divides into only two terminal branches instead of three. True supernumerary bronchi are rare and can only be diagnosed by demonstrating the normal segmental bronchus in addition to the supernumerary bronchus. The distinction is of importance where surgery is contemplated. Other abnormalities may also occur in the region of the right upper lobe. Thus the right upper lobe bronchus may arise from the trachea and not from the right main bronchus (Williot, 1958). An abnormal bronchus arising from the trachea and dividing into apical and anterior segmental branches to the right upper lobe has been described (Franchel, Merlier, Morelec, and Polsvert, 1955, Case 2). In this case the right upper lobe bronchus was narrower than usual. Diverticula must be distinguished from aberrant bronchi, and the classification of Morel, Madray, Layssol, and Coll (1957) emphasizes this. They distinguish (1) true accessory lung with three lobes; (2) supernumerary lobe, as described here; (3) displaced apical segmental bronchus; (4) culde-sac or diverticulum; (5) a communication with a cystic, degenerative region. The last two are acquired lesions. Type 5 is in many cases probably the result of a pulmonary lesion which has tracked into the trachea, while the true diverticulum (Type 4) develops from the trachea itself. CASE REPORT