The main fissure of each lung is indicated by a curved line which starts behind at the level of the second dorsal spine, (Fig. XVI, 7., Fig. XVII, 6.)the apex of the main lower lobe of each lung, therefore, being situated immediately below and external to this dorsal spine. When the arms fall naturally to the side of the body, the line representing the main fissure cuts across the infraspinous surface of the scapula, and crossing then the fifth rib in the mid-axillary line, terminates in front at the inferior border of the lung at the level of the sixth chondro-costal junction. When the arms are extended above the head, the inferior angle of the scapula slides upwards and outwards on the chest wall. In this position the main fissure is represented by a line from the second dorsal spine which passes downwards and outwards to the inferior angle of the scapula, and then forwards to the termination of the fissure at the inferior border of the lung.
The smaller or upper fissure of the right lung separates off from the main fissure in the mid-axillary line, (Fig. XVI, 8.) and passes almost transversely forwards along the lower border of the fourth rib and costal cartilage to the anterior border of the lung. It is thus manifest that the anterior aspect of the chest, above the level of the sixth costal cartilage, corresponds to the upper two lobes of the right lung and to the upper lobe of the left lung, whilst the posterior aspect of the chest below the level of the second dorsal spine corresponds to the right and left main lower lobes.