The trachea on reaching the chest divides into the two bronchi, one going to each lung. As seen in fig. 200, they then divide and subdivide again and again till the tubes are scarcely larger than a hair, when they are termed bronchioles. These, after a short course, suddenly change their character, becoming dilated into sack-like ends, and presenting depressions, cups, or pits on their walls as shown in fig. 201.