In =vesicular emphysema= the smallest bronchial tubes and the air cells have become distended beyond the natural standard and remain permanently so, the lung tissue having lost its power of contraction. If such a lung is inflated and dried, and a thin slice taken from the surface of the emphysematous part the size of the minute orifices on the cut surface will show its condition. These fine openings are only the air cells cut across, and in their healthy state they will admit no larger object than the point of a needle or a fine bristle. They are slightly larger in adult and especially in old horses than they are in the young. If affected by emphysema they will often admit a hempseed or even a small pea. On opening a healthy chest the lung collapses, contracting on itself and expelling the contained air; if the lung is emphysematous the diseased portion does not collapse and if the entire lung is affected it continues to fill the chest and may even bulge outward after it has been opened. The color of the emphysematous lung is of a brighter red than are the healthy portions. If a diseased lung has been left exposed to the air for twenty-four to thirty-six hours and then cut across in all directions, the diseased lobules may be distinguished at a glance by this lighter shade, and such light portions if near the surface will be found to correspond to elevations above the general level of the lung. If the diseased lung is placed in water it floats on the surface like an inflated bladder scarcely at all sinking into the fluid. If the lung is blown full of air the emphysematous part is first filled causing the bulging on the surface to be still more marked than before. Vesicular emphysema rarely affects an entire lung; it is usually confined to the anterior lobes and to the thin lower and posterior borders of the organ. =Interlobular emphysema= is the extravasation of air into the connection tissue between the lobules owing to rupture of the air cells, and smaller bronchial tubes. It may occur independently of the vesicular emphysema but more frequently, it is preceded by that form and results from it. It is manifested on the surface of the lung by irregularly formed transparent elevations movable from one place to another under the pressure of the finger contrary to what is the case in vesicular emphysema. These vary from the size of a pea to that of a hen’s egg. When the air exists in the cellular tissue between the lobules, it appears as intersecting lines circumscribing irregular spans, and seemingly dark colored to a superficial glance but seem to be transparent on a closer examination. Like the elevations on the surface these collapse on being pricked.