Laennec's View regarding the former mode of development of the affection, is essentially an important one. We do not, however, believe that the long retention of the breath is, in itself, the principal cause of forcible expansion of the air-cells; we are much more inclined to attribute it to the very deep and forcible inspirations which at length follow the expirations, and, in illustration of this view, we may refer to the nature of the inspirations in croup, in the bronchial catarrh of children, and in hooping-cough. Besides inducing forcible dilatation, they may also cause a paralysis of the contractility of the pulmonary tissue, and, consequently, a stagnation of air in the dilated pulmonary vesicles.

Emphysema may, however, be developed in cases where none of these injurious influences have been present; and it may occur gradually in persons leading a sedentary life. In these cases the less frequent but the proportionally deeper inspirations are the more to be regarded, because they take place with very little action of the diaphragm (abdominal respiration), as the occupation of such persons requires a bent position compressing the abdominal cavity, and, at the same time, power-ful exercise for the arms. A paralyzed and atrophied condition of the diaphragm is here of the greatest importance; for the hinderance to the abdominal respiration thus induced is compensated by the strained activity of the other great respiratory muscles; and this circumstance is in accordance "with the facts that the dilatation of the thorax is most marked in its upper segment, and that emphysema is primarily and most fully developed in the upper lobes, and especially in their anterior portion.

The thickening of the walls of the dilated air-cells arises, in our opinion, principally from the tissue adjacent to them becoming broken down by compression, and fusing with the cell-walls themselves; nevertheless if the dilatations increase from the persistent pressure exercised by the dilated cells on one another, atrophy of the contiguous walls will ensue, and the cells will unite to form larger cavities, much in the same way as we occasionally observe to occur in contiguous bronchial sacs.