The Dyspnoea, Conditional On Emphysema, depends on several causes.

a. The excessive accumulation of air in the pulmonary vesicles hinders the proper filling of the capillaries ramifying on their walls by the pressure it exerts on them, and thus interferes with the vitalization of a sufficient quantity of blood.

B. In the higher degrees of emphysema numerous capillaries become obliterated, not only in the walls of the dilated cells, but also in the surrounding atrophied tissue, - a condition which induces the above consequence in a still higher degree.

y. The diminished contractility of the pulmonary tissue and the constantly labored inspirations which then become necessary in consequence of the imperfection in the chemical process of respiration, allow of only a very imperfect emptying of the pulmonary cells and consequently give rise to the permanent stagnation of air no longer fit for the purpose of respiration, which, in its turn, also tends to prevent a sufficient ventaliza-tion of the mass of the blood.

The impermeability of the capillaries depending on the two first-named conditions gradually, but unfailingly, leads to disease of the right side of the heart in the form of active dilatation, which proceeds to affect the venous system; the venosity and cyanosis which ensue from these changes constitute the leading grounds for the immunity of asthmatic persons from tuberculosis.

The impermeability of the capillary vascular system, moreover, gives rise to the anaemic state of emphysematous lungs, thus rendering it an impossibility that oedema, stasis, hemorrhage, or pneumonia, should be developed in them.

It is easy to understand how it proves fatal. It kills by finally inducing paralysis of the lungs, by asphyxia from the accumulation of air no longer fit for the process of respiration, by paralysis of the heart, or by vascular apoplexy of the brain.

Emphysema interlobulare is the only form which, strictly speaking, deserves the name of emphysema; it consists in an accumulation of air in the cellular interstices of the pulmonary lobules. It can only result from the rupture of one or more pulmonary vesicles, and the escape of air from them into the adjacent cellular interstices, if, indeed, we except the spontaneous development of gas into the interlobular cellular tissue, which is not altogether impossible.

We consequently find air-bladders in the cellular interstices, and especially on the surface; they vary in number and size, and are characterized by their paleness, transparency, and round or rather oblong form; they may be made to move in the direction of the interstices, and to run into one another, so as to form ridges which ramify in the same direction superficially, and into the body of the lung; sometimes they circumscribe and, as it were, insulate the lobules, and as they are broadest on the surface, and as their size diminishes in proportion to the depth to which they penetrate within the substance of the lung, they present a wedgelike shape. When they are very small and closely crowded together, they present the appearance of froth. On making a section of a portion of dried lung, we find the interstitial tissue presenting irregular cellular spaces of larger or small size, heaped, without order, on and around one another, and perfectly different from the adjacent air-cells. As has been already mentioned, most of the air is usually found accumulated in the peripheral interstices, so that the pleura presents a puffed up, vesicular appearance. The air often makes its way into the cellular tissue uniting the pleura to the lung, peeling off large patches, and forming flattish, convex, movable air-bladders; and, in these cases, it is to be feared that some of these bullae may be ruptured, and that the air may be extravasated into the pleural sac. In other cases the extravasated uir may penetrate into the substance and towards the root of the lungs, and pass into the cellular tissue of the mediastinum, and from thence into the neck, and thus cause general emphysema.

This condition usually co-exists with a puffy state of the lungs, but never with well-marked vesicular emphysema. It is most common in children; and is occasioned in them, as well as in the rarer instances in which it occurs in adults, by very rapid, deep inspirations, or by long retention of breath when great muscular exertions are made, requiring a fixed condition of the thorax. It is most commonly situated in the upper part of the lobes, and especially along their anterior edges.