Under the term pulmonary emphysema we comprehend, according to Laennec, two different conditions, of which one (and by far the more important one) is not fairly entitled to this name; but this inaccuracy leads to no error, because, in using the terms emphysema vesiculare and emphysema inter-lobulare, we indicate the seats of the two diseases, and thus distinguish one from the other.

In emphysema vesiculare we have a morbid condition of the peripheral portion of the respiratory organs analogous to that which we have already described as dilatation of the bronchi, and even of the trachea. Had Laennec done nothing else for medical science, his discovery of this diseased condition, and of the causes giving rise to it, would have sufficed to render his name immortal.

Vesicular emphysema consists in a permanent dilatation of the pulmonary vesicles, and the respired atmospheric air is actually contained within them, and does not, as in ordinary emphysema, become extrava-sated into the interstitial texture.

It not unfrequently arises very rapidly as a vicarious development of pulmonary parenchyma in cases where a great portion of the lung has become impermeable; and it appears more especially to be produced in a high degree during the last moments of existence, as a consequence of the labored inspirations which then occur. Thus in diffused hepatisation we find the edges of the inflamed lobes puffy and emphysematous, and in the higher degrees of pulmonary tuberculosis, the same condition is observed in the interstitial parenchyma between the tubercles, and in the superficial stratum of the lung. In like manner it developes itself as a sequence of those acute and chronic diseases which prove fatal by paralyzing the nervous apparatus presiding over the chemical process, and in which there is the most laborious action of the respiratory muscles, deep inspiration, and an insatiable thirst for air; or it arises as a consequence of a sudden check to expiration, as in hemorrhages from the air-tubes, when the bronchi become obstructed by blood.

In such cases we observe the following appearances: the emphysematous portion of the lung is puffed up, and conveys to the hand a peculiar feeling, which may be compared to that of a cushion filled with air; is pale, varying in tint, from a palish red to a dull white color, and is perfectly anaemic; is dry, collapses rapidly on being cut, but on pressure crepitation is indistinct and dull; it floats on the surface when placed in water; its cells are more or less dilated, and their walls are attenuated in proportion to the rapidity with which the morbid change has been developed. Finally there is sometimes extensive laceration of the dilated cells, and the emphysematous portion of the lung then presents the appearance of a torn network swollen with air. This form of emphysema seldom attains this degree, except on the anterior edge and towards the base; and at these parts it gives rise to the escape of air beneath the pulmonary pleura, which consequently peels off from the lung.

This form of emphysema, in so far as relates to the attenuation of the walls of the dilated cells, presents an analogy with senile atrophy of the lungs.

Another form of vesicular emphysema developes itself slowly, gradually spreading itself over a large portion of the lung, till it finally involves the whole organ; it arises, in part, from other causes than those already mentioned, and constitutes a substantive disease of the lung, which, as Laennec remarks, unquestionably gives rise to most of the so-called nervous asthmas.

It presents many varieties in degree and extent. By degree, we refer to the extent of the dilatation of the pulmonary cells; it must, however, be remarked, that in emphysema of long standing, we always simultaneously find several degrees of dilatation, and that it is only during the commencement of the disease that the dilatation is observed to be uniform. The pulmonary cells may be dilated to the size of a millet-seed or pin's head, or to that of a hemp-seed, a pea, or even a bean, and, in proportion to the size which they attain, they deviate the more from their original shape. At first the disease is a genuine, simple dilatation of the cells, and when the cell-walls become to a certain extent thickened and rigid, it may be regarded as an active dilatation of the cells somewhat analogous to hypertrophy of the lungs. In higher degees, on the contrary, the dilatated cells unite to form larger spaces, their walls becoming atrophied by the pressure they exert on one another. Such hemp-seed, pea, or bean-sized cells always present a very irregularly sinuous, but on the whole a roundish form, and exhibit a singular arrangement; for on their inner surface there are elevated ridges, projecting to various heights within the cavities of the dilated cells, traversing them in various directions, and forming boundaries and imperfect partition-walls to the different sinuosities. We likewise perceive delicate threads, either extending across the cells, or hanging free in their cavities: these cover the elevated ridges and the remains of the contiguous walls of the pulmonary cells. The pressure exerted on the adjacent tissues, which gives rise to their atrophy, is proportional to the dilatation of the cells; and the cell-walls becoming thick and rigid, the emphysematous lung, when a section is made, either does not collapse at all, or collapses very slowly.

Moreover, this form of emphysema occurs most frequently, and is most highly developed, in the peripheral portion, and along the edges of the lungs: it is not unfrequently associated with bronchial dilatation; and this, amongst other signs, establishes the affinity of these two diseases. It either attacks a small portion of the lung only, being confined especially to the anterior edge of one or other of the upper lobes, or else it spreads over a whole lobe, or a whole lung, or even both lungs.

In cases of emphysema of both lungs, the association of all the anatomical signs presents the following picture of the disease:

Barrel-shaped dilatation of the thorax, with permanent depression of the intercostal spaces; great dorsal curvature of the spine; hypertrophy of the respiratory muscles; and a clear sound on percussion.

On opening the thorax, the lungs expand beyond the walls of the chest, are seen to be remarkably large, and do not collapse under the pressure of the atmosphere.

On their surface, and especially at their anterior edges, we find round prominences as large as a hemp-seed or a pea, either standing alone or arranged in groups, and which are nothing more than the dilated pulmonary cells which have been already described.

The lungs have a very peculiar, soft, elastic feeling, which may be compared to that of a cushion filled with down.

On being cut they collapse very slowly, and the air escapes sluggishly, with a very diffused sound, scarcely amounting to crepitation, and somewhat resembling that of air slowly escaping from a pair of bellows.

Their tissue is pale throughout, anaemic, and singularly dry.

When only one lung is emphysematous, then only the corresponding half of the thorax is dilated; but an important fact in this case is the displacement of the mediastinum and the heart towards the opposite side. Finally, if only individual portions of the lung are emphysematous, they may, if they are very numerous, and the disease is highly developed, prevent by their pressure the expansion of the neighboring healthy cells, and thus retain them in a state of persistent compression.

The conditions giving rise to the production of emphysema, and its pathogeny in general, although much labor has been devoted to the affection, are still far from clear. Laennec regards it as a consequence of his so-called dry catarrh with pearl-colored secretion, and explains it in a mechanical manner: this secretion, and the catarrhal puffiness of the mucous membrane, obstruct the bronchi in such a manner, that although they allow the inspired air to enter the pulmonary vesicles, the diminished energy of the act of expiration presents an impediment to its escape, and hence a portion of it is retained. In the succeeding inspirations new air is again conveyed to the pulmonary vesicles, whose dilatation is thus effectually accomplished, the expansion of the inspired air, from its elevated temperature, doubtless contributing to the result. Moreover, a prolonged retention of the breath during parturition, a stool, or in blowing wind-instruments, may give rise to emphysema.

In opposition to the view that emphysema is a consequence of catarrh, cases have been adduced in which either no catarrh had existed, or when it only followed the dyspnoea as a symptom of pre-existing emphysema; in reference to the last-named causes, it is alleged to have occurred in persons who have never been exposed to these diseases. Hence, a spontaneous dilatation of the pulmonary vesicles has been assumed, which at one time occurs as premature atrophy with attenuation of the cell-walls, while at another time, from equally unknown causes, it is associated, as in other hollow organs, with hypertrophy of the cell-walls.