2 Ziemssen's Cyclop., vol. v. p. 373.
It may be said, then, that while in partial or local emphysema the alteration in the air-vesicles may be effected by extraordinary efforts brought to bear upon healthy cell-walls, in general or lobar emphysema, on the other hand, it may be produced by ordinary efforts acting upon weakened and diseased cell-walls. The morbid change is probably not in all cases alike, being sometimes fatty, sometimes fibroid, degeneration, and in other cases of a kind not ascertained.
In addition to other considerations, the markedly hereditary nature of emphysema in not a few instances would of itself render the existence of some constitutional predisposing cause highly probable. On this point A. T. H. Waters3 quotes the observations of Greenhow and Jackson. Out of 42 cases collected by Greenhow, 23 showed an hereditary tendency, and in 28 reported by Jackson, 18 were of emphysematous parentage. In stating his belief that substantive or general emphysema is the result of some degenerative process, Waters bases it on the following considerations: 1st. The high degree of development which the disease often reaches, without any previous history of violent or long-standing cough, in connection with either bronchitis, whooping cough, or any similar affection. 2d. The frequency with which the disease attacks the whole of both lungs, and the uniform character of the morbid changes often observed throughout all parts of the lungs. 3d. The hereditary nature of the disease, as shown by observations. 4th. The manner in which the disease is influenced by certain remedial measures which are known to act beneficially on other diseases attended with degeneration of tissue.
3 Diseases of the Chest, pp. 122, 123.
As to the nature of the immediate exciting cause of emphysema, whether in the general or local form, different views have been maintained. The most important of these are the inspiratory and expiratory theories.
The former of these theories, that in accordance with which the disease is referred to inspiratory action, was maintained by Laennec, and under the influence of his authority was at one time generally accepted. In accordance with this view, the existence of bronchitis is an important factor in the production of emphysema, as undoubtedly it often is in the lobular form. The dilatation of the air-vesicles was attributed to their over-distension by inspiratory efforts allowing the free entrance of air, the escape of which was impeded by bronchial mucus. Inspiration was thus regarded as a more powerful act than expiration, which was considered too feeble to drive the air beyond the accumulated mucus. In this way the air was supposed to accumulate in gradually increasing amount within the cells, which thus became distended.
But in opposition to this view it has been shown by Hutchinson's researches that Laennec was wrong in supposing inspiratory power to be greater than that of expiration; and it is further opposed by the researches of Mendelssohn and Traube, and those of Gairdner, which have shown conclusively that the presence of a pledget of mucus in a bronchial tube, so far from causing distension of the air-vesicles to which it leads, must ultimately ensure their collapse. The collapse thus occasioned, which is most common in the lower parts of the lungs, may lead, partly perhaps through inspiratory pressure, to vicarious emphysema in the upper portions, which receive a relatively larger quantity of air, in accordance with Williams' theory of negative inspiratory pressure.
It is true, then, as maintained by Laennec, that bronchitis may occasion emphysema, but the emphysema does not occur in the vesicles to which the affected tubes directly lead, nor from the force of inspiration applied to these vesicles, as Laennec taught, but in other portions of the lungs.
The expiratory theory affords a more satisfactory explanation of emphysema than does the inspiratory theory, and one more completely in accordance with the physiology of respiration and the anatomy of the thorax.
In ordinary expiration, in which the lungs are uniformly and equably compressed by the chest-walls, there is nothing tending to force air into one part of these organs more than into another, and thus produce emphysematous dilatation. But in forced expiration, such as occurs in the act of coughing, it may be plainly seen, if the chest be uncovered, that the air is driven upward to the top of the lungs, so as to produce a perceptible bulging in the supra-clavicular region. This bulging is notably increased in the coughing-spells of emphysematous subjects; and this fact is urged by Sir William Jenner both as throwing light upon the expiratory act as a principal factor in the disease, and as accounting for the special frequency of emphysema in the upper parts of the lungs. The explanation of this phenomenon is found in the circumstance that in the strong expiratory efforts of coughing the abdominal muscles force the diaphragm upward, and thus compress the lungs from below; at the same time the strong lateral anterior and posterior thoracic walls resist pressure, while the superior part of the thorax, covered over with fascia, but not completely protected by a bony structure, offers least resistance. To this unprotected part of the lungs and to the free margins and borders, which contain normally the smallest amount of air, will the strong currents produced by violent expiratory efforts be driven, so as to cause distension of their vesicles. Thus, the frequent coughing-spells of bronchial catarrh, so commonly associated with emphysema, give rise to the expiratory efforts which are the immediate cause of the emphysema.