We shall consider the abnormal conditions of the respiratory organs under three heads: -

1st. As they occur in the air-passages; namely, the larynx, trachea, and bronchi. 2d. As they occur in the pleurae; and - 3d. As they occur in the lungs.

The deviations from the normal state occurring in the thyroid and thymus glands will be noticed in a Supplement.

I. - Abnormal Conditions Of The Air-Passages

1. Deficiency And Excess Of Formation

An entire deficiency of the air-passages invariably occurs in cases in which the lungs are absent. A partial deficiency, as for instance of the trachea, may occur without the lungs being necessarily absent, the bronchi in such cases being given off directly from the larynx. Under this head we must notice imperfect development of the air-passages, dependent on the deficiency of certain parts entering into their structure, as of some of the laryngeal cartilages or tracheal rings; and their malformations arising from the arrest of development both in length and width.

An excess of formation is seen in the duplication occurring in double monsters, in which either the upper or the lower portion of the pulmonary apparatus is doubled; and here we must also place the occurrence of a supernumerary third bronchus, occasionally noticed on the right side in persons otherwise normally developed. Finally, we sometimes meet with supernumerary laryngeal cartilages and tracheal rings, thereby increasing the distance to the tracheal bifurcation.

2. Deviations In Size

In noticing the calibre of the air-passages, we shall omit the consideration of those cases in which the whole apparatus is either extremely developed, or where, on the other hand, it appears in an undeveloped condition with its walls in an attenuated state, and shall at once proceed to the subject of acquired dilatations and contractions. In noticing the thickness of the walls of the air-passages, we shall have to consider the hypertrophy and atrophy of the various structures entering into their composition.

A. Morbid Dilatations Of The Air-Passages

These occur in variously recurring forms in the larynx, trachea, and bronchi, and sometimes occasion a dilatation of the whole apparatus; much more commonly, however, the dilatation affects only single portions of the respiratory organs, as, for instance, the bronchi, where, by the way, this morbid change is most frequently noticed.