A certain degree of condensation is natural to the lungs of children; it sometimes occurs in adults as an individual peculiarity, and is then often associated with smallness of the lungs and pleural sacs. It is also present as a transitory condition during pregnancy.

It only comes within the limits of pathology, when it has become permanent and highly developed, and offers a persistent impediment to the capillary circulation through the lungs.

Such a degree of condensation may arise when the abdomen becomes enlarged and encroaches on the thoracic cavity, but in children it is more frequently dependent on lateral depression of the thorax consequent on atrophy of the great respiratory muscles, or on rachitis affecting the chest; it may also arise from spinal curvature, distension of the pericardium, enlargement of the heart, large aneurisms, adventitious products, etc.; or from the pressure exerted by an accumulation of air or fluid in the cavity of the chest, from pleuritic exudation, or from bronchial dilatation; and, according to the various exciting causes, it may occur simultaneously in both lungs, or only in one, or merely in certain portions of the pulmonary tissue, as in cases of rapidly developed emphysema, where we not unfrequently find single lobules compressed in the centre of the emphysematous portion, or in cases of atrophy of the external respiratory muscles, where single circumscribed portions of lung are found in a state of condensation under the bent anterior ends of the ribs.

There are different grades varying from simple increase of density characterized by augmented consistence and compression of the pulmonary tissue, and by a stasis and hyperemia depending on obstructed circulation, to such a degreee of compression as to destroy the air-cells, to arrest the capillary circulation, and to give rise to atrophy of the texture of the lung.

The most intense compression of the lung occurs in cases where there is abudant pleuritic effusion. With the alterations in position and form, to which allusion has already been made, the lung always becomes denser and gradually becomes impermeable to air, and, finally, even to the passage of blood along its capillaries. If the lung still contains blood, its red color gives it such a similarity to flesh, that this condition has received the name of carnificatio pulmonis, but at a subsequent period it becomes of a dirty brown, or, more commonly, of a bluish-gray or lead-color, and is tough and leathery, and sinks in water.

If the state of extreme compression persist for a length of time, the pulmonary tissue finally becomes obsolete, that is to say, it becomes converted into a cellulo-fibrous tissue, - a condition altogether distinct from atrophy of the pulmonary tissue.

Excessive condensation of the lungs gives rise to consequences similar to those of emphysema; it impedes the capillary circulation, and thus occasions stasis in the trunk of the pulmonary artery, giving rise to active dilatation in the heart, and consequently to venosity and cyanosis. Hence, like emphysema, it affords a remarkable immunity from tuberculosis, especially when associated with curvature of the spine.

There is a peculiar form of anomalous condensation of the pulmonary tissue, probably dependent on a congenital bronchial catarrh or catarrhal pneumonia, and consisting in a deficient development of the lungs of new-born children, in which certain portions of those organs retain their foetal condition after birth. It is termed atelectasis of the lungs, and presents various degrees of obstruction to the closure of the foetal passages, namely, the ductus arteriosus and the foramen ovale, thus giving rise to predominance of the right side of the heart and to cyanosis.