The pulmonary artery is. probably more liable to embolism than any other vessel. The embolus, may be derived from thrombi in the right side of the heart or in the veins: it may consist of fat, or it may have origin in tumours or parasites which have penetrated the veins.

Oil or fat embolism of lung.

Fig. 357. - Oil or fat embolism of lung. The fat occupies the smaller arteries and extends into the capillaries which, towards the right, form a network injected with oil. (Thierfelder.) x 90.

In a considerable proportion of cases the haemorrhagic infarction results, but by no means in all (see p. 106). In the case of Fat embolism there may be many small haemorrhages with oedema, or even a more considerable area of haemorrhage. (See p. 109 and Figs. 33 and 357).

There are many cases of pulmonary embolism without any development of the infarction, and, it may be, without any pronounced appearances at all. If the embolus be large enough to obstruct the main artery of the lung, then there can be no haemorrhage, as the whole lung is cut off from its blood supply through the pulmonary artery. When smaller branches are occluded, the infarction may be absent. Cases of sudden death after parturition occur from embolic obstruction of the lungs without any infarction. The suddenness of death in some of these cases seems to indicate that it is not due entirely to the obstruction in the lungs and consequent non-aeration of the blood. It seems rather due in great measure to the fact that, as the blood is prevented from reaching the left ventricle in sufficient amount, the brain and medulla oblongata are deprived of their proper nourishment. In such cases the right ventricle will be found dilated, while the lungs are pale and probably over-inflated by the violent but ineffectual respiratory efforts. These sudden deaths after confinement used to be ascribed to shock before Virchow pointed out their true significance.