Radioscopic characteristics common to oedema and to con-
gestion of the bases. — ^These passive conditions are seen always
on the screen as diffuse shadows, ordinarily not very deep
and of very variable extent. They are distinguished by
certain characteristics from shadows produced by effusion,
even though this is slight. The opacity is never as intense
as that produced by effusion. The permeabiUty of the lung
having been retained in part, the air which penetrates into
the alveolars modifies the shadow produced by excess of



VASCULAR PROCESSES 101

serous fluid. In inspiration the base becomes sufficiently
cleared for one to distinguish plainly the convex contour of
the diaphragm and the notching of the costodiaphragmatic
sinus. The movements of respiration have kept their
regularity and their extent. It is then certain that there is
no question of effusion.
INFARCT.^-Infarct of the lung, when it affects a sufficient
area, can by radioscopic examination present a very percep-
tible abnormal shadow. Contrary to what the anatomic
aspect of the lesions would lead one to suspect, this shadow
is not very opaque and does not usually show contours
clearly defined. It does not have the frank triangular form
and should not be cdnfused, for example, with the pneumonic
triangle. It has neither its appearance, its intensity, nor
its position. It shows ordinarily on the screen as a blurred
spot with shaded contours, situated most often in the full
pulmonary field, but rather in the lower part of this field.
