area, and is usually surrounded by a zone of induration.

By gently palpating with the finger-tips over the softened area, a fluid
wave may be detected--_fluctuation_--and when present this is a certain
indication of the existence of fluid in the swelling. Its recognition,
however, is by no means easy, and various fallacies are to be guarded
against in applying this test clinically. When, for example, the walls
of the abscess are thick and rigid, or when its contents are under
excessive tension, the fluid wave cannot be elicited. On the other hand,
a sensation closely resembling fluctuation may often be recognised in
œdematous tissues, in certain soft, solid tumours such as fatty tumours
or vascular sarcomata, in aneurysm, and in a muscle when it is palpated
in its transverse axis.

When pus has formed in deeper parts, and before it has reached the
surface, œdema of the overlying skin is frequently present, and the skin
pits on pressure.

With the formation of pus the continuous burning or boring pain of
inflammation assumes a throbbing character, with occasional sharp,
lancinating twinges. Should doubt remain as to the presence of pus,
recourse may be had to the use of an exploring needle.

_Differential Diagnosis of Acute Abscess._--A practical difficulty which
frequently arises is to decide whether or not pus has actually formed.
It may be accepted as a working rule in practice that when an acute
inflammation has lasted for four or five days without showing signs of
abatement, suppuration has almost certainly occurred. In deep-seated
suppuration, marked œdema of the skin and the occurrence of rigors and
sweating may be taken to indicate the formation of pus.