ydatid cyst_. The liver is by far the most
common site of hydatid cysts in the human subject.

With regard to the further life-history of hydatids, the living elements
of the cyst may die and degenerate, or the cyst may increase in size
until it ruptures. As a result of pyogenic infection the cyst may be
converted into an abscess.

The _clinical features_ of hydatids vary so much with their situation
and size, that they are best discussed with the individual organs. In
general it may be said that there is a slow formation of a globular,
elastic, fluctuating, painless swelling. Fluctuation is detected when
the cyst approaches the surface, and it is then also that percussion
may elicit the "hydatid thrill" or fremitus. This thrill is not often
obtainable, and in any case is not pathognomonic of hydatids, as it may
be elicited in ascites and in other abdominal cysts. Pressure of the
cyst upon adjacent structures, and the occurrence of suppuration, are
attended with characteristic clinical features.

The _diagnosis_ of hydatids will be considered with the individual
organs. The disease is more common in certain parts of Australia and in
Shetland and Iceland than in countries where the association of dogs in
the domestic life of the inhabitants is less intimate. Pfeiler, who has
worked at the _serum diagnosis of hydatid disease_, regards the
complement deviation method as the most reliable; he believes that a
positive reaction may almost be regarded as absolutely diagnostic of an
echinococcal lesion.

The _treatment_ is to excise the cyst completely, or to inject into it a
1 per cent. solution of formalin. In operating upon hydatids the utmost
care must be taken to avoid leakage of the contents of the cyst, as
these may readily disseminate the infection.

A _blood cyst_ or haematoma results from the encapsulation of
extravasated blood in the tissues, from haemorrhage taking place into a
preformed cyst, or from the saccular pouching of a varicose vein.

A _lymph cyst_ usually results from a contusion in which the skin is
forcibly displaced from the subjacent tissues, and lymph vessels are
thereby torn across. The cyst is usually situated between the skin and
fascia, and contains clear or blood-stained serum. At first it is lax
and fluctuates readily, later it becomes larger and more tense. The
treatment consists in drawing off the contents through a hollow needle
and applying firm pressure. Apart from injury, lymph cyst