y muscular effort. Haemorrhagic cysts
may also develop in the substance of these goitres.

* * * * *

_Effects on the Trachea._--The trachea may be _displaced laterally_
when the enlargement of the gland affects one lobe more than the
other; or it may be _compressed and narrowed_ from side to side--the
_scabbard trachea_--when both lobes are about equally affected and the
enlargement extends posteriorly so as almost to surround the
air-passage (Figs. 278, 279). The third effect is that of _softening
of the cartilaginous rings_ of the trachea so that the air-tube,
instead of having a considerable degree of elastic resiliency, is soft
and flaccid and readily yields to pressure. Under these conditions an
alteration in the attitude of the patient, from the erect or sitting
to the recumbent position, would appear to be sufficient to permit of
a compression of the trachea.

Further changes in the trachea consist in catarrh and engorgement of
the blood vessels of its mucous membrane, attended with an abundant
secretion of mucus, which, if it accumulates behind a narrowed segment
of the trachea, may still further encroach on the lumen.

_Pressure on other Structures._--The _recurrent nerve_ may be pressed
upon intermittently causing spasms and choking, or continuously
causing abductor paralysis and hoarseness.

The gullet is rarely compressed; if marked difficulty in swallowing
develops, some additional factor should be suspected, notably
carcinoma at the junction of the pharynx with the oesophagus. The
carotid arteries are displaced laterally beneath the sterno-mastoids
without detriment; the superficial veins--anterior and external
jugular--are greatly distended in those cases in which the goitre
grows downwards behind the sternum.

_Clinical Features._--The symptoms vary widely in different cases, and
their severity is not proportionate to the size of the goitre. The
disfigurement produced by the swelling is often the only cause of
complaint. In some cases the symptoms are due to the pressure of the
enlarged thyreoid on surrounding structures. In others toxic effects,
in the form of cardiac, nervous, muscular, and general metabolic
disturbances, predominate, and are due to absorption of excessive or
abnormal thyreoid secretion. This thyreoid toxaemia varies in degree;
in the milder cases it merely amounts to a nervousness or
excitability that may unfit the patient for occupation; it reac