n. It is sometimes found more satisfactory to dissect out
the cyst through an incision below the jaw, and in the event of
recurrence this should be undertaken.

Cystic tumours, similar to the sublingual ranula, form in the other
glands in the floor of the mouth--for example, the incisive gland,
which lies just behind the symphysis menti, as well as in the apical
gland on the under aspect of the tip of the tongue. The latter is
distinguished by the fact that it moves with the tongue. In rare cases
children are born with a cystic swelling in the floor of the
mouth--the so-called _congenital ranula_. It is usually due to an
imperfect development of the duct of the submaxillary or sublingual
gland.

#Solid Tumours--Mixed Tumours of the Parotid.#--The most important of
the solid tumours met with in the salivary glands is the so-called
"mixed tumour of the parotid." This was formerly believed to be an
endothelioma derived from a proliferation of the endothelial cells
lining the lymph spaces and blood vessels of the gland. A more
probable view is that it develops from rests derived from the first
branchial arch an not from the parotid. The matrix of the tumour is
made up of cartilaginous, myxomatous, sarcomatous, or angiomatous
tissue, the proportion of these different elements varying in
individual specimens, and it may include some portions that are
adenomatous. A gelatinous substance forms in the intercellular spaces
of the tumour, and may accumulate in sufficient quantity to give rise
to cysts of various sizes. There is reason to believe that the tumours
of the parotid previously described as adenoma, chondroma, angioma,
myxoma, and many of the cases of sarcoma, were really mixed tumours in
which one or other of these tissues predominated.

The tumour usually develops in the vicinity of the parotid, and
presses on the salivary tissue, thinning it out and causing it to
undergo atrophy.

_Clinical Features._--The mixed tumour is usually first observed
between the ages of twenty and thirty. It is of slow growth and
painless, and forms a rounded, nodular swelling, the consistence of
which varies with its structure. The skin over the swelling is normal
in appearance and is not attached to the tumour (Figs. 263, 264). Only
in rare cases does paralysis result from pressure on the facial nerve.

[Illustration: FIG. 263.--Mixed Tumour of Parotid.]

[Illustration: FIG. 264.--Mixed Tumour of the Parotid of over twenty
years'