#Lymphangioma.#--A lymphangioma is a swelling composed of a series of cavities and channels filled with lymph and freely communicating with one another. The cavities result either from the new formation of lymph spaces or vessels, or from the dilatation of those which already exist; their walls are composed of fibro-areolar tissue lined by endothelium and strengthened by non-striped muscle. They are rarely provided with a definite capsule, and frequently send prolongations of their substance between and into muscles and other structures in their vicinity. They are of congenital origin and usually make their appearance at or shortly after birth. When the tumour is made up of a meshwork of caverns and channels, it is called a _cavernous lymphangioma_; when it is composed of one or more cysts, it is called a _cystic lymphangioma_. It is probable that the cysts are derived from the caverns by breaking down and absorption of the intervening septa, as transition forms between the cavernous and cystic varieties are sometimes met with. The _cavernous lymphangioma_ appears as an ill-defined, soft swelling, presenting many of the characters of a subcutaneous hæmangioma, but it is not capable of being emptied by pressure, it does not become tense when the blood pressure is raised, as in crying, and if the tumour is punctured, it yields lymph instead of blood. It also resembles a lipoma, especially the congenital variety which grows from the periosteum, and the differential diagnosis between these is rarely completed until the swelling is punctured or explored by operation. If treatment is called for, it is carried out on the same lines as for hæmangioma, by means of electrolysis, igni-puncture, or excision. Complete excision is rarely possible because of the want of definition and encapsulation, but it is not necessary for cure, as the parts that remain undergo cicatrisation. [Illustration: FIG. 76.--Congenital Cystic Tumour or Hygroma of Axilla. (From a photograph lent by Dr. Lediard.)] The _cystic lymphangioma_, _lymphatic cyst_, or _congenital cystic hygroma_ is most often met with in the neck--_hydrocele of the neck_; it is situated beneath the deep fascia, and projects either in front of or behind the sterno-mastoid muscle. It may attain a large size, the overlying skin and cyst wall may be so thin as to be translucent, and it has been known to cause serious impairment of respiration through pressing on the trachea. In the axilla also the cystic tumour may attain a considerable size (Fig. 76); less frequent situations are the groin, and the floor of the mouth, where it constitutes one form of ranula.