Cystic tumors are subdivided according to the nature of their contents and the method of their origin. One group is composed of clotted blood within cavities resulting from the laceration of tissues or in preformed spaces. If the cyst primarily is merely a rent, the wall becomes thickened in time from a growth of the limiting tissues, and the blood-clot, of which the tumor was chiefly composed, may remain or become absorbed. If the latter event occurs, its place of deposit may become obliterated by a fusion of the walls of the cyst, or may persist from the subsequent addition of serum. The cystic tumor whose contents are extravasated blood is the hæmatoma, familiar instances of which are met with in the hæmatoma of the dura mater, of muscle, of the vulva, and the polypoid hæmatoma of {116} the uterus. The latter is the long retained and constantly enlarging blood-clot, due to the adherence of portions of the placenta after childbirth. The second group of cystic tumors has for its contents a more watery fluid, and to this the term hygroma is applied. This watery fluid lies, for the most part, within preformed cavities, and its accumulation is connected with a dilatation of these cavities. Instances are met with in the tumors resulting from the accumulation of fluid in the membranes of the brain or spinal cord, and in the ventricles of the former or in the central canal of the latter. These lead to the congenital cystic tumors of the cranium or spine, with watery contents. The ganglion, the house-maid's knee, as also the hydrocele of the tunica vaginalis, are regarded as hygromata. The hydrocele of the neck and elsewhere in the subcutaneous or intermuscular connective tissue is now removed from the hygromata to the tumors which arise from lymph-vessels. A like transfer of other hygromata might be made in accordance with the prevailing views concerning the cavities in which the watery fluid is accumulated.