and hair. In the more complicated cysts brain-substance, striated muscular fibres, and bone have been found. The cysts grow slowly; occasionally they suppurate and rupture spontaneously, in which event the character of the escaping material indicates the nature of the cyst.

The testicle is usually found atrophied from pressure and functionally useless. If the cyst is extirpated, the testicle should be removed with the tumor. Extirpation of the tumor without castration has not yielded satisfactory results.

XXX. RETENTION-CYSTS.

ALL true tumors are composed of new tissue produced from matrices of embryonic cells. All inflammatory swellings are composed of, or are derived from, pre-existing tissue. It remains for us to discuss in this section a form of swelling composed of a sac of pre-existing tissue, with an accumulation of some one of the secretions or excretions of the body as its contents.

Definition. A retention-cyst is a swelling due to the retention in a pre-existing space of a physiological secretion or excretion by obstruction of the outlet of a gland.

The enlargement of a part should be named in accordance with the histogenetic source of its cellular elements, according to which a "hypertrophy" consists of a numerical increase of the tissue-elements of a part or an organ. The term "tumor" should be restricted to

a localized production of tissue independently of mature normal cells; "inflammatory swellings" should include all enlargements consisting of cells derived from the blood or by proliferation of mature tissue, or of accumulations of serum or synovia in pre-existing spaces; and "retention-cysts" should occupy the ground covered by the definition preceding this paragraph.

The greatest confusion exists in the minds of the student in differentiating, from etiological and pathological standpoints, between the different kinds of cysts; this confusion is largely due to the manner in which the subject is treated even in the most recent text-books. A cystoma is a true tumor in which both walls and contents are new products derived from a tumor-matrix.

We have seen that all tumors undergo cystic degeneration by regressive metamorphoses or by the cells producing a secretion which accumulates in the tumor-tissue, owing to the absence of an excretory duct. A cyst may also form in consequence of the extravasation of blood into tumor-tissue or into normal tissue; and, lastly, many so-called "pseudo-cysts" are produced by transudations into preexisting serous spaces. It would be just as proper to call a hydrops of the knee-joint a "hydrothorax," or a hydrocephalus a "cyst," as a hydrocele.

Pathological accumulations of synovia or of serum in serous cavi

ties and in parasitic cysts do not come under the head of retentioncysts. They are inflammatory products, and have no place in a treatise on tumors. The writer will therefore exclude from this section the hydroceles, diverticula, bursæ, neural cysts, and parasitic cysts. A true retention-cyst can form only in organs that produce a physiological secretion or excretion which is discharged by an outlet upon the skin or upon a mucous or serous surface; in other instances the secretion is absorbed at the place where it is produced.

The only instance in which, normally, a glandular secretion is discharged into a serous cavity is furnished by the Graafian follicles of the ovary. The secretion of the follicles of the thyroid gland in a normal condition is absorbed; but if, for any reason, absorption is suspended, the follicles become dilated and eventually form retention-cysts.

Histology. The cyst-wall is composed of the connective tissue, basement membrane, and epithelial lining of the follicle, tubule, acinus,

FIG. 452.-Wall of atheromatous cyst (after Boyce): a, fibrous wall; 6, epithelial layer; c, horny amorphous transformation of epithelium. (Obj. 4 inch, without eye-piece.)

or duct which has become obstructed. The amount of connective tissue as compared with the normal structure of the part affected varies greatly. If the obstruction is acute and the part on the distal side continues to secrete, the pre-existing spaces, according to the activity of the physiological function of the part affected, dilate rapidly, resulting in distention of the gland or duct, with thinning of the wall. If the obstruction forms slowly and the amount of the retained secretion.

accumulates slowly, the cyst-wall is often enormously thickened by the formation of new connective tissue. The best illustration of the former condition is furnished by acute hydronephrosis, and of the latter by sebaceous cysts. The epithelial cells which line the cyst-wall correspond in structure and manner of arrangement with the epithelial cells which exist normally in the lining of the obstructed space.

Cysts of glands lined by stratified epithelium show stratified layers of squamous epithelium (Fig. 452). If the cyst forms in a duct or a gland lined by columnar epithelium, the cyst, at least in its early stages, is lined by columnar epithelium.

FIG. 453-Section of the wall of a cyst of the vagina (after Schröder). The external surface is the pavement epithelium of the vagina; the internal, the cylindrical epithelium of the cyst.

In branchial cysts, as well as in retention-cysts of other tubes or ducts lined by similar epithelium, the cyst-wall is always found lined by ciliated epithelium. Through great pressure the columnar epithelium is often flattened, resembling squamous epithelium, but it always retains its intrinsic capacity to produce, under more favorable auspices, cells of its original type.

Retention-cysts result from mechanical obstruction of the outlet of glands, leading to the accumulation of the secretion behind the point of obstruction. If the obstruction is located near the point at which the secretion is produced, the cyst forms at this point, as is the case in obstruction in a ductlet of an acinus of a gland. If the obstruction is located in a duct some distance from the point at which the secretion is produced, the obstructed duct becomes distended and forms the wall of the retention-cyst.

The cyst-contents are subject to various changes. If inflammation. of the cyst-wall occurs, the contents of the cyst are modified by the addition of inflammatory products. Hemorrhage into the cyst, according to its amount, may simply stain or may constitute the bulk of the

cyst-contents. In cysts lined by stratified epithelium the product of epithelial degeneration forms the well-known atheromatous material, which is subject to still further changes. In young cysts this material appears as a hard mass composed of cells arranged in concentric layers, while in old cysts the cells disintegrate and the detritus is suspended in a serous fluid, presenting the appearance of a thin emulsion. The addition of fat- and cholesterin-crystals further modifies the appearance of the cyst-contents. In mucous cysts the mucoid material is frequently transformed into a clear serous fluid. Cysts frequently become isolated from the gland in which they originated by complete obliteration and detachment of the duct. In retention-cysts that have not been the seat of inflammation the outside of the cyst-wall is surrounded by

FIG. 454.-Chronic interstitial nephritis (after Boyce): a, glomerulus with connective-tissue cell-proliferation; b, commencing cystic dilatation of renal tube; c, fibroid glomerulus. (Obj. inch, without eye-piece.)

a delicate, loose, vascular layer of connective tissue which supplies the cyst with blood-vessels, and which is such an important structure in removing cysts by enucleation-the pericystium.

Etiology. The mechanical obstruction which is invariably the cause of retention-cysts may be—1. Inflammation; 2. Cicatricial stenosis; 3. Tumors; 4. Flexion of a duct, and 5 valvular closure; 6. Altered secretion; 7. Impaction in the duct of a foreign body, a concretion, or a parasite. By far the most frequent cause of mechanical obstruction is inflammation and its consequences.

The effect of inflammation in the production of an obstruction to the