In asserting cyst to be a substantive new growth, with a distinctive elementary groundwork, we exclude all accidental cyst formations, that is, capsules and sheaths forming around foreign bodies, extravasate, or entozoa (cysticercus, for example); as also cyst-like disease of hollow organs consequent upon the closing and obliteration of their excretory ducts and orifices, - for example, in the gall-bladder, the Fallopian tube and uterus, the vermiform appendix, the sebaceous and muciparous glands. Certain gland-elements, however, and in particular those of the thyroid gland, demand an especial consideration, inasmuch as these hollow bodies represent a certain stage in the process of cyst formation, having the same elementary structure, an& being susceptible of ulterior cyst-like development.

Let us begin with the results of an examination with the naked eye of perfect cysts, and in particular of the exquisite specimens so frequently met with in the ovaries.

We have the simple (unicancellated) and the compound cyst (Muller's compound cystoid). The first is sufficiently characterized by its name. Compound cysts declare themselves by phenomena which induced Hodgkin to distinguish them in two classes, although types of both very commonly coexist in the same formation. The first comprehends a cyst-formation with cysts of a secondary order in the parietes of a voluminous (parent) cyst; and these secondary cysts involve, in like manner, cysts of a tertiary order within their parietes. These filial cysts project upon the outer surface of the parent cyst, rather than upon its inner surface, where they are in a degree flattened. The wall of the parent cyst often appears separated, receiving the secondary cyst, as it were, in a chink.

Such a formation is to be distinguished from a group of simple cysts developed in mutual juxtaposition, some one of which predominating in size, flattens the contiguous smaller ones. A group of smaller cysts in an ovary may readily mislead; making it seem as if the fibrous capsule of the ovary were but the wall of a cyst, and as if the smaller cysts interposed between it and a contiguous larger one, were secondary cysts.

The repetition of this process of secondary cyst-formation frequently leads to a very complex cyst-formation, wherein, however, for the most part, a cyst pre-eminent in size, reveals itself as the parent or primary cyst in whose parietes the cysts of the second order become developed. Sometimes the primary cyst is so prolific of this secondary cyst development within its parietes, as to endow the latter with a considerable thickness. It may even cause them here and there to degenerate into a tumor, consisting of an aggregate of cysts, collocated like facetted pouches in the breadth of the cyst-wall, and presenting a polyedrical cell-structure, in and upon the walls of which smaller cysts arise. Occasionally a cyst-wall, so constituted, further degenerates (owing to a rupture of the secondary cysts, and to their bursting into the cavity of the parent cysts) into a multilocular cell or network.

Every cyst is of course competent to represent a parent cyst in relation to its own ulterior cyst production.

The second category comprises cysts in which secondary cysts arise upon the internal surface of the parent cyst, and grow into its cavity. They are sessile upon a broad base, or more often upon a neck or pedicle; in which case they mostly represent pear- or wedge-shaped tumors. They often so luxuriate in number, and at the same time grow to such a size as nearly to fill a parent cyst of considerable magnitude. In rare instances, a solitary cyst of this kind so increases as singly to fill up the space of the parent cyst, causing the sac to consist, down to the base of the filial cyst, of two contiguous layers.

These secondary cysts become developed in the internal layer of the parietes of the parent cyst, and have a sheath derived from the internal membrane, from which the secondary cyst can, with care, be separated. In the pedunculated, pear-shaped cysts, it furnishes, in a state of involution, the pedicle into which the pouch, or wedge-like cyst, projects with a conical tapering end.

They are either simple or compound, according to one or other type. In their wall, namely, reside cysts of an ulterior, that is, a tertiary formation, which grow more or less outwardly or inwardly; the former acquiring a shallow-lobed, blackberry-shape, and appearing cellular within. The pear-shaped cysts commonly consist of several parallel pouches of various lengths. Along with these are found, on the inner surface of the parent cyst, in varying numbers, the smallest vesicles, just cognizable with the naked eye. In one instance these were found on the inner surface of an extensive ovarian cyst, mixed up with, for the most part, naked yellowish incrustations the size of a poppy- or a millet-seed.

The difference between these two types of the compound cyst is obviously not essential, but depends only upon the seat of development of the secondary cyst. Hence, the very common concurrence of the two types.

There also occur, on the inner surface of the cysts, both parent and secondary, ramified cauliflower excrescences, flattened, or fungoid, or pedunculated. These are scattered singly, or grouped together, or knotted in masses. Sometimes they luxuriate in and by the side of the said secondary cysts, to such an extent as to fill both these and the parent cyst, rupturing the latter, and, in the frequent cases of ovarian cyst, invading the peritoneal cavity. In the ruptured secondary cysts we often recognize their sheaths folded back, and reflected over the cauliflower vegetation.

Besides the variations alluded to, there is much that is worthy of note in and about these excrescences.

1. They consist of a very delicate membranaceous growth folded and rolled up in their pedicle, projecting about and especially above it in various ways, branching out into numerous villous and bulb-like processes, or into the semblance of a plaited frill. Upon the said processes are again seated delicate villous flocculi. They are highly vascular, and have a blood-loaded aspect.

2. Here and there they frequently carry, especially at the extremity of their branchlets, a just cognizable, poppy-seed-sized, limpid, or semi-opaque vesicle, or a hemp-seed-, or a pea- or bean-sized cyst.

3. More frequently still they bear upon their twigs solid, though soft, whitish, roundish, or, from mutual compression, indistinctly facetted corpuscles; or else tougher, white, opaque tubercula, mostly of the bigness of millet or hemp-seeds. Here the entire excrescence is commonly white, the small broad-based ones resembling delicate stellar horny warts, whilst the larger and more extensively clustered ones constitute an unyielding tumor, superficially stellate, studded with the aforesaid tubercula upon its peripheral villous structure, permeated throughout its cut surface by fibrous threads (the pedicles). In this tumor, the bloodvessels have become destroyed.

4. Besides the more extensive excrescences, smaller ones are commonly seen, resembling a nap of extremely delicate villi, or of finely pedunculated tubercula.

5. At the same time the internal investment of the cyst has, in expanded parts, often a very finely reticulated aspect; or it reveals very minute fissure-like grooves, not a few of which are surrounded by an elevated ridge-like brink. Out of these is here and there seen to rise a simple or branched excrescence. In other places, the internal layer of the cyst-wall is seen raised into a flattened vesicle, which, like the said grooves, often displays minute fissured openings. Internally is sometimes plainly discerned a convoluted mass of bulb-shaped excrescences, or else a very minute network.

With these are associated larger vesicles, rising into pedunculated wedge-shaped pouches, which contain a very delicate cancellated structure, and frequently exhibit roundish or angular chink-like openings, out of which delicate felt-like excrescences occasionally project. The size of the cysts varies greatly from that of a just cognizable vesicle to that of a sac of from 1 to 2 lines in diameter. The compound cysts may, of course, attain to a very considerable magnitude, a notable portion always appertaining to the parent cyst.

The free space of the cysts hitherto described, is commonly occupied by a serous synovia-like, or a thicker glutinous, or glutino-lardaceous, so termed, colloid moisture.

Examined under the microscope, the following additional light is thrown upon the above appearances.