The last of these categories has not usually constituted an element in the consideration of the mode of formation of cysts, and the sub ject has in this way been divested of the difficulties which it must otherwise present. Many accurate observers have expressed a belief that cysts were a consequence of the irritation occasioned by a foreign body ; in this way a large proportion of these organs must be entirely excluded from consideration, or must be treated of under the term acephalocyst. Another class of observers, admitting the exist ence of the foregoing, have added another variety :—they have assumed that the parietes of an alveolus of cellular tissue are attacked by some " morbid qffection" by which all com munication with the adjoining cells is cut off; that the parietes of this alveolus, under the in fluence of irritation, acquire the power of secret ing a product entirely different from that which they furnish in their natural condition ; that the accumulation of this morbid product causes a progressive distention of this small cavity, and a thickening of the cellular lamin in the midst of which the tumour is developed : in other words, that the tumour so produced acts in the same manner as a shot or other body in troduced from without. In the opinion that all cysts are so produced, they are fortified by the belief that, by the process of maceration, of inflammation, or of suppuration, it is possible to reduce the parietes of these organs to their " original element, cellular tissue." Such was the opinion of Morgagni, Haller, Louis. The opinion propagated by Bichat, that a certain uniformity in structure obtains in all cysts, that they are all analogous to serous mem branes, will, it is believed, be found incorrect ; there are many cysts which in structure and function are essentially different from serous tissues, for instance, some are fibrous, cartila ginous, osseous, others are cutaneous, others covered with hair.
Our first class contains the greater number of those subcutaneous tumours which are so commonly seen under the integuments of the cranium, the face, and some other parts of the body, and which contain rneliceric, athero matous, steatomatous, or other matter. It has been over and over again demonstrated that those follicles which open upon the surface of the body may have their aperture obliterated : the secretion from the internal surface of the organ may still proceed, and they occasionally attain a considerable volume; in this way " steatomatous" tumours are produced. The matter contained in these tumours has been analysed by Thenard, who obtained the fol lowing results.
One bundred parts submitted to desiccation were reduced to forty, which treated by alcohol were, in fact, dissolved : the alcohol in cooling deposited a fatty matter, which was easily melted and was similar to adipocire. The residuum, which formed sixteen parts, was of an albuminous nature; consequently there were twenty-four parts of adipocire. This adipocire did not crystallise like that of the biliary cal culus in man ; it was deposited in flakes like those of putrid animal matter dissolved in alcohol : yet, in the matter of the cyst, it was in the form of very brilliant micaceous lainm. These cysts frequently appear very thick, but this great thickness is a consequence of their being almost constantly lined by an inorganic coat, which is sometimes susceptible of being divided into laminw ; when this coat is re moved,there remains a very thin cellular mem brane. 'If the lining membrane be irritated,
the secretion as well as the membrane may be modified ; and the variety of these subcuta neous tumours is thus explained.
Other cysts differently formed appear to ar range themselves most naturally in this class ; of such are those which succeed to the ob struction of a salivary duct, ranula for instance ; those which succeed to a fistulous canal, and are produced by the obliteration of the orifices of such canal ; the mucous tissue by which the canal was previously invested becomes changed in its organization, and a serous cha. racter is acquired :—those which are occasionally produced in the luncrs by the obliteration of .
the canal of communication between a tuber cular cavity and a bronchus ; in this case also a serous membrane is developed within the cavity.
The second class.—Every foreign body, fluid or solid, formed within or derived from with out the animal economy, induces in that eco nomy an effort at expulsion. Whether the body be a shot, a bullet, or other projectile, or whether it be extravasated blood, stone in the bladder, the fcetus in extra-uterine pregnancy, acephalocysts, tubercle, or other heterologous or analogous formation ; in all cases irritation or inflammation is developed, for the purpose of expelling or isolating the nocuous body. If it be in its nature irritating, it excites in flammation, and is expelled with the pus which has been secreted around it; if it have no mechanically or chemically irritating property, it may remain in the midst of the organ, some times passing from cell to cell, obedient al ways to a kind of eccentric movement ; some times nature isolates it by organising around it a cyst which is adherent by its external surface to the surrounding tissues, but which is free and smooth internally,—furnishing a fluid by which many of these bodies may be broken down, and as soon as they are removed, the walls of the cyst become reduced into cellular tissue by absorption.
Frequent opportunities are afforded for ex amining these structures in the cellular tissue. 'When a certain quantity of a succulent fluid is accumulated in this structure, if it cease to increase, the parietes of the cavity which con tains it continues to be the seat of a chronic inflammation by which the formation of a cyst is determined. Until the organisation of this cyst is perfected, the surrounding cellular tissue continues red and indurated; but as soon as the organ is completed, this redness and in duration are commonly in progress of dis sipation ; in some cases, however, they remain, and then it occasionally happens that the cyst participates in the morbid action, and the in terior of the cyst may have a pseudo-membrane developed on its surface. Cysts so developed are at their commencement soft, not very con sistent, and may be easily detached from the surrounding structure. The inflamed stratum, between the cyst and the adjacent healtivr tissue, gradually acquires a greater density and more power of resistance, at the same time that it becomes thinner, and contracts a more intimate union with the proper membrane of the cyst. When the organisation of this spe cies of cyst is completed, the membrane is whitish, opaque, more or less thick, and as a point of comparison, denser, and thicker than a serous membmne, and it presents a surface somewhat similar to that membrane.