In making a third class, it must be obvious that we incline to the opinion of Delpech, " that certain cysts do not proceed from an accidental and mechanical modification of the cellular tissue," but that they are so many new organs, so many newly developed tissues, which do not possess either the same degree or even the same kind of vitality as the surround ing parts.
In this class we range those which contain a serous or sero-mucous fluid, which are de veloped in various parts of the body. Their parietes are sometimes transparent, at others opaque; upon their inner surface they uSually present a kind of tomentum or velvet-like tex ture, sometimes it presents hair. Their ex ternal surface is sometimes free on all sides except that upon which the vascular commu nication obtains, sometimes they are com pletely adherent. They are observed free and almost floating in the cerebral cavities, in the kidney, the liver, the lungs, and in all serous cavities.
We also include in this class certain syno vial cysts, which are observed around the articulations of the hand, of the foot, some times of the knee, and in the neighbourhood of the sheaths of tendons. Some persons have been disposed to refer the origin of these organs to a displacement of the synovial mem brane which has yielded at this point; but ob servation has demonstrated that they are cysts with dense and fibrous external, and serous internal parietes, developed in the cellular tissue surrounding the normal synovial sac.
ln the same class we place a species of cyst developed, so far as we yet know, under the anterior annular ligament of the carpal articu lation,— more rarely in the vicinity of the tibio-tarsal articulation, but always around sy novial sacs or tendons, and essentially con stituted of small white bodies, in appearance similar to small grains of boiled rice.
Of the serous cysts, we may frequently find some very small, and, as nearly as may be, empty, the membrane being puckered and plicated, and in contact with itself at points where the Om meet. At a certain period of their existence there is scarcely a particle of fluid accumulated in them, and of course the first exaggerated exhalation which has place will be lodged without any obstacle in the cavity, the plicm will be effaced, and the pa rietes removed to a certain distance, the one from the other. It is probable that this pro portion between the cyst and its contents is maintained until some irritation shall acce lerate the exhalation, much as in the serous cavities of the body. This exhalation is some
imes so abundant and rapid that the parietes become irritated and inflamed, and these tunics, at first characterised by so much tenuity, may, by the pure and simple effect of their rapid development, or as a consequence of their relation with very moveable organs, or by the effect of accident, to which they are exposed, become susceptible of almost unlimited trans formation.
We believe, therefore, that all the varieties composing this class owe their existence to irritation ; in the synovial the irritation is spe cific and caused by pressure,—in the serous, we believe it to be of another kind,—in many of them it is similar to that which presides over the development of hydatids: the only difference between certain of them, those, for instance, which are so nearly isolated, having merely a vascular communication, and an hydatid, is perhaps simply, that their existence has not been sufficiently prolonged to permit with safety the rupture of this umbilical cord, if I may so term it, by which they are con nected to the surrounding tissues. We must now endeavour to explain the circumstances 'under which these cysts are developed.
The experiments and observations of Cru veilhier shew, in the most convincing manner, that humidity, abundance, and the bad or ve getable quality of the nourishment of an animal, are unequivocal means of producing acephalocysts. If by the concurrence of these circurnstances acephalocysts may be produced, it must be evident that by the agency of the same causes a modification of existing tissues, —irritation, in fact, of a specific kind, has been excited by which a state favourable to their developrnent has been produced. Admitting then that by such means a particular kind of irritation may be set up in certain tissues, we must go further; that irritation must be suf ficient to cause the exhalation of a particle of lymph, that lymph, as in tbe case of a pseudo membrane, becomes organised, acquires step by step an individual existence, it will be the minimum of organisation and independent vitality, but still, when its separation is achieved, it will be a living being. Supposing this idea to be correct, it may follow that a variety of modifications of such products, more or less independent, may be in a similar manner pro duced.