Colloid, colloid substance, is a sufficiently common heterologous formation. It is requisite, however, to state, that under this term substances have been brought together which, in a physical and chemical respect, are not perfectly uniform; for instance, the colloid of the thyroid gland, the substance of collonema, on the one side, and on the other, gelatinous cancer. Further inquiry may, however, show such differences to be but modifications and gradations of the same substance.

Moreover, the occurrence of colloid abuts so closely upon the physiological, that it is difficult to define its pathological significance. Thus, it accompanies the often mere passing development of the thyroid gland, the secretion of certain follicles undergoing occasional cyst-like development, especially at the cervix uteri; and again it forms the contents of glandular growths in the progress of cyst-like dilatation. In other instances the appearance of colloid is too obviously of pathological import to admit of any doubt. It constitutes both innocent and malignant new growths.

Colloid is a semi-fluid adhesive substance, resembling a saturated solution of gum or glue, or a fruit jelly. It is seldom colorless, ordinarily of a honey or pale wine-color, but often brown, or green, sometimes black. With all these tints it is clear and pellucid, and only now and then turbid, flocculent. Microscopically examined, it displays, in smaller or greater number, elementary granules, nucleated forms, nucleated and non-nucleated cells, together with parent-cells, in rare instances (even in colloid of the thyroid gland) the pouch-like formations mentioned under the head of metamorphosis of blastemata, and even caudate cells.

With respect to chemical composition, the reactions are those of various gradations of casein, of pyin, of certain kinds of mucus.

Colloid

Colloid is, for the most part, found accumulated in hollow organs, in follicular, alveolar, cystoid spaces, and so seldom free within a texture, that the former mode of its occurrence has been regarded as pathognomonic of its true character.

1. It is most frequently met with in the thyroid gland, so frequently, indeed, that few thyroid glands are examined in which more or less of it is not here and there detected. It is accumulated in the cyst-like dilatations of already existing acini, as well as in others of new growth. The disease represents lymphatic, and in further development, cystic goitre. It occurs, moreover -

2. In simple cysts (whether new growths or morbid developments of pre-existent hollow organs, for example, cysts of the kidneys consecutive to Bright's disease), and also in compound cystoids, - of the ovary, for instance.

3. In the pituitary gland, as a pale amber-colored layer interposed between its two lobes, believed by Wenzel to be the cause of epilepsy.

4. In serous sacs, as a remarkable transformation of a fibro-croupous exudate into colloid. Andral has witnessed this in a pleural, and we ourselves in peritoneal exudates.

5. A colloid substance constitutes collonema, and its kindred, benign, new growths. (See Sarcoma).

6. A colloid resembling the vitreous secretion of mucous follicles forms the contents of alveoli, and of their endogenous cysts, in innocent and malignant new growths, - sarcomata and cancers, especially true alveolar cancer.

A question of great interest is, whether colloid is secreted as such.

(a.) Several facts, especially the appearance of colloid in the Mal-pighian bodies of the kidneys, but likewise the transformation of the aforesaid exudates into colloid, afford conclusive evidence that, under some unascertained conditions, albumen and fibrin become converted into colloid.

(b.) Other facts render it probable that it is the product of an altered function of secreting gland-cells, or of the action of anomalous cells, parent-cells, aveoli, and cysts.

(c.) The colloid of the thyroid gland in its voluminous occurrence, as endemic goitre, merits an attentive consideration on account of its character of exclusiveness in relation to tuberculosis. The alienated habit of body acquired in endemic goitre, may, indeed, be indicative of a change in the crasis; although as to the nature of such change, its relation to the function of the thyroid gland, and its character of antagonism with tuberculosis, we are altogether in the dark.

Colloid undergoes many spontaneous changes. Besides its resorption, as observed in colloid of the thyroid gland, it becomes, in cyst, diluted by the thin secretion from the cyst-wall, or else, under gradual extinction of the cyst, condensed, and eventually changed into a brittle substance resembling dried glue. Lastly, it achieves -

(a.) A remarkable conversion to molecular fat, becoming yellowish, turbid, opaque, and unctuous (colloid of the thyroid gland, gelatin of alveolar cancer).

(b.) In a few instances, cretefaction, - in colloid of the thyroid gland. These cases are, however, quite distinct from the cretefaction and ossification of fibrinous exudates within the strumous thyroid gland.