MORPHOLOGY. - The preceding details are abundantly sufficient to prove the glandular nature of the thyroid. This truth, which elder anatomists saw clearly, though rather afar off, we, by more intimate and minute scrutiny, are enabled to confirm and establish in every par ticular. Let us take in order the several parts of the secretory apparatus of the thyroid as we have described them, and see whether they do not exactly correspond to homologous parts in any undoubted gland. The recep tacular cavities, enclosed by envelopes of homogeneous membrane, manifestly represent the tubes of the kidney or testis, or the ter minal vesicles of the salivary gland. The limitary tissue is identical in appearance in each ; but disposed in the one so as to form shut sacs ; in the others to ensure a pervious canal or outlet, for the secretion. In the true glands the limitary tissue or basement membrane supports, on its interior, a layer of epithelium, essentially consisting of nuclei and granular matter. The same tissue in the thyroid is lined internally by a similar layer, which differs only in the smaller quantity of granular matter interposed among the nuclei, and in both cases the formation of perfect cells (i. e. with envelopes) is uncom mon. The exterior of the limitary tissue is in contact, in all the true glands, with a capillary plexus, from which the materials for the nutrition and growth of the epithelium are furnished. The same disposition exists in the thyroid. A certain amount of fibrous tissue, or some modification of it, penetrates more or less extensively among the elementary parts of the true glands, and serves to pack and unite their component parts together ; this also we have had occasion to describe in the thyroid. With respect to a supply of lym phatic vessels and nerves, it is sufficient to state, that as far as ordinary dissection can go, the thyroid is similarly circumstanced with the other glands ; and there is no reason to doubt that the actual arrangement of these parts in all is the same. The parallel, thus exactly sustained in every particular, warrants us in regarding the thyroid as the TrapaiEtypa of the class of ductless glands, and it may be not without advantage, if we take occasion to note, in the other organs belonging to the same class, how gradually the strongly marked characters are laid aside, till the glandiform organ passes into a modification of vascular structure. In the supra-renal capsules the limitary membrane, though described by an eminent anatomist as usually forming tubes of various length, enclosing celloid epithelial particles, is, according to my observation (which is however but limited), very faint, or entirely absent. I have never been able to discover it in the human adult or foetal sub ject ; and in the sheep the cortical structure has seemed to consist simply of rows of celloid nucleated particles, without any enclosing membrane. Conceding, however, that in these organs the limitary tissue does exist, there can be no doubt that it is much more feebly de veloped than in the thyroid, while, as a set off, the epithelial particles are more perfectly formed. In the case of the thymus the limitary tissue is well marked, and resembles very much, in its arrangement, that of the con glomerate salivary gland ; but the epithelium is remarkably modified. The nuclei exist, but
almost alone. No granular matter of any pecu liar properties is formed around them, prepa rative to, and evidencing the existence of, secretory action. In the spleen both these alterations coincide. The epithelium is reduced to mere nuclei aggregated in masses round prodigiously developed venous radicles, and the limitary tissue is discoverable nowhere, not even surrounding those curious spherical masses of nuclei the white corpuscles of Mal pighi. The only circumstance I know of,, which seems to indicate that even in the spleen a kind of secretory action does occasionally take place, is, that there frequently occur, in the parenchyma, some peculiar yellow cor puscles, which are most abundant in fishes, and of which I gave a detailed account some years ago. The very presence, how ever, of these (for they are by no Ineans constant), as well as their condition, argue strongly that secretion is not the appointed function of the splenic parenchyma. From this, the last and lowest of the ductless glands, we descend to erectile tissue, in which the venous portion of the sanguiferous system is even more highly de veloped than in the spleen, and where the intervening parenchyma is still further reduced in quantity ; yet, from the phenomenon of afflux of blood to the part, it must be'regarded, I think, as exerting an attractive force, as is unquestionably the case in the spleen.* Returning from this digression to the con sideration of the thyroid, we may lay it down as fully established, that it is a gland whose secretion is formed, and collects, in closed cavities. Now from this fact we are able easily to deduce another, viz., that the secre tion when formed is capable of being absorbed from the receptacular cavities ; for otherwise these would go on enlarging and distending themselves indefinitely, as in fact they do under certain morbid conditions. But, though there may be various causes concerned in the production of bronchocele, yet I think it must be regarded as proved, by Mr. M'Clellan's inquiries, that certain waters are adequate of themselves to produce this disease ;—it would appear that when they are drunk, some in gredient or principle is supplied to the blood, which, being in excess, is straightway elimi mated by the epithelium of the glandular cavities of the thyroid, and thus collects in, and distends them. If a patient in whom this has occurred be removed from using this unwholesome water, and if the natural ab sorbing power be aided by the influence of iodine, then, the supply of the substance for which the thyroid has a special attraction being cut of the excess collected in the receptacular cavities returns speedily to the circulating current, which now, being in a minus condition as to this principle, readily resumes it. In such cases there seems, in fact, to take place a very analogous process to that which occurs in ordinary fattening and emaciation ; if an excess of oily matter exist in the blood, old fat vesicles enlarge, and new ones are formed ; if the reverse is the case, the oil in the fat cells is readily absorbed into the blood circulating in the capillary loops around them.