The thyroid gland is of a red or reddish yellow colour, of tolerably firm consistence, and gives to the touch the sensation of granulations. Cruveilhier, thus describing, proceeds as follows : " This organ presents all the anatomical characters of glands, and, like them, is separated by dissection into glandular grains ' (doubtless meaning the acini of Malpighi) ; "but there is, be tween these glandular grains and those of ordinary glands, this difference, that in the thyroid gland the glandular grains communi cate with each other, while in the others they are independent." He then details the result of mercurial injection, to show that the glan dular grains, or granulations, have a vesicular structure, and communicate with each other ; this latter statement, however, is certainly erroneous, as we learn from more accurate modes of investigation. The presence of a certain amount of secreted fluid, in the natural condition of the tissue of the thyroid, and the accumulation of a similar material in larger quantities under certain morbid con ditions, the eminent French anatomist justly regards as evidence of the thyroid possessing a secreting apparatus ; but, at the same time, faithful to the results of accurate dissection, he acknowledges that no excretory duct can be found leading either into the trachea, the ventricles of the larynx, or the foramen cmcum of the tongue, whither earlier ob servers had, with too nice refinement, sought to trace its course. Far truer and more phy siological than such straining after uniformity is the conclusion he adopts: " I think that there exist, in the economy, glands without excretory ducts, such as the thymus, the supra renal capsules, and the thyroid gland. The li quidproduced in the gland is absorbed entirely, and fulfils unknown uses." I have thought it worth while to follow this accurate and trust worthy anatomist through his account of the structure of the thyroid, though it be some what antiquated, partly for the sake of the confirmation it affords to the results of a more recondite and powerful analysis, and partly that we may observe how securely we may trust Nature's own teachings, even when they may appear, for a time, contradictory to established doctrines, as doubtless it must once have been considered that a gland should exist unprovided with an efferent duct, I now proceed to give a more detailed account of the structure of the thyroid gland. Its surface is somewhat uneven,— a natural condition which is often greatly exagge rated in hypertrophy of the gland ; it is tra versed by several large branches of the nutrient arteries, which ramify over it before they plunge into its substance. A thin fibrous expansion, continuous with the sheath of the cervical vessels on each side, forms a capsule which invests the gland, and from whose inner surface septa dip into the interior, dividing its substance into lobes and lobules much after the manner of a conglomerate gland ; these fibrous septa are often well seen in sections of hypertrophied specimens. A thin slice of the thyroid, examined under a low power of the microscope, displays its con stitution very perfectly and readily (fig. 733.) It is seen to be made up of closed vesicles, aggregated together in groups of various size by the fibrous expansions just described. The form of these vesicles is primarily spherical ; but many, perhaps the majority, are more or less affected by mutual pressure, being trian gular, elongated, ovoid, or oblong. They are all perfectly closed, the wall being formed by an homogeneous limitary membrane, which is easily traced all round, and can never be seen passing off' into a neck, or blending with the envelope of an adjacent Where a number of vesicles lie closely crowded to gether, the homogeneous envelopes are of course in contact, or separated only by the interjacent vascular plexus ; but those form ing the surface of a group are invested by a thin expansion of fibrous tissue derived from the general capsule. The diameter of the vesicles of the human thyroid I have found to range from rasa inch to A.
inch ; in the bullock, from inch to inch ; the greater number averaging about inch, in this animal as well as in the pig. In the mesian column I found, at least in one instance, that the structure was essentially the same as that of the thyroid itself, only that there was a much greater amount of fibrous stroma, which resembled more nearly ordinary areolar tissue, containing both the white and yellow element.
The vesicles are lined internally by an epithe lial stratum, consisting usually of nuclei set closely together in a scanty basis substance (fig. 734.), which is either feebly granular, or of a somewhat oily aspect. The nuclei are at
once recognised by the practised eye as exactly resembling those of the true glands. Their nucleoli are not always visible, and vary very much in number—from one to four or five. The nuclei are, however, always vesicu lar, bounded by a strongly marked envelope and have a mean diameter of th inch. It has been observed by Mr. Simon, and I have occasionally had the opportunity of confirming the remark, that the nuclei, instead of remaining in their primitive condition, pro ceed to the further stage of cell development ; this he has noticed both in man and in several of the lower animals. I should say that it is certainly a circumstance of rather rare oc currence ; but it is worth remarking, that it may be artificially produced by adding to the specimen some coagulating re-agent, which speedily solidifies a film of albuminous plasma around the nuclei, and thus produces very good imitations of cells. The epithelium of the thyroideal cavities often assumes the form of small vesicles larger than the nuclei (see figs. 736, 737, 738.), and easily distinguished it is, however, certain, that they are not de veloped upon pre-existing nuclei. The layer of epithelium is generally of no great thick ness, not occupying more than one eighth or one sixth of the distance from the envelope to the centre of the cavity ; in the rabbit, however, it appeared to be more abundant, encroaching considerably on the interior, which, in this instance, was not filled with the characteristic glistening secretion. In a sec tion prepared in the ordinary way, a large quantity of epithelium is broken up, and may be seen strewn over the field. Not unfre quently, however, the nuclei adhere firmly together; and sometimes, as in the pig, I have seen the greater part of the lining of a cavity detached entire.
The contents of the cavities are for the most part a clear, somewhat refracting, homo genous material, which is manifestly the pro duct of secretion, and fills all the spaces not occupied by the epithelium : this fluid is some times contained in small vesicles to inch diam. (figs. 739. and 740.) which have a from them.* The diameter of these in a hu man subject averaged inch; in a bullock, about inch. They are, in their natural state, perfectly spherical, but often somewhat angular from mutual pressure. Their contents are a very faintly granular or pellucid material, which does not surround a nucleus except iu some rare instances, where there may be seen an imperfect trace of one. These vesicles, which I thus name to distinguish them from the nucleated cells occasionally met with, exist in the glandular cavities, sometimes alone, sometimes mingled with the ordinary form of epithelium in varying proportion. I am in clined to believe that they originate in the nuclei, which undergo a kind of expansion, at the same time losing their nucleoli. This opinion needs further confirmatory evidence; well marked, structureless envelope, but are destitute of any thing like epithelium. They may be seen occasionally in the interior of the glandular cavities, and also floating free in the field of view, having been perhaps detached from cavities opened by the section. The exact import of this circumstance goes not appear ; for I cannot regard them as newly formed glandular vesicles, developed within the original ones in an endogenous manner. Were this the case they would occur more frequently, and would exhibit some traces of epithelial lining. Large crystals, sometimes of well marked prismatic, sometimes of octo hedral, form, are seen occasionally in the glandular cavities. They are generally single in each, and I have no other guide than their form to lead me to any opinion respecting their chemical constitution. I have seen, in a human thyroid, some large oval or circular corpuscles about inch diam., consisting of coarse granular matter not surrounded by any distinct envelope, and of an opaque dead white colour. These were perhaps ab normal formations ; yetin a tortoise, where the gland was quite healthy, similar corpuscles, and more numerous, were observed. The clear fluid material, contained within the glandular cavities, is generally spoken of as of an albuminous nature. This opinion seems con firmed by two analyses of the gland, made by my friend Mr. Beale, which may be regarded (after allowance is made for the areolar tissue, vessels, envelopes, and epithelium) as express ing pretty correctly the chemical nature of the secretion which forms so large a part of the whole bulk. These analyses I will presently quote, but will first detail a few observations of my own, as to the effects of certain re agents on the fluid in question.