The condition in which the epithelium of the thyroideal cavities is usually found is worth observing, and seems susceptible of a probable interpretation. It may be stated as a pretty well established fact, that the nucleus of a cell is the essential part, that in it resides that influence, or is developed that force, which produces all the phenomena of growth and assimilation ; that, so long as the nucleus persists, the energy of the cell, if one has been formed, continues to be manifested ; but, if it has disappeared, the active life of the cell is at an end. It is also certain that the comple tion of a cell, i.e. its being surrounded by an envelope, is by no means an essential circum stance ; that all the purposes of cell life may be effected by the mere aggregation of granular matter around a nucleus ; that, in fact, the cell wall or envelope is of no importance, or but very little, in the metabolic changes which are produced, and that its presence, when it exists, seems merely to denote a certain per manence of duration in the particle. Many examples of the correctness of these state ments, will occur to every one who is in the habit of examining the glandular organs in man or the lower animals. I believe we may also advance a step beyond these doc trines, and regard it as very probable that, when we find an epithelial structure con sisting principally of bare nuclei, with but a scanty interposed quantity of granular matter, the secreting action there effected is of a rapid and simple kind ; the destined pro duct being quickly formed and thrown off, and not slowly evolved within the chamber of a cell. The following instances may be re ferred to, as illustrating the extreme varieties of secretory action in reference to this par ticular. The cells in which the spermatozoa are formed must be of considerable per manence, the development of the filaments taking place gradually, and in one instance, as Mr. (Ioodsir has shown, only being completed in the sperroatheca of the female. The biliary cells of various annelids are at first filled with pale granular contents ; but gradually these are replaced by the characteristic molecules with which the cell becomes at last distended, and thus remains, often for a long time. The cells of the kidney of the common snail, which are very perfectly formed, enclose, within a well marked envelope, an opaque white mass of uric acid, which, after a long time, may increase to such a degree as com pletely to fill the cell. These cells are very permanent.* In contrast to these instances, wherein complex and elaborate products are formed in complete cells by a secretory action of a slow and deliberate character, we may refer to (I) the absorbent glands, which doubtless produce some change in the fluid which traverses them ; this, however, must necessarily be a rapid process, and, accordingly, scarce any cells are formed ; the mass of the glandular paren chyma consisting of very perfect nuclei ; (2), the terminal hepatic ducts, whose walls, as I have shown, consist almost entirely of nuclei set close together, and which, by means of these effective agents, eliminate the actual biliary secretion from the varying, partly biliary, partly oily, fluid formed by the hepatic cells on their exterior ; this process of elimi nation is, I believe, continually going on, and is not so much of a metabolic as of an ab sorbing nature; the essential change being probably effected by the hepatic cells of the lobules; (3) the nucleated tissue forming the principal part of the villi, which scarce ever developer cells,* but is constantly attracting the chylous fluid through the basement mem brane from the cavity of the gut, and permit ting it to pass off by the efferent lacteals (here scarce any chemical change appears to take place) ; (4) the cineritious matter of the cerebral hemispheres, which, amid the rapid alternations of sensation, thought, and volition, must be undergoing incessant change, consists, in by far the greater part, of nuclei and granular matter, the fully formed vesicles being few and far between.
Applying, now, these views to the case of the thyroid, there seems reason to believe that the ordinary condition of its epithelium is such as to adapt it for rapid and transitory action ; so that a large amount of secretion may be quickly thrown into its cavities on any sudden occasion ; which again would easily transude through the thin epithelial layer and homo geneous tunic, when the time of action had passed by.
It may also be remarked, as consonant with the views above stated, that as respects its chemical nature the secretion of the thy roideal cavities is of a simple kind, not appa rently requiring much elaboration. It seems, in fact, to be a mere modification of albuminous matter containing, it may be, some gelatine, but strongly contrasting with the highly wrought products of the renal and hepatic laboratories. This implies that the change effected by the thyroideal epithelium on the liquor sanguinis supplied to it is by no means considerable.
USE.- Cruveilhier, about fifteen years ago, briefly says, " the use of the secre tion of the thyroid is unknown ; " nor can the anatomist of the present day give a much more satisfactory account. So en veloped in mystery the use of the gland seems to have been always regarded, that inquirers have been more willing simply to confess their ignorance than is usually the case, and fewer speculations and hypotheses have been broached respecting this than regarding other points which promised at first sight to be of easier solution. It is scarcely worth while to mention the opinion, which supposes the thyroid to have any essential connexion with the larynx, either as pouring into it, through supposed ducts, a fluid fitted to lubricate the lining membrane, or, as Sir A. Carlisle sup posed, forming a protection to the delicate organs of the voice, against the variations in temperature of the external air. There seems no doubt that the relative position of the thyroid to the larynx is quite unimportant, so far as the function of the organ is con cerned. This is borne out by the variations of its site which occur in birds, and by the results of morbid action ; since prodigious goitre does not induce disease of the larynx, except in a mechanical way, i. e. by injurious pressure.
Passing over more crude conceptions, we come to consider a theory which has been propounded by Mr. Simon, and which has certainly every claim to our careful attention, both from the character of its author, and as it is the only one yet promulgated which can be said to be even probable. He considers that the thyroid acts as a diverticulum to the cerebral circulation ; exercising, at the same time, its secretory function in an alternating manner with the encephalic nervous mass. His words are, " What diversion is to the stream of blood viewed quantitatively, alter native secretion would be to the composition of blood viewed qualitatively ; and I should conceive that the use of the thyroid gland, in its highest development, may depend on the joint exercise of these two analogous func tions. I should suspect not only that the thyroid receives, under certain circumstances, a large share of the blood which would other wise have supplied the brain, but also that the secretion of the former organ bears some essential relation (which chemistry may here after elucidate) to the specific nutrition of the latter ; that the gland,—whether or not it appropriates its elements in the same proxi mate combination as the brain does,—may, at all events, affect, in a precisely similar degree, the chemical constitution of the blood tra versing it ; so that the respective contents of the thyroid and cerebral veins would present exactly similar alterations from the characters of aortic blood. Finally, I should suppose that these actions occur only, or chiefly, during the quiescence of the brain, and that when this organ resumes its activity the thyroid may probably render up again from its vesicles to the blood, in a still applicable form, those materials which it had previously diverted from their destination." This theory mainly rests on the circum stance that the thyroideal arteries arise in close proximity to the cerebral, and this is I think sufficiently constantly the case to form a strong argument in its favour. It must be remembered, however, that variations in the place of origin of the arteries both of the brain and thyroid, do occur without, as far as we know, any interference with the full discharge of the functions of the gland ; and it may also be considered probable that the purpose of a diverticular stream would have been better attained, if the origin of the vessels had been below instead of above the point of giving off of the arteries to the brain. Besides, however, this argument, two others may be mentioned which at least favour the same view. One is, that no special characteristic principle appears to exist in the secretion of the thyroid, but that it is a mere modification of albuminous matter,—this seeming to imply that no special use is served by the secretion of the gland, that it is not elaborated for the sake of pro ducing any peculiar effect on some other part, but that it is simply secerned from the circu lating current for a time, to return and mingle with it again in a condition but little altered from its primitive one of blood-plasma. The other argument is drawn from the con dition of the epithelium, which, as we have before remarked, seems adapted for rapid and transitory action, so that it might quickly secrete a large amount of material on any diminution of the nutrient processes in the brain. These arguments may be allowed to possess some weight. Before, however, this theory can be regarded as at all established, a more sure and discriminating chemistry must prove some relation of composition to exist between the secretion of the thyroid and the grey nervous matter. Till this is done we can but deal with the question afar off, without bringing it to an exact issue.