Morgagni, in his Advers. Anat. (1723) dis cusses two questions respecting the thyroid ; one as to whether the gland is double or single, i. e. whether the lobes are connected by an isthmus or not ; this he decides, as re spects man, positively in the affirmative. The other vexata questio, as to whether the thyroid be provided with a duct or not, he confesses to be yet undetermined. He notices the ex istence of vesicular cavities in enlarged thy roids, which he justly supposes to be the natural cavities (nativi acini) dilated by their accumulated secretion. From his examina tion of the secretion of the thyroid, " mollem quendam, et obliniendo lubricandoque ido neum, succum communi isti amygdalarum oleo longe consimilem;" and from observing the thyroid to be exposed to the pressure of contracting muscles, as is the case with some other undoubted glands, he inclines to con sider it probable that the gland has some duct opening into the pharynx, the oesophagus, or into the top of the trachea.
Santorini (Observ. Anat. 1751), recognizes the thyroid as a single gland, and makes men tion of its median column as previously known to Morgagni, though it was probably dis covered by Bidloo or Lalonette. He details the failure of his efforts to discover a duct, though he had several times detected an orifice at the anterior angle of the glottis, into which a bristle could be passed ; and yet re marks that the thyroid gland may be urged to expel its secretion by the pressure of the sub-hyoidean muscles, the throbbings of the carotids, and the contractions of the oeso phagus.
Haller (Element. Physiolog. 1766), in his account of the thyroid, gives a good descrip tion of the median column, and of the several varieties which it presents ; four times only has he found it absent ; most frequently existing on the left side ; sometimes, however, on the right. He relates some experiments of Lalonette, in which it appeared as if the glandular cavities had been distended by inflation with air, and also the lymphatic vessels proceeding front them. The result of this coarse proceeding he explains, and probably cor rectly, by supposing that the distended cavities were those of the areolar texture, and not the secreting vesicles. He remarks that, even
according to Lalonette's testimony, no secre tion can ever be pressed out of the thyroid gland into the cavity of the larynx ; or if any appear it seems to be nothing more than the contents of some mucous follicles. After detailing the struggles and efforts of various anatomists to discover an efferent duct, he states at last that several inquirers, among whom he mentions Ruysch in particular, had adopted the only possible remaining opinion, that a peculiar fluid was elaborated in the gland, which being received into the radicles of the veins, was returned into the blood. This view, which laborious, and thoughtful, and sagacious men were then slow to entertain, is now universally adopted ; and it seems cer tainly a matter of wonder that it was not sooner arrived at. May we not, however, question whether, in regard to other glands, a process somewhat similar does not also occur, —whether certain complementary products of secretory action are not formed in the gland, and afterwards absorbed and carried off by venous and lymphatic radicles ? Meckel's description of the thyroid is as complete as could be accomplished by the most consummate anatomical skill, while un aided by the achromatic lens. I need not refer to his well known pages, further than to notice a suggestion which he offers, viz., that as the median column is much more deve loped in the infant than in the adult, the excretory duct may exist at that period (in the median column), and become obliterated as age advances.
In proceeding beyond this period, we come to the anatomists of our own day ; several of whom have advanced our knowledge con siderably respecting the thyroid and other ductless glands. To none, however, are we more indebted than to Mr. Simon ; whose masterly and philosophical Essay on the Thymus contains the best account of the anatomy and physiology of these organs that has yet been given.