Liq. Potasste, added to a thin section pre pared for the microscope, rendered it much more transparent, partially dissolving the epi thelium, and leaving a quantity of oily matter diffused throughout it. Acetic acid now added to the specimen pretty nearly restored it to its former appearance, but did not bring into view any precipitated protein. Acetic acid, alone, dissolves in part the epithelium of the vesicles, and renders the fibrous tissue more transparent. Liq. Anzmonice dissolves the epithelium in great part, but does not alter the transparent contents of the cavities ; nor does liq. potassre or acetic acid. Solution of iodine does not materially affect the epithe lium of the cavities, but renders it more opaque. Strong nitric acid at first renders the epithelium more opaque and granular, but does not manifestly affect the contained secretion. After a time it colours this material bright green or yellowish green, and disengages a great many bubbles of gas. A saturated solution of Dichloride of mercury, even after long maceration, does not seem materially to affect the secreted contents of the vesicles; it makes their peripheral stratum of epithelium quite opaque ; but the interior still appears transparent and glistening.
The chief conclusion deducible from the above results is, that the secreted material of the glandular cavities of the thyroid is not ordinary fluid albumen ; as otherwise it would certainly be coagulated by the agents em ployed. The effect produced by nitric acid is also worthy of notice, though I cannot ex plain tke meaning of it.
The analyses given above testify to the presence of a large quantity of fibrinous and albuminous matters in the gland, and leave no doubt that its secretion is a protein com pound; it is, however, unfortunately impos sible to procure a sufficient quantity apart from other substances to analyze correctly ; and the exact nature of the thyroideal secre tion consequently still remains unknown. Thus much, however, seems to be ascertained, or rendered very probable. (1) That the secreted material is of an albuminoid nature. (2) That it is not in the state of ordinary fluid albumen. (3) That gelatine is sometimes an ingredient of the secretion; (it was found in the gland of an ox, but not in that of the human subject, and consequently could not have been derived from the fibrous tissue). (4) That though crystals of triple phosphate and of oxalate of lime occur in the cavities, no urea nor lithic acid, nor in fact any special organic compound, can be detected.* Vessels.— The vascular supply of the thy roid is very abundant, and completely justifies Cruveilhier's opinion, that more than a mere process of nutrition is carried on in the gland.
The arteries which are distributed chiefly to this organ are very constant in their number, and tolerably so I think in their respective dimensions, though in this respect they vary inversely with regard to each other. They
ordinarily arise, as has been well remarked by Mr. Simon, just beyond the points where the arteries to the brain are given off from the large trunks,—a circumstance which he con ceives to be very significant of the function of the gland which they supply ; — the two superior thyroideal arising one on each side from the external carotids, almost immediately after the bifurcation of the common carotids, and the inferior thyroideals from the intra scalenal portion of the subclavian, almost opposite the point where the vertebrals are given off. A fifth thyroideal artery occa sionally exists,—that named after Venbauer ; taking its origin from the arch of the aorta or the arter.innominata. The superior thyroid not unfrequently takes origin a little lower down from the division of the common carotid, or even from its trunk ; or it may arise higher up from a common trunk with the lingual. It courses first forward and inward, when it is covered only by the deep fascia and platys= ; but it soon turns vertically downwards, and runs beneath the sub-hyoidean muscles to the upper extremity of the gland, where it divides into three branches ; one of these runs be tween the thyroid gland and the trachea, a second skirts the external border of the lateral lobe, while the third, running along the inter nal border, forms an anastomosis with the cor responding branch of the opposite side.
The inferior thyroid is noticed by Cruveil bier as one of the arterial branches most liable to vary in its origin,—an opinion which, emanating from a less high authority, I should have been inclined to question. It may arise, acccording to him, from the common carotid, the arch of the aorta, or the arteria innomin ata. (The supra-scapular often, less commonly the posterior scapular, and sometimes even the internal mammary, spring from the com mencement of the inferior thyroid, which is therefore called the thyroid axis.) Its course is peculiar ; it runs at first straight upwards, then comes downward, and again ascends to reach the inferior extremity of the lateral lobe of the gland. It passes in front of the trachea, and behind the great vessels and yaps nerve : the connecting cord ofthe sympathetic descends behind it to the middle cervical ganglion when it exists, which is then almost constantly found, as it were, seated astride upon the vessel, exactly on the convexity of its first curve. Like the superior thyroid, it has three terminal branches, one running along the in ferior border of the gland, another breaking up over the posterior face of the lateral lobe, and a third which penetrates between the gland and the trachea, and anastomoses with the one of the opposite side along the upper border of the isthmus. (Cruveilhier).