Among the various families of the Saurian the thyroid is found to occupy different posi tions ; in some being single and mesial; others double ; in some it lies high in the neck, in others low. Even in the same family its arrangement is not always uniform ; thus among the Lacertidee the gland is single, and of considerable breadth in the true lizards while in the Monitor it is double. Among the Iguanicke, likewise, a similar variety prevails. The Gecholidee, Chama.leonido, and Scificidce present the same general form as the true lizard. In the Chamaeleon it is rather higher (nearer the os hyoides) than in the other families, and is overhung by the sacciform dilatation of the larynx.
In the Amphisbania and Ophidia "the gland lies just above the base of the heart, between the right and left carotid arteries. It is a little hidden by the thymus of each side ; and in those genera which possess a fit body this large organ lies conspicuously in front of both the thymus and thyroid." In the Batrachian order there has been found, in the common Frog, on each side a small glandular body, which Mr. Simon de clares is unquestionably possessed of true thyroid structure. They are situated on the carotid arteries, just beside the cornua of the hyoid bone. Huschke conceived these bodies to have their origin in the shrinking of the bronchia', and endeavoured to establish that the thyroid generally had its origin in the transformation of the branchial arches in the early embryo. This hypothesis, however, Mr. Simon well remarks, appears refuted by the existence of the gland in a perenni-branchiate animal, the Menobranchus, where it consists of two symmetrical portions connected with the inferior border of the os hyoides, one on each side.
In the class of Fishes there seems yet some doubt whether a true thyroid gland really exists. Mr. Simon believes that he has discovered the organ in many fishes, enumerating the Carp (Cyprinus), Pike (Esox), Cod (Gallus), Had dock (Illori.hua), Whiting (11ferlangus), Eel (Anguilla), Sturgeon (Accipenser), Shark (Squalls), and Skate (Raia);— it seems also to be present in the Anableps, Exoccetus, Cal lorhynchus, and Lamprey (Petramyzon), but the evidence for its existence is less conclusive. It may occupy, he states, either of three posi tions; (1) as a single organ situated in the median line in connection with the basibran chials, and supplied with blood from the first branchial vein while yet within the gills. (2) " In the Gadidm the gland is double. One portion lies on each side, not, as in the last case, at the anterior extremity of the first branchial arch, but near its posterior or ver tebral end. Here it occupies part of a recess which is bounded by the gill below, and above by the outer extremity of that transverse fold of mucous membrane which limits the extent of the palate. It is merely covered by mucous membrane, which leaves it apparent to the eye without need of any express dissection. Its vascular supply is reflected to it from the ophthalmic artery, which arises before the formation of the systemic aorta from the first branchial vein close to the origin of the pro per encephalic artery." (3) " In the carp, anableps, pike, and exoccetus, the gland is placed at the inner extremity. of the same duplicature of mucous membrane, and more toward the palate, so as to lie upon the fibres of the pterygoid muscle." Though there is this variety of situation, yet Mr. Simon re gards these several organs as constantly agree ing in one point, viz., in deriving their vascular supply from the first bronchial vein, and thus being brought into connection with the ence phalic nervous centre, by their nutrient streams having origin from a common source. Pro
fessor Owen dissents from the view that the pseudobranchim are the analogues of the thy roid gland. He states that in osseous fishes they are not diverticular to the cerebral circu lation, but only to the and in most cases are subsidiary, in this respect, to the cho roid vaso-ganglion. The sublingual gland of Retzius is the organ which Professor Owen considers as most nearly representing the thyroid, though he suggests a doubt as to whether, by reason of its relations to the heart and great vessels, it may not more properly be regarded as the analogue of the thymus.
It is with some hesitation that I proceed to mention the results of my own examination of some specimens from the three classes pointed out by Mr. Simon, in which he seems to regard the presence of a thyroid as un doubtedly ascertained. I have, however, care fully examined the structure with the micro scope in every instance, and I believe I may refer to the observations, so far as they go, as free from any material error. In the Skate I have found the organ described by Retzius as a salivary gland, and by Mr. Simon as a thy. roid, occupying the situation well described by the latter, and lying exactly upon the terminal division of the branchial aorta. It was of a faint reddish gray tint, and presented to the unaided eye the appearance of a conglomerate gland. No excretory duct, however, was ob served proceeding from it. In structure it consisted of numerous vesicles aggregated to gether. The form of these was mostly circular ; some were elongated, and many variously altered by mutual pressure. Their diameter was about_ to inch (fig. 744. A). The limiting envelope of the vesicles presented a good example of homogeneous membrane. It was lined internally by a pretty thick stratum of epithelial substance, which in some instances was so abundant as almost to fill up the cavity. The epithelium consisted of nuclei and a very large quantity of rather coarse granular ma terial, which quite obscured the nuclei them selves (fig. 744. B.) There were also a few granular cells, and, in the interior of many of the vesicles, imperfect prismatic or octohedral crystals could be discerned. I could not discover, among the glandular structure, any tubes resembling excretory ducts ; so that I am much inclined to believe it has no relation to the salivary organs, but belongs to the class of ductless glands. A gland, however, it assuredly is, and not a mere vaso-ganglion. Besides this body I discovered at some dis tance behind it, just at the junction of the branchial arches anteriorly, a small light red dish mass, which was covered in by a thin fascia, and by the mucous membrane, and could not be seen till the latter was dissected off. Its structure was almost exactly similar to that of the organ just described, cm sisting of vesicles about ,4 to m inch diameter (fig. 745.), with a thick interior stratum of epithelium resting on a beautiful homogeneous limitary membrane. The pscu dobranchia, situated on the anterior wall of the spiracular canal, is manifestly of entirely different structure to the organs described. It consists of small plicae of mucous membrane, covered by a kind of pavement epithelium. In a Dog-fish (Spinax) the pseudobranchia was very small, but distinct. 1 could discover no trace of the sublingual gland, or of the small one behind it which I found in the skate.