THYROID GLAND. 



J109 



sides were large, ^ to ^ inch diameter, closely 

 aggregated together, and very variously altered 

 from the spherical form by mutual pressure. 

 There was very little investing areolar tissue. 

 The epithelium consisted of a single row of 

 nuclei imbedded in granular matter, which 

 was more abundant than usually is the case. 

 In some parts this had almost disappeared, 

 and was replaced by delicate vesicles larger 

 than the nuclei which lay closely together in 

 contact with each other. Whichever of these 

 forms the epithelium assumed, it did not en- 

 croach much on the cavities of the vesicles, 

 which were large and filled with some trans- 

 parent non-refracting fluid. 



Most of the cavities also contained one to 

 three yellowish coarsely granular globules, 

 _j__ to -gi-tf in. diameter ; these existed in va- 

 rious stages of development. A fine large 

 octohedral crystal was also seen in one of 

 the cavities ; but there were no prisms of 

 triple phosphate (fig. 743.). 



Among the various families of the Saurians 

 the thyroid is found to occupy different posi- 



Fig. 743. 



One side of a vesicle from thyroid of Tortoise. 

 The epithelial stratum consists of a single row of 

 nuclei, imbedded in a more abundant quantity of 

 granular matter than usual. 



tions ; in some being single and mesial, in 

 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 Lacertida: the gland is single, and 

 of considerable breadth in the true lizards 

 while in the Monitor it is double. Among the 

 IguanidtE, likewise, a similar variety prevails. 

 The Crec/iotidcs, ChameeleonideB, and Scincidce 

 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 Opliidia "'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 fat 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 

 branchiae, 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 pcrcnni-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 Fia/ics 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 (Eso.r), Cod (Gaff us), Had- 

 dock ( Morr/ma), Whiting (Merlangus), Eel 

 (AnguiUa), Sturgeon (Accipenser'), Shark 

 (Syua/us), and Skate (Rnia); it seems also 

 to be present in the Anableps, Exoccetus, Cal- 

 lorhynchus, and Lamprey (Petromyzon), but 

 the evidence for its existence is less conclusive. 

 It may occupy, he states, either of three posi- 

 tions ; (l)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 Gadidae 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 

 duplicatufe 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 branchial 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 

 pseudobranchioe 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 ophthalmic, 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 



4u 3 



