THYROID GLAND. 



1111 



pseudobranchia was absent, I found, by the 

 aid of the microscope, between the first and 

 second basibranchials, a small mass, which 

 consisted in great part of fat, but contained 

 also some large vesicles closely resembling 

 those of a real thyroid. Their diameter 

 varied from -^-^ to -fa inch. They had an ho- 

 mogeneous envelope lined by an epithelium, 

 consisting of small non-nucleated pellucid cor- 

 puscles, and surrounding a cavity filled by a 

 transparent somewhat refracting fluid. 



In a Ptenronectid I found the pseudobranchia 

 quite free, uncovered by mucous membrane, 

 and projecting a series of small distinct leaf- 

 lets into the branchial cavity. It differs essen- 

 tially from that of the cod or whiting, and 

 showed no tendency to assume a solid glan- 

 dular form, but was manifestly a real though 

 minute gill. 



In a Carp I found the pseudobranchia with 

 some difficulty. It was situated very deeply 

 between the anterior part of the upper ex- 

 tremity of the first branchial arch, and the 

 posterior border of the pterygoid muscle. Its 

 structure was entirely that of a gill, consisting 

 of parallel folds of a membrane, arranged 

 transversely to a median axis, and overlaying 

 a vascular plexus. These folds were covered 

 \vith a kind of scaly epithelium, which was 

 often detached in large pieces, and mingled 

 with some circular cells, closely resembling 

 mucous globules. I could find no trace of the 

 sublingual nor of other adjacent gland. 



From the few facts now detailed I think it 

 may be concluded, (1) That there is no evi- 

 dence from their structural characters to prove 

 that the pseudobranchiae are the representa- 

 tive of the thyroid. (2) That in some in- 

 stances organs which seem to be of a totally 

 different kind are found, which resemble 

 very closely the structure of the thyroid 

 when it unquestionably exists. Mr. Simon 

 lays stress upon the circumstance, that the 

 pseudobranchia? and the sublingual gland re- 

 ceive their vascular supply from the same 

 source, viz. the 1st branchial vein ; this, how- 

 ever, is not constantly the case, the pseudo- 

 branchiae in osseous fishes, according to Pro- 

 fessor Owen, serving only as diverticula to the 

 ophthalmic, and not to the cerebral circulation; 

 so that on this ground they cannot be sup- 

 posed to have similar functions. For a philo- 

 sophical discussion, however, of the analogy 

 and homology of the pseudobranchia to the 

 thyroid, 1 would refer to Professor Owen's 

 Lectures, vol. ii. p. 270. note. 



MORPHOLOGY. The preceding details are 

 abundantly sufficient to prove the glandular 

 nature of the thyroid. This truth, which elder 

 anatomists saw clearly, though rather afar off, 

 we, by more intimate and minute scrutiny, are 

 enabled to confirm and establish in every par- 

 ticular. Let us take in order the several parts 

 of the secretory apparatus of the thyroid as 

 we have described them, and see whether they 

 do not exactly correspond to homologous 

 parts in any undoubted gland. The recep- 

 tacular cavities, enclosed by envelopes of 

 homogeneous membrane, manifestly represent 



the tubes of the kidney or testis, or the ter- 

 minal vesicles of the salivary gland. The 

 limitary tissue is identical in appearance in 

 each ; but disposed in the one so as to form 

 shut sacs ; in the others to ensure a pervious 

 canal or outlet, for the secretion. In the 

 true glands the limitary tissue or basement 

 membrane supports, on its interior, a layer 

 of epithelium, essentially consisting of nuclei 

 and granular matter. The same tissue in 

 the thyroid is lined internally by a similar 

 layer, which differs only in the smaller quantity 

 of granular matter interposed among the 

 nuclei, and in both cases the formation of 

 perfect cells (i. c. with envelopes) is uncom- 

 mon. The exterior of the limitary tissue is in 

 contact, in all the true glands, with a capillary 

 plexus, from which the materials for the 

 nutrition and growth of the epithelium are 

 furnished. The same disposition exists in the 

 thyroid. A certain amount of fibrous tissue, 

 or some modification of it, penetrates more or 

 less extensively among the elementary parts 

 of the true glands, and serves to pack and 

 unite their component parts together ; this also 

 we have had occasion to describe in the 

 thyroid. With respect to a supply of lym- 

 phatic vessels and nerves, it is sufficient to 

 state, that as far as ordinary dissection can go, 

 the thyroid is similarly circumstanced with the 

 other glands ; and there is no reason to doubt 

 that the actual arrangement of these parts in 

 all is the same. The parallel, thus exactly 

 sustained in every particular, warrants us in 

 regarding the thyroid as the Trapddeiyfjia of 

 the class of ductless glands, and it may be not 

 without advantage, if we take occasion to note, 

 in the other organs belonging to the same 

 class, how gradually the strongly marked 

 characters are laid aside, till the glandiform 

 organ passes into a modification of vascular 

 structure. In the supra-renal capsules the 

 limitary membrane, though described by an 

 eminent anatomist as usually forming tubes of 

 various length, enclosing celloid epithelial 

 particles, is, according to my observation 

 (which is however but limited^, very faint, or 

 entirely absent. I have never been able to 

 discover it in the human adult or fcetal sub- 

 ject ; and in the sheep the cortical structure has 

 seemed to consist simply of rows of celloid 

 nucleated particles, without any enclosing 

 membrane. Conceding, however, that in these 

 organs the limitary tissue does exist, there can 

 be no doubt that it is much more feebly de- 

 veloped than in the thyroid, while, as a set off, 

 the epithelial particles are more perfectly 

 formed. In the case of the thymus the 

 limitary tissue is well marked, and resembles 

 very much, in its arrangement, that of the con- 

 glomerate salivary gland ; but the epithelium 

 is remarkably modified. The nuclei exist, but 

 almost alone. No granular matter of any pecu- 

 liar properties is formed around them, prepa- 

 rative to, and evidencing the existence of, 

 secretory action. In the spleen both these 

 alterations coincide. The epithelium is reduced 

 to mere nuclei aggregated in masses round 

 prodigiously developed venous radicles, and 

 *B 4 



