1110 



THYROID GLAND. 



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 T inch (jig. 744. A). The 

 limiting envelope of the vesicles presented a 



Fig. 744. 



Fig. 745. 



from thyroid of Skate. 



A. Vesicle, -yi^ in. in diameter. 



B. Several of the ^uclei imbedded in diffused 

 mottled substance. 



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, con- 

 sisting of vesicles about -^ to -^ inch 

 diameter (fie,. 745.), with a thick interior 

 stratum of epithelium resting on a beautiful 

 homogeneous limitary membrane. The pseu- 



A. vesicle from sublingual gland of Skate, diam.^ in. 

 It contains abundance of nuclei and granular 

 matter, with delicate vesicles. 



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. 



In the cod and whiting the pseudobranchia 

 is situated, as Mr. Simon has described it, near 

 the upper extremity of the first branchial 

 arch on each side. It lies in 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. Its structure is peculiar; Mr. 

 Simon regards it as a thyroid ; but from this 

 opinion I feel obliged to differ. The following 

 description is taken from examination of the 

 organ in the Cod, but applies equally to that 

 in the Whiting. It is a body of light red 

 colour, semi-transparent aspect, flattened so 

 as to present two faces, about one line and a 

 half in thickness, and having two borders, 

 one convex and slightly notched, the other 

 somewhat concave. The surface is slightly 

 uneven or nodulated. It is enclosed in a cap- 

 sule, through which some large vessels are 

 seen ramifying. Its general aspect is that of 

 "glandular flesh," and certainly not of a mere 

 congeries of vessels. In structure it appears 

 to consist of parallel folds of homogeneous 

 membrane, beneath which is spread a vascular 

 plexus, and which are covered by an unusually 

 developed epithelium. This epithelium appears 

 under the form of granular cells of an oval, 

 circular, or irregular form, not distinctly nu- 

 cleated, and having a diameter of - g ., i 1 666 inch. 

 These do not appear to constitute a mere in- 

 vestment, but to form a layer of some thick- 

 ness, filling up the intervals between the 

 adjacent processes or folds of homogeneous 

 membrane. These organs, manifestly con- 

 structed after the type of gills, evince thus a 

 tendency to assume a glandular structure ; 

 yet I can see no sufficient reason for sup- 

 posing them to represent the thyroid, from 

 which they differ so entirely in structural 

 characters. In the whiting I could detect no 

 trace of a sublingual gland, nor of the small 

 posterior one; but in the Eel, where the 



