300 MR. SIMON ON THE COMPARATIVE ANATOMY OF THE THYROID GLAND. 
them in very few instances, and draw my inferences from those only which I can 
quote with entire confidence. 
The positions in which I have found the gland are the following, viz. — 
1. In those cartilaginous fishes* which possess it, as likewise in the Eel, it is a 
single organ, and is situated in the median line, in connexion with the anterior surface 
of the cartilages, which bind together the branchial arches of the opposite sides of the 
body. Within this line it may be more or less removed from the mouth of the ani- 
mal, sometimes advancing so far upward as to come into contact with the lingual 
bone; but its position is always defined, as the spot where the great trunk of the 
branchial aorta distributes its terminal branches. It lies in the angle of this bifur- 
cation, and sometimes (as in the Sturgeon) extends some little way behind it; it is 
covered by the sterno-hyoid or sterno-maxillary muscle, and also by the mylo-hyoid 
and genio-hyoid, when these are present. Situated at the anterior extremity of the 
first branchial arch, it receives its supply of blood by means of a recurrent branch 
given off in this direction by the first branchial vein, while yet within the gill. It 
never receives the smallest share of supply from the branchial artery with which it is 
in contact. 
2. In the Gadidce 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 vertebral end. Here it occupies part of a recess which is bounded by the gill be- 
low, 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 proper encephalic artery. 
3. In the Carp, Anableps, Pike and Exocetus, the gland is placed at the inner ex- 
tremity of the same duplicature of mucous membrane, and more toward the palate, 
so as to lie upon the fibres of the pterygoid muscle. It requires more dissection, in 
order to be made apparent ; in the Carp especially it is at considerable depth, 
being hidden by the extraordinary thickness of the soft palate, and imbedded 
between the surface of the pterygoid muscle and the outer extremity of the branchial 
bone. 
Amid these differences in the relative anatomy of the gland there is constancy in 
respect of one point, which I am inclined to consider essential ; the organ, whether 
placed in the median plane or otherwise, whether at the vertebral, or at the hyoidal 
extremity of the gill, is always so placed as to receive its blood from the first bran- 
* The questionable trace of a thyroid, noticed in the Petromyzon marinus, was in a depression of the base of 
the skull, between the palate and the posterior confluence of the branchial veins; it would resemble the above 
in being single, but otherwise would be nearer to the thyroids of the osseous fishes. 
