MR. SIMON ON THE COMPARATIVE ANATOMY OF THE THYROID GLAND. 301 
chial vein, and is thus brought into connexion with the encephalic nervous centre by 
a certain community of vascular supply. 
Nothing could at first sight appear more capricious and uncertain than the pre- 
sence of the gland in fishes ; it would seem almost hopeless to account for its being- 
found in the Carp, yet not in the Tench ; in the Eel, yet not in the Gymnotus, &c. 
Further research, however, tends to justify the belief that its distribution is go- 
verned by a simple and uniform law ; namely, by the existence or non-existence of 
another organ, with which its presence alternates. Since this law, if discoverable, 
must be an important step toward ascertaining the function of the thyroid gland, I 
beg leave to olfer a few observations on the organ alluded to. 
In many fishes (take for instance any one of our common Acanthopterygii or Pleu- 
ronectse) there exists a peculiar appendage to the branchial apparatus, first described 
by Broussonet*, and subsequently with more detail by Meckel ~f\ It has the 
aspect of a minute supplementary gill, and is situated between the outer extremity 
of the palatine duplicature of mucous membrane on the one hand, and the dorsal 
attachment of the first true gill on the other^;. It must not be confounded with the 
large opercular gill, found in some fishes (as for instance very largely in the Sturgeon), 
from which it essentially differs. 
The vascular supply of this small appendage is a point of extreme interest in its 
history. Its vessels communicate on the one hand with the systemic veins about the 
base of the cranium, on the other, by a single long trunk with the first branchial 
vein. Rathke§ and Cuvier|| consider the former to be its afferent, the latter its 
efferent vessel. But it surely must be erroneous to describe any part of the circula- 
tion in a fish as commencing in a systemic vein, and attaining a branchial vein without 
having previously traversed the heart : and I cannot doubt that the supply of this 
branchiola is after the analogy of organs in the systemic circulation, — that it com- 
mences, namely, in the branchial vein, and terminates in those veins which are con- 
veying blood to the hearty. 
The supplementary gill occupies in those fishes which possess it, exactly the same 
relative position as the thyroid gland in the sub-brachian Malacopterygii : a careless 
examination of the Cod, Haddock, or Whiting, where the gland shines through the 
membrane, might almost lead an observer to suppose that the supplemental branchia 
was there, as in the Pleuronectse ; so similar is the arrangement in the two cases. 
Equally exact, according to the above argument, would be the agreement of the two 
* Mem. de l’Acad., 1785, p. 174. f Vergleich. Anat. vol. vi. p. 179. 
X It is often a little obscured by the free edge of the palate overhanging it. 
§ Uber den Kiemen-apparat und das Zungenbein, 1832, p. 53. || Anat. Comp. 
The supplemental gill, like the thyroid gland, never derives a branch from the branchial artery ; injection 
driven by the bulbus arteriosus never directly reaches it ; but it admits of ready and complete injection by the 
systemic aorta. 
