302 MR. SIMON ON THE COMPARATIVE ANATOMY OF THE THYROID GLAND. 
organs in respect of their vascular supply ; both deriving their afflux of blood from 
the first branchial vein, both rendering it up again by the readiest channel toward 
the heart, both as it were participating in the cerebral circulation. 
But (it may be contended) this arrangement, which is comprehensible for the gland, 
is not equally so for the gill ; why should this organ, which, in regard of structure at 
least, is clearly branchial, derive its supply of blood from a stream no longer requiring 
aeration? It cannot assist the respiratory process, which is already accomplished for 
the blood arriving there : it cannot do more than maintain the previous oxygenisation 
of the fluid which traverses it. I can conceive no other theory for the supplemental 
gill, in agreement with the preceding facts, than one which interprets it as a diver- 
ticular appendage to the first branchial vein, and contemplates its uses in reference 
to the parts supplied by that vessel. 
Such then is the organ which, — obtaining the same nutritive supply as the thyroid 
gland, and occupying a correspondent position, — found only where that gland is 
absent, absent only where it is found, — may fitly be considered as its partial and 
rudimentary representative*. 
The general results of the preceding inquiry are as follows ; viz. 
1. The thyroid gland, or an organ representing its place and office, may be found 
in all vertebrate animals. 
2. It does not appear (as is the case with some organs) to belong to these animals, 
merely in behoof of the greater general completeness of their organization ; but seems 
to have particular reference to that specific character, which binds them together as a 
natural section of the animal kingdom, — namely, the aggregation and more perfect 
development of their nervous centres ; for 
3. The gland, shifting its position most variously, yet always maintains an intimate 
relation to the vascular supply of the brain ; always is so nourished that it can alter- 
nate a greater or less nutrition, according to the activity or repose of that nervous 
centre. 
4. The organ, which in certain fishes represents a rudimental form of the thyroid, 
is plainly a mere diverticulum to the cerebral circulation : in the remaining fishes 
(where a more perfect thyroid exists), as likewise in the ascending scale of vertebrate 
animals, there is no essential change from the vascular organization of the branchiola ; 
there is simply the super-addition of a glandular structure. The thyroid is but a 
higher development of the branchiola : what was a mere capillary plexus, now has 
gland-cells intermingled with its texture. Probably then the use of the secretory 
actions occurring in the thyroid gland is to be found in harmony with the obvious 
* The Sturgeon, in addition to its thyroid, has a very small supplementary gill; so small, however, and so 
rudimentary, that this fish cannot be considered an exception to the general rule, but rather as an additional 
confirmation of its accuracy. 
