A^VSCULAR SYSTEM OF FISHES. 269 



body, which divides into the artery for the brain (encephalic), and 

 that for the eye (ophthahiiiic). The pseudobrancliia is thus a kind of 

 rete mirabile for both the cerebral and ojihthalmic circulation in the 

 Sturgeon* : in osseous fishes it stands in that relation to the eye only, 

 and is most generally associated with the more immediate ophthalmic 

 'rete mirabile,' called ' clioroid gland' {Jig. 56. o). The pseudo- 

 branchia coexists with the hyoid uniserial gill in most Plagiostomes ; 

 and in those that have the spiracula it is developed, as in the Stur- 

 geon, on the anterior wall of each of those temporal outlets from the 

 branchial cavity : its ' vena arteriosa' supplies the eyes and part of 

 the brain ; and it is important, also, in reference to a true and clear 

 idea of the function and homology of the 'pseudobranchia' in fishes 

 generally, to bear in mind that it co-exists in the Plagiostomes, 

 Chimajroids, Sturgeons, and some osseous fishes, with the vaso- 

 ganglion supplied by vessels from the anterior branchial veins, 

 which lies between the anterior basi-branchials and the sterno- 

 hyoid muscles.f Besides the small nasal and orbital arteries, and 



* See XXI. 1841, pp. 41 — 67. 75. for a most valuable and exact specification of 

 the structure, relations, and varieties of the Pseudobranchia?. 



f This body has already been alluded to in connection with the salivary system 

 (p. 230.). Mr. Simon's opinion, that it is the 'thyroid gland' of Cartilaginous 

 Fishes is more in accordance with its nature as a vaso-ganglion, and its relative 

 position. But since it co- exists in Cartilaginous Fishes with tlie actual homoloTue 

 of the pseudobrancbiffi of Osseous Fishes, these cannot be, as Mr. Simon contends 

 the thyroid glands of such fishes. That the parts which the accomplished author of 

 the paper cxvi. describes as the thyroid glands in the Exoca;tus, Pike, Anableps, 

 Carp, and some species of the Cod tribe, are the same bodies which JMiiller had 

 previously described as 'pseudobranchia.-' in those fishes, will be manifest bv com- 

 paring the descriptions of the two anatomists. " In the Gadidm the gland is 

 double ; one portion lies on each side, not as in the last case [the sublingual vaso- 

 ganglion of the Sturgeon is alluded to] at the anterior extremity of the firstbranchial 

 arch, i)ut near its posterior or vertebral end." (Simon, Philos. Trans. 1844, p. 300. 

 Compare with this jMiiller, xxi. 1841, p. 47. tab. iii. fig. 13.) 



I'rof iNliiUer's description of the 'pseudobranchia;' of tlie Car]) is as follows: 



' Die verborgenste Lage hat das Organ bei Cyprinus Carpio nnd Carassuis. Y,s ist 

 nicht bloss von dem beweglichen dicken Gaumenorgan bcdeckt, sondern selbst von 

 knochen verhiillt. Man lindet es nach Wegnahmc des contractilen CJaumen- 

 organs zwischen dem hintern Ende des quercn Gaumen-muskels und den oberii 

 Schlundknochen." (xxi. 1841, p. 47.) Mr. Simon's account of the tliyroid gland 

 in the same species is as follows : — 



" In the Carp especially it is at considerable depth, being hidden by the extraor- 

 dinary thickness of the soft palate, and ind)edded between the surface of the ptcrv- 

 goid muscle and the ouier extremity of the branchial bone." (Philos. Trans. 1844 

 p. 300.) 



It is obvious that all the parts described as 'thyroid glands' by IVIr. Simon, in 

 Osseous Fishes, are the pseudobranchia? ; and the author recognises that homology 

 in regard to the free gill-likc forms of pseudobranchia; described by 15roussoiu.t 

 and ^lin:Vc\ m AcanthoiJteri/fiii and Pleitrmwctidtv. That these ' psei'idobranclda; ' 

 receive arterialised blood, and arc thus essentially distinct from the hyoid or oper- 

 cular uniserial fin in the Sturgeon, I.epidosteus, 8:c., bad been clearly demonstrated 

 by Prof. Midler (1. c. 1841). There remains, then, to consider the relations of the 

 'pseudobranchia;' of Fislies with the thyroid glands of Mammals. Tiiese may be 

 either relations of analogy or of homology. The former would be determined by 



