270 LECTURE X. 



the liyo-opercular, from wliicli the proper ophthalmic artery is de- 

 rived, the carotids are usually sent off from the ' circulus aorticus.' 

 In the Chiraa^ra the carotids are transmitted directly from the an- 

 terior branchial veins ; and, in the Pike, the artery of the pectoral 

 fins (brachial) is transmitted from the common trunk of the two 

 anterior branchial veins. In the Myxines an anterior, as well as a 

 posterior, aorta is continued from the common confluence of the 

 branchial veins. In all higher fishes the posterior aorta is the only 

 systemic trunk so formed. 



This aorta extends beneath the bodies of the vertebra? along the 

 abdomen and through the htemal canal to the end of the tail. In 

 many Cyprinoid fishes it dilates beneath each abdominal vertebra 

 into a sinus. It gives off intercostal arteries, and, with less regu- 

 larity, numerous small branches to the kidneys. The first principal 

 visceral branch is the ' cajliac,' which sometimes, as in the Burbot, is 



showing that the parts in question performed the same functions in the two 

 classes. 



Mr. Simon conceives that the thyroid gland " maintains an intimate relation to the 

 vascular supply of the brain." The pseudobranchia, however, are not diverticula to 

 the cerebral circulation in Osseous Fishes, but only to the ophthalmic circulation, 

 and in most cases are subsidiary, in this respect, to the choroid vaso- ganglion. The 

 internal carotid or encephalic artery in Osseous Fishes is a division of an artery 

 sent off from a part of the ' circulus aorticus,' formed by the three posterior pairs 

 of branchial veins : the artery of the pseudobranchiae is given off from the anterior 

 branchial vein, or from the hyo-opercular artery, or from both : its subsequent 

 modification cannot therefore affect the currents of circulation through the gills, 

 with whose efferent vessels it has no connection, and from which vessels the ence- 

 phalic arteries are derived. In the Sturgeon and Plagiostomes, indeed, the pseudo- 

 branchiae may, and doubtless do, act as diverticula to the cerebral circulation ; 

 but it is precisely in these fishes that Mr. Simon transfers the name and function of 

 thyi'oid gland to another vaso-ganglionic body, viz. lletzius's sublingual gland. 

 Mr. Simon was acquainted with the existence of this body, and also of the supple- 

 mentary opercular gill in the Sturgeon, but he seems to have overlooked the pseudo- 

 branchiae, which, in this fish, hold precisely that relation to the cerebral vessels 

 which would have best supported Mr. Simon's determination of the pseudobranchia» 

 as the thyroid gland. I consider, however. Professor Midler's comparison of the 

 pseudobranchiae in the Sturgeon with the ento-carotid plexuses of the Ruminants 

 to be a truer and more natural view of their analogies. As to the question of 

 ' homology,' which is to be determined by consideration of the organic connections, 

 relative position, and development of the parts in question, it is obvious that if, 

 by a consideration of these characters, we admit the sublingual body of the higher 

 Cartilaginous Fishes to be, of all the vascular ganglions in or near the head, that 

 which most nearly repeats the homological conditions of the thyroid gland of higher 

 Vertebrates, we cannot regard the vascular ganglions, which maintain a diametrically 

 opposite position in regard to the branchial arches, as being also parts answerable to 

 the thyroid glands. 



If any thing were wanting to convince an anatomist holding that view, viz. 

 that the pseudobranchiae are the homologues of thyroid glands, of the fallacy of 

 such a view, it would be the fact of their co-existing in some fishes with a vaso- 

 ganglion having a much better title to be so considered, and which is actually 

 described by the ingenious author of the paper in the Philosophical Transactions as 

 the thyroid gland in those fishes ; though its relations to the heart and great ves- 

 sels in the Plagiostomes and Sturgeons seem to give it more claim to be regarded 

 as the homologue of the thymus. 



