38 Proceedings of the Royal Physical Society. 
There exists an unfortunate confusion regarding the nomenclature of the 
parts into which the anterior end of the cardiac tube develops. The science 
of morphology being in its very essence an evolutionary science, we are 
compelled, in accordance with its general principles, to found our interpreta- 
tion of the structure of the more highly evolved forms upon the facts of 
structure of the more nearly primitive forms. Similarly, we must base our 
nomenclature, which is merely the verbal expression of homology, upon the 
conditions met with in the more primitive forms. The most nearly primitive 
of the Gnathostomatous Vertebrates are, as is almost universally admitted, 
the Elasmobranch Fishes, and probably even those morphologists who are 
inclined to minimise the claims of this group to the title primitive will admit 
that, so far as heart structure is concerned, they are more nearly primitive 
than other Gnathostomes. Now the nomenclature of the parts of the heart 
in these fishes is perfectly clear. The part of the heart which specially 
concerns us here, the part which passes forward from the ventricle to the 
anterior limit of the pericardiac cavity, is what Gegenbaur christened conus 
artertosus.t 
This name is generally accepted by workers on the anatomy of Fishes, 
and we are therefore, in my opinion, bound to accept it for the homologous 
structure in Vertebrates above Fishes, in spite of the fact that the name 
was originally borrowed from a not necessarily homologous structure in the 
heart of man.? The conus arteriosus of the Elasmobranch is the morpho- 
logically anterior portion of the primitive cardiac tube: it is intercalated 
between the ventricle and the ventral aorta. The structural characteristics of 
the primitive conus which mark it off from the ventral aorta in front of it are 
two in number. (1) Its endocardiac lining is thickened to form cushion-like 
longitudinally arranged ridges (frequently four in number) which project into 
its lumen; and (2) its muscular wall contains striped muscle fibres which, like 
those of the rest of the heart, are rhythmically contractile. The primitive 
' 1866, Jenaische Zertschrift, i. p. 374. Joh. Miller had already shown (Dec. 1844, Abh. 
Berlin Akad. Wiss.) that, under the name bulbus arteriosus (the name given at that time 
generally to the structure lying in fishes between ventricle and ventral aorta), there were 
included two physiologically different organs—the organ bearing that name in Elasmo- 
branchs and Ganoids being physiologically a part of the heart, rhythmically contractile 
like the rest of that organ, possessing striped muscle fibres in its wall and containing valves 
_ in its interior, whereas the similarly named organ in the Teleost was merely a part of the 
ventral aorta, becoming passively dilated on the contraction of the ventricle, containing no 
valves in its interior, and possessing only smooth muscle fibres in its walls. 
* Cf. 1894, Langer, Morph. Jahrb., xxi, p. 42. The chief objection to using the term 
bulbus cordis for this part of the heart is its similarity to, and therefore liability to be 
confused with, bulbus arteriosus. The whole point in Gegenbaur’s giving it a special name 
was to distinguish it sharply from the bulbus arteriosus, 
