540 MULLER ON THE STRUCTURE OF THE GANOIDS, 
In the preceding treatise the muscular dilatation of the ¢run- 
cus arteriosus in the Selachii, Ganoids, and the osseous fishes, is 
assumed as of equal importance, and I have confined myself 
merely to the valvular differences in the interior of this swelling, 
which is also sufficient in a zoological point of view. However, 
on a further anatomical and physiological examination of the 
importance of this swelling, we obtain the unexpected result, 
that in the osseous fishes it has a remarkably distinct structure, 
which has not the slightest resemblance to that of the Ganoids 
and Selachit. The fact may be briefly expressed as follows :— 
The muscular tunic in the arterial trunk of the Selachii and Ga- 
noids is a true heart, destined for pulsation like the auricle and 
ventricle, and also agrees with these in its minute structure. 
The bulb of the arterial trunk of the osseous fishes however is 
no segment of the heart, no portion of the active central organ ; 
nor does it pulsate like a heart, but is nothing further than the 
much-thickened commencement of the artery, in which a pecu- 
liar layer of the arteries is swollen to an enormous thickness. 
It was generally admitted by anatomists that the muscular 
substance of the arterial trunk in osseous fishes and Selachii 
was identical. Tiedemann asserts that in cartilaginous and os- 
seous fishes it contracts, and that the contraction followed that 
of the ventricle. I have myself long considered the above por- 
tion as identical in both. But if we reflect on the function and the 
action of the valves in the one and in the other, we at once begin 
to doubt. In those fishes in which there are several rows of 
valves inside the muscular arterial trunk, the muscular tunic of 
the trunk has evidently the importance of an accessory heart of 
an elongated ventricle. When it contracts it empties its blood 
into the true arteries, just as the cardiac bulb of the heart of a 
frog does. The valves are then separated from the artery by the 
pressure of the blood; the uppermost extend by their margins 
as far as the point where the muscular tunic of the artery ceases, 
above them the artery remains full, but the muscular arterial 
trunk at the moment of the interval of the cardiac pulsation 
would be withdrawn from the arteries by the pressure of the 
blood. In the osseous fishes this is exactly reversed. In them 
the valves lie between the ventricle and the bulb of the artery. 
When the ventricle contracts, the bulb and the arteries are di- 
lated. If-the bulb could contract in a pulsatile manner, as in 
frogs, the blood would be forced out of the bulb into the nearest 
