PISCES. 
shape, but sometimes, as for example in the 
Sturgeon, resembling the tricuspid valve of the 
heart in Mammalia, having chorde tendinee 
passing from its margin to the muscular walls 
of the ventricle. 
In other cases again, as in the Sun-fish 
( Orthagoriscus Mola), the auricular aperture 
is guarded by four valves, two small semilunar 
valves being placed at right angles with and on 
the auricular side of the two large semilunar 
valves that usually exist in this situation. 
The branchial artery (fig. 522, B) which arises 
from the ventricle is very different in character 
from an artery of ordinary appearance, its walls 
being exceedingly thick and muscular, and fre- 
quently fasciculated internally ; its cavity is more- 
over dilated so as sometimes to equal in capacity 
that of the ventricle itself. This dilated portion 
of the branchial artery, to which the name of 
bulbus arteriosus has been given, is in fact 
almost equivalent to a second ventricular cham- 
ber, and doubtless by its contractile power 
forcibly assists in propelling the blood through 
the gills. 
The origin of the bulbus arteriosus is always 
guarded by strong valves, of which there are fre- 
quently only two of a semilunar form, but occa- 
sionally the valve is made up of four semilunar 
membranous folds. 
But it is not only at the commencement of 
the bulbus arteriosus that valves exist, these 
defences being frequently multiplied in this 
portion of the circulating system of Fishes in a 
very extraordinary manner. Thus, in the Stur- 
geon there are three series of semilunar valves, 
two at the commencement and one at the ter- 
mination of the bulb, the last being the strong- 
est and most perfectly formed. Those at the 
base are much thickened at their margins, 
which are attached to the parietes of the bulb 
by small chorde tendiner. The two lower 
valves are each made up of four semilunar 
folds, whilst in the upper one there are five. 
In the Skate ( Raia Batis) there are five dis- 
tinct sets of valves, increasing in size to the 
last row, which is at the termination of the 
bulb. 
The most probable reason of this unusual 
Heart and principal vessels of Perch. 
989 
fortification of the valvular apparatus of the 
branchial artery in Fishes is that, owing to the 
depth to which some of them descend or at 
which many races dwell habitually, as for ex- 
ample, the ground-frequenting Skates, the pres- 
sure upon the surface of the gills must render 
the passage of the blood over the branchie very 
difficult, so that unusual care has been taken in 
strengthening the bulbus arteriosus itself, and 
likewise the valvular structures in its interior. 
The bulbus arteriosus ultimately resolves itself 
into the branchial artery, which gives off the 
trunks that supply the venous blood to the 
ills. 
" Vascular system.—The general course of the 
blood during its circuit through the body of a 
fish has been already described in a preceding 
article (see CIRCULATION); we shall therefore 
limit ourselves in this place to describe the dis- 
position of the principal vas- 
cular trunks, and the manner 
in which the circulating fluid 
is distributed to different parts 
of the system. The vessels 
formed by the division of the 
branchial artery run in a deep 
groove along the convexity of 
each branchial arch external to 
the branchial vein, which runs 
in the same groove, taking an 
opposite course. The bran- 
chial vein, as we have already 
seen, is formed by collecting 
all the venules from the bran- 
chial lamine of the corre- 
sponding gill, and thus carries 
only the blood which has un- 
dergone the process of respira- 
tion. The branchial artery and 
the branchial vein are therefore 
placed under precisely inverse circumstances 
with respect to each other; the former dimi- 
nishing continually in size as it mounts upwards 
towards the dorsal aspect of the gills by giving 
off arterioles to the branchial laminz ; the latter 
increasing in bulk as it proceeds in the same 
direction, owing to the constant accession of 
little veins derived from the fringes of the gills. 
In the Skates there are two branchial veins to 
each gill, which, however, ultimately become 
united into one trunk. 
No sooner do the branchial veins issue from 
the dorsal extremities of the branchial arches 
than they assume the texture and the function 
of arteries, and ultimately all joining with 
each other and with those of the opposite 
side, they constitute by their union the aorta, 
by which the blood is distributed to the general 
system. Before their union into a single aortic 
trunk there are arterial vessels given off from 
the branchial veins themselves; thus the ante- 
rior give off, even before they leave the branchial 
arch, several vessels to supply the head and ad- 
jacent parts, while the heart itself and the neigh- 
bouring region beneath the throat likewise re- 
ceive their supply of arterial blood through a 
twig derived immediately from a branchial vein. 
The aorta, formed, as has been stated, by the 
union of all the branchial veins without the inter- 
