PISCES. 
which is interposed between the skull and the 
surlace of the brain. 
Organs of respiration—The respiration of 
Fishes is purely aquatic, the oxigenization of 
the blood being accomplished throughout the 
entire class by its exposure to the oxygen 
dissolved in the surrounding medium as it 
passes through the network of extremely mi- 
nute vessels that is spread out over the exten- 
sive surfaces furnished by the gills or branchiz. 
These organs consist of vascular fringes or 
lamine placed on each side of the neck, over 
which, in the great majority of species, the 
water taken in at the mouth is made to pass 
as it issues through the opercular cavities ; 
and in this way the branchial surfaces, being 
perpetually bathed with aerated water, perform 
the same office as the lungs of an air-breathing 
animal. 
But while respiration is thus accomplished 
throughout the whole class by means that are 
essentially similar, there are several modifica- 
tions in the mechanical arrangement of the re- 
Spiratery apparatus, each of which will demand 
our especial notice. 
Throughout all the extensive division of os- 
seous Fishes (with the exception of the Lopho- 
branchii) the construction of the breathing 
organs will be found to accord with the fol- 
lowing general description. To the external 
convex surface of each of the four branchial 
arches (fig. 522) is attached a double series of 
flat, elongated, cartilaginous lamine, tapering 
gradually towards their extremities, the whole 
forming a crescent-shaped pectiniform frame- 
work, over which is spread the highly vascular 
membrane that constitutes the respiratory sur- 
face. On making a transverse section of the 
gill it is found that towards their base, whereby 
they become attached to the branchial arch 
(fig. 520, b), the two series of branchial lamine 
are united to each other, and, moreover, the 
structure of each leaflet of the branchia becomes 
apparent. The branchial artery (c), whereby 
the blood is brought to the gills for the purpose 
of respiration, is seen running along the con- 
vexity of the supporting arch in the middle of 
the base of the branchial lamine opposite each 
pair, of which it gives off two branches, which 
pass outwards to the end of the substance 
which unites the two layers of gills at their 
bases, and then severally subdivide, one of the 
ramuli extending along the internal margin of 
each branchial lamina to its extremity, the other 
retrograding to its base. From these two ra- 
muli minute transverse vessels are given off, 
which distribute the blood over the general 
surface of the laminz, and ultimately form the 
branchial veins, from which the systemic artery 
is continued. Besides the respiratory lamine 
the branchial arches support a series of un- 
vascular processes, which project from their 
concave margins, and serve to prevent sub- 
stances taken into the mouth from escaping 
through the branchial fissures and thus getting 
among the gills; these processes in the Mullet 
(Mugil chelo) are extremely beautiful, forming 
long and delicate fringes along the concavity 
985 
of each branchial arch, adapted to bar the 
Fig. 520. 
passage of minute or finely 
comminuted food through 
the branchial interspaces. 
These internal unvascular 
appendages to the branchial 
arches act, therefore, the 
same part as the epiglottis of 
mammiferous animals. Fre- 
quently there are likewise 
tubercular projections from 
the contiguous margins of 
the concave surfaces of the 
branchial arches, for the pur- 
pose of preventing the gills 
from becoming too closely 
approximated to each other, 
and thus interfering with the 
free circulation of the blood 
over their surfaces. 
The mechanism of the 
respiratory process is, there- 
fore, in these osseous Fishes 
exceedingly simple. The 
water which is constantly 
taken into the mouth passes 
through the branchial fis- 
sures, and is forcibly driven 
by the simultaneous action 
Diagram of the cir- 
culation of the 
blood through the 
branchial leaf- of the branchial arches of the 
lets. os hyoides, of the palato- 
b, section of temporal flaps, and of the 
branchial arch; a, 
branchial artery ; 
c, branchial vein; 
d, d, e, e, the arte- 
rial and venous 
trunks derived 
from them. 
opercula, through the inter- 
spaces of the gills, and thus 
passes out through the wide 
fissure upon the side of the 
neck beneath the branchios- 
tegous membrane. 
In such genera as have this external opening 
very large and patulous, as it is, for example, 
in the Herrings and numerous other races, the 
death of the fish ensues almost immediately 
on its removal from its native element, not so 
much on account of a deficiency of oxygen 
wherewith to aerate the blood in the branchiz, 
seeing that that might be derived from the 
atmosphere, but because the gills, being no 
longer floated out, collapse, and thus, by pre- 
venting the passage of blood through the deli- 
cate vessels which ramify over the branchial 
lamin, put a stop to the circulation as com- 
pletely as strangulation could do; but in some 
genera a provision is made to permit of a more 
lengthened existence out of the water where 
the habits of the fish render such an arrange- 
ment necessary. In the whole tribe of Eels, 
for example, the external fissure is removed 
very far back and reduced to a very small yer- 
tical slit, converting the cavity wherein the 
branchize are lodged into an elongated cham- 
ber, wherein a considerable quantity of water 
can be retained : in such Fishes, therefore, the 
circulation of the blood is by no means put a 
stop to by taking the fish out of the fluid it 
usually inhabits, but, on the contrary, many 
species can exist for a considerable length of 
time in the air, and even make their way to 
a distance from their native ponds, the water 
