Comparative.] 
the female lays her fpawn in fome place that is mod fecure, 
while the male emits his fperm upon the female fpawn. 
After railing up the black peritonaeum in filhes, there 
comes in view an oblong white membranous bag, in which 
there is nothing contained but a quantity of elaftic air. 
This is the fzuimming-bladder ; it lies clofe to the back-bone, 
and has a ftrong mufcuiar coat, whereby it can contract 
itfelf. By contracting this bag, and condenfing the air 
within it, fifli can make their bodies fpecifically heavier 
than water, and fo readily fall to the bottom; whereas, the 
mufcuiar fibres cealing to act, the air is again dilated, and 
they become fpecifically lighter than water, and fo l'vvim 
above. According to the different degrees of contraction 
and dilatation of this bladder, they can keep higher or 
lower in the water at pleafure. Hence flounders, foies, 
raia or Ikate, and fuch other fillies as want this fac, are 
found always groveling at the bottom of the water: it 
is owing to this that dead filh (unlefs this membrane has 
been previoufly broken) are found fwimming on the fur- 
face w'ith their bellies uppermoft; for the back-bone can¬ 
not yield, and the diftendcd fac is protruded into the ab¬ 
domen, and the back is confequently heaviell at its upper 
part, according to their pollure. There is here placed a 
glandular fubftance, containing a quantity of red blood; 
and it is very probable that the air contained in the fwim- 
ming-bladder is derived from this fubftance. From the 
anterior part of the bag go out two procejfes or appendices, 
which, according to the gentlemen of the French acade¬ 
my, terminate in their fauces. In a variety of other filli¬ 
es we find communications with fome parts of the alimen¬ 
tary canal, particularly the oefophagus and ftomach. The 
falmon has an opening from the fore-end of the air-bag 
into the oefophagus, which is furrounded by a kind of 
mufcuiar fibres. The herring has a funnel-like paflage 
leading from the bottom of the ftomach into the air-bag; 
but it is not determined whether the air enters the air-bag 
by this opening, or comes out by it: the latter, how'ever, 
feems to be the more probable opinion, as the glandular 
body is found in all fillies, whereas there are feveral with¬ 
out this paflage of communication. At the fuperior part 
of this bag there are other red-coloured bodies of a glan¬ 
dular nature, which are connected with the kidneys. 
From them the ureters go down to their infertion in the ve- 
fica urinaria , which lies in the lower part of the abdomen ; 
and the urethra is there produced, which terminates in the 
podex. Tliefe laft-mentioned parts have not hitherto been 
obferved in fome fpecies of fifli; whence authors too haf- 
tily denied them in all. Tliefe creatures have a membra¬ 
nous diaphragm, that forms a fac in which the heart is con¬ 
tained. It is very tenfe, and almoft perpendicular to the 
vertebrae. 
The heart is of a triangular form, with its bafe down¬ 
wards and its apex uppermoft; which iituation it has be- 
cairfe of the branchial. The heart has but one auricle, one 
ventricle, and one great artery. The lize of the auricle 
and that of the ventricle are much the fame; the artery 
fends out numerous branches to the branchiae or gills. 
And, what is rather curious, this artery, inftead of fup- 
porting all parts as in the frog, is diftributed entirely upon 
the gills, every branch terminating there, and becoming 
fo extremely fniall as at laft to efcape the naked eye. The 
branchice lie in two large flits at each fide of the head, and 
feem to be all that bears any analogy to lungs. Their form 
is femicircular; they have a vaft number of red fibriike 
ftanding out on each fide of them like a fringe, and very 
much referable the vane of a feather. Tliefe branchiae 
are perpetually fubjeff to an alternate motion and prelfure 
from the water; and we may here remark, that we have 
not found any red blood but in places fubjeff to this alter¬ 
nate preflure. This obfervation will help us in explain¬ 
ing the affion of the lungs upon the blood. Over tliefe 
gills there is a large flap, allowing a communication ex¬ 
ternally; by which the water they are obliged to take into 
their mouths with their food finds an exit without palling 
into their ftomach; it is owing to tliefe flaps coming fo far 
Vol. I. No. 42. 
666 
down, that the heart is faid commonly to be fituated in 
their heads. The blood is collected again from the gills 
by a vaft number of fmall veins, fomewhat in the fame 
manner as in our pulmonary vein; but, inftead of going 
back to the heart a fecond time, they immediately unite 
and form an aorta defeendens without the intervention of 
an auricle and ventricle. Hence a young anatomift may 
puzzled to find cut the power by which the blood is pro¬ 
pelled from the gills to the different parts of the body ; 
but the difficulty will be confiderably leflened when we 
cortfider the manner in which the blood is carried through 
the liver from the inteftines in man and quadrupeds. The 
aorta in fifties fends off branches which lupply all the parts 
of the body excepting the gills. From the extremity of 
thofe branches the blood returns to the heart fomewhat in 
the fame manner as in the former clafs of animals; only there 
are two inferior venae cavae, whereas the former has but one. 
In the abforbentfyjlem in fifties, we fliall take the had¬ 
dock as a general example: for the other filhes, particu¬ 
larly thole of the fame fhape, will be found in general to 
agree with it. On the middle of the belly of a haddock, 
immediately below the outer lkin, a lymphatic vcffel runs 
upwards from the anus, and receives branches from the 
parietes of the belly, and from the fin below the anus; 
near the head this lymphatic paffies between the two pec¬ 
toral fins; and, having got above them, it receives their 
lymphatics. It then goes under the fymphyfis of the two 
bones which form the thorax, where it opens into a net¬ 
work of very large lymphatics, which lie clofe to the pe¬ 
ricardium, and almoft entirely furrotinds the heart. This 
net-work, befides that part of it behind the heart, has a 
large lymphatic on each fide, which receives lymphatics 
from the kidney, runs upon the bone of the thorax back¬ 
wards, and, w’hen it has got as far as the middle of that 
bone, it fends off a large branch from its infide to join the 
thoracic duff. After detaching this branch, it is joined 
by the lymphatics of the thoracic fins, and foon after by 
a lymphatic which runs upon the fide of the fifli. It is 
formed of branches, which give it a beautiful penniform 
appearance. 
The laffeals run on each fide of the mefenteric arteries, 
anaftomofing frequently acrofs thofe veffiels. The recep- 
taculum into which they enter is very large, in proportion 
to them ; and conlifts at its lower part of two branches, 
of which one lies between the duodenum and ftomach, 
and runs a little way upon the pancreas, receiving the lym¬ 
phatics of the liver, pancreas, thofe of the lower part of 
the ftomach, and the laffeals from the greateft part of the 
fmall inteftines. The other branch of the receptaculum 
receives the lymphatics from the reft of the alimentary 
canal. The receptaculum formed by tliefe two branches 
lies on the right fide of the upper part of the ftomach, 
and is joined by fome lymphatics in that part, and alfo by 
fome from the found and gall-bladder, which in this fillt 
adheres to the receptaculum. This-thoracic draff takes its 
rife from the receptaculum, and lies on the right fide of 
the oefophagus, receiving lymphatics from that part; and, 
running up about half an inch, it divides into two duels, 
one of which paffies over the oefophagus to the left fide, 
and the other goes ftrait upon the right lide, paffes by the 
upper part of the kidney, from w hich it receives fome 
fmall branches, and foon aftervyards is joined by a branch 
from the large lymphatic that lies above the bone oi the 
thorax ; near this part it likewife lends off a branch tojom 
the duff of the oppofite fide ; and then, a little higher, 
is joined by thofe large lymphatics from the upper part of 
the gills, and from the fauces. 
The thoracic duff, after being joined by thefe veffiels 
communicates with the net-work near the orbit, where its 
lymph is mixed with that of the lymphatics from the pof- 
terior part of the gills, and from the fuperior fins, belly. 
See. and then, from this net-work, a vefiel goes into the 
jugular vein juft below the orbit. This laft veffel, which 
may be called the termination of the whole fyftem, is very 
fmall in proportion to the net-work from which it rifes; 
8 H and 
A N A T O M Y. 
