28 G L A N D U L A R O R G A N S AND
S E C R E T E D LI QJJ O R S IN FISHES: 2g
It iias been long known, that in the flat fiflies there is no fwimming-bladder.
In a few long-iliaped fiilies, as in the mackrel, I have alfo found it wanting.
It is likewife known, that in many fiihes the air-bag communicates by a duifl
with the osfophagus.
On examining this matter, I have found in a fturgeon a round hole, nearly
one inch in diameter, in the upper and back part of the ftomach, by which it
communicates with a very large air-bag. The hole is furrounded by thin muf-
CLilar fibres, placed between the membranes of the ilomach and air-bag, which
decuiTate at oppohte fides of the hole; and, no doubt, have the effed of a
fphinder mufcle (z).
In the falmon, I have found a hole fo large as to admit readily the largeft-iized
goofe-quill, leading diredly through the coats of the cefophagus into the airbag.
The cefophagus in this fiili has a very thick mufcular coat; but the fibres
of that coat do not feem to form a diftind fphinder around the hole («).
In the pike, in different kinds of carp, in the perca arenarea, in the conger,
duds of confiderable length lead from the cefophagus into the air-bag; and if,
as in the carp, there are two air-bags, the dud leads to the pofterior bag, from
which there is a paffage into the anterior (¿).
In the common herring, the under part of the ftomach has the ihapc of a
funnel; and from the bottom of this funnel a fmall dud is produced, which runs
between the two milts or the two roes to its termination in the middle of an
oblong-fliaped air-bag (i . ) No valves are found in thefe air-bags.
When we carry our refearches no farther, we very readily conclude, that the
air found in the fwimming-bladder has paiTed into it through the duds I have
been defcribing. And thefe feem well fuited for the purpofe; for, as in the
common horizontal fituation of the fiih their beginning is at the upper 'part of
the ftomach or cefophagus, we can conceive that the air which we fee them take
in at their mouth when they afcend, or that the air which may, by fome more latent
procels, he tlifengaged from the water, is applied to thefe duds ; and that
the fifh, by an inftind of nature, cfiftin^ttifhes the irritation of air from that of
water, and propels the air into the air-bag, but excludes the water. We certainly
diftinguiih air from water contained in our inteilinum rcQum.
But I muft next obferve, that in the cod and haddock, although the air-bag
is very large, and its fides remarkably ftrong, I have not been able to difcover
any communication of it with the mouth, cefophagus, ilomach, or inteftines.
N o intermediate dud is difcoverable by difledion. The air-bag is not enlarged
b y blowing into the alimentary canal; nor can we empty the air-bag without
buriling it. Further, a red-coloured organ,, the furface of which is very extenfive,
as it is compofcd of a vaft number of leaves or membranes doubled, is
found on the inner fide of the air-bag of the cod haddock, &c.: but in
thofe fiilies where the air-bag communicates with the alimentary canal, this red
body
See Tab. XV. fig, 1.2.3.
(Í) See Tab. XV. fig. 2. Tab. XV
(a) Tab.XV. fig.i,[Ln<lTab.XXIX.F.
(0 Tib. XV. Cg. 3. (J) T»b, XV, fig. -ii
bbdy is either very fmall and fimple in its ilrudure, as in the conger eel; or
entirely wanting, as in the flurgeon, falmon, herring, carp.
Hence there is fome reafon to fuppofe, tlaat the air may be fecreted from this
red body, fomewhat in the way it fecms to be fecreted into the fwimmingbladders
of aquatic plants, or perhaps into the air-bag of the egg of a bird as the
chick grows (i).
This, however, I Ihall leave as a mere hypothefis, perfuaded that moil readers
will rather fuppofe that the cod, haddock, &c. have an air-dud, which has
as yet efcaped obfervation.
T o fuch, another queilion will occur, to wit, What is the ufe of this red
body ? Docs it, like the gills, receive fomewhat ufeful, or difcharge fomewhat
hurtful, to the animal ? And are we to fuppofe that the air-bag not only ferves
to render the body of the fifli fpecifically lighter, but alfo that the air received
into it is of benefit to the conftitution, by adding fomewhat ufeful, or by taking
up fomewhat hurtful?
C H A P T E R IV .
A DE S C R I P T I O N of the SY S T EM of LY M P H A T I C ABSORBENT
V E S S E L S in FISHES.
IN this chapter, I ihall defcribe the courfe and terminaticins of the ladeal and
lymphatic veiTels; firil, in the nantes pinnati, of which no account has been
yet given ; and then in the pifces of Linnaeus. In a fubfequent chapter, I iliall
propofe fcch obfervations and expcrlrr.,-»" Js lerve to throw farther hght on the
lymphatic fyftem and fubjea of abforption in general.
T . i;
IN the fkate, my chicf example of the nantes pinnati, the trad of the ali^
mentary canal is remarkably iliort ( / ) . When we open the fmall inteiline,
we obferve fuch large and numerous valvule conniventes, that, on a calculation,
the length and furface of the villous coat is found to be much greater than
at firil fight we would fuppofe (g).
The principal ladeal veíTels are fituated near the large branches of the celiac
and mefenteric arteries and veins; and the principal lymphatic velTels of the
H afliilant
(i) Tlif air.bagofan
atmorpUcrc after tlic glaii
is over tlic top of tlie vagin» of the lien ; apd the air it c{
covered by apcillcle.
« before incubation is taken in from the
(/) Se« T»b. XVUI. and XIX,