O B S E R V A T I O N S O N T H K
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8. Applying to man what we have obfcrved of tiîc veíTcls and circulation in
fiflies, we in the firft place receive ñrong confirmation of an opinion I have always
taught, That our arteries are of a mufcular nature ; and that their adivity
is ciTential in circulation, fecretion, and other important offices.
In the next place we will conclude, that the alternate prelTure of our diaphragm
and abdominal mufcles in refpiration is not, as fome have fuppofed, the principal
caufe of the motion of the blood through the liver ; but that the motion of the
blood and fecretion of tlie bile depend chiefly on the mufcular flrudure and action
of the branches of the vena portarum. Nay, in the large branches of the.
mefenteric veins and vena portarum of an ox, I can by dilTeélion demonftrate a
truly mufcular coat, confiding of two diftind layers. In the external layer the
fibres are longitudinal, in the internal they are tranfverfe or circular (?/).
That refpiration and other external impulfes allift, is highly probable, as the
difeafes of the chylopoetic vifcera are moft frequent in fedentary perfons.
9. In their abdominal vense cavas, and alfo between their venae cavoe hepaticae
and vens cavze, there are large receptacles of blood, and the abdominal trunks of
the caviE and feveral of their branches are larger than their terminations.
In the Seal I have difcovered a fimilar dilatation where the vense cavas hepática:
terminate. As in fiflies the preiTure of the water upon the gills muft be
very much more than the prelTure of the air upon the lungs of land animals, fuch
receptacles were perhaps neceffary when they defcend to a great depth ; juft as
in the Seal they are wanted when the refpiration of the air is interrupted by his
diving.
There feems, however, to be reafon for fufpeÄing, that fome other more
latent purpofes are accompliflied by this ftructure. This appears from the
analogy of the lacteal and lymphatic fyftems ; in which we fliall find afterwards
there are ftill larger receptacles.
10. The lail remark 1 fliall make is, That, the circulation of the blood being
carried on in the cartilaginous fiflies in the fame manner as in the ofi"eous or
Pifces of Linnoeus, and the whole mafe tif.blood palTmg through their gills, they
muß breathe regularly and uninterruptedly to furniih blood to the brain and
other organs, or they cannot polTefs the pulmo arbitrarius as is fuppofed by
Linn^us : So that there appears no jufl: reafon for clafling them with the Amphibia.
In the animals which are commonly reckoned amphibious, to wit the tortoife,
the frog, the lizard, and the ferpent, a part only of the mafs of blood paiTes
through the lungs. In the frog and common fmall lizard (x), branches are fent
ofl^from the aorta ; which, if we may judge from their fize, convey about one
third part of the whole mafs. In the tortoife, the ferpent, and fuch of the
lizard tribe as have two auricles and ventricles, a greater proportion paíTes
through the lungs. In the fea tortoife, the heart and large vciTels of which are
delineated in Table IV. the blood from the lungs paiïes into the left auricle,
and from it into the left ventricle. From the left ventricle it is tranfmitted, by
tranfverfe
(•;) SetTab.in, ' (x) LInnauü Syft. Nat, G. 121. Sp.4i-and47.
C I R C U L A T I O N IN F I S H E S . 21
tranfverfe canals or holes in the feptuni between the ventricles, into the right
ventricle, where it is mixed with tlie blood which is fent from the vense cav«
through the right auricle. All the arteries therefore take their rife from the right
ventricle ; and the pulmonary arteries are confiderably fmaller than thofe which
fupply the place of our aorta (_)>).
In all thefe animals, therefore, every part of the body may receive a confiderable
portion of blood, although the refpiration and free paflTage of the blood
through their lungs be interrupted. Hence they are not under the fame neceff
i ty with the mammalia, birds, and fiflies, of breathing frequently, regularly^
or alternately j or they enjoy the pulmo arbitrarius.
C H A P T E R in .
O f the GLANDULAR ORGANS and SECRETED LIQUORS in
FISHES.
w "E fliall next confider fome remarkable circumilances relating to the
glandular organs and fecreted liquors in fiflies.
T .
Of Liquors fecreted on the external Surface of Fîfhes,
' p H E furface of fiflies, cfpecially of fucíi as live in the fea, is defended by a
quantity of vifcid flime ; a great part of wfucii, as has been long known, is
poured out in the oiTeous fillies by the branches of two du£ls which are placed
upon their fides. Thefc duils, I have obferved, are continued upon the head
and upper jaw ; and others of a fimilar ftruflure are added upon the under
jaw {%).
In the flcate, numerous orifices, placed pretty regularly over the furface, have
been obfcrved by Steno to difcharge this flimy matter. With refped to thefe
iaft, I have remarked fome memorable circumftances. Firft, I have difcovered
one very elegant ferpentine canal between the fldn and mufcles, at the fides of
the five apertures into the gills (a). Fartlier forwards it furrounds the noftrils ;
then it pafTes from the under to the upper part of the upper jaw, where it runs
backwards as far as the eyes. From the principal part o f this dud,, in the under
fide or belly of the fifli, there are not above fix or eight outlets ; but from the
^ upper
(,) See Tab. IV. and .lie EitpUna.ion of it,
(.') See Tab. VJ, and VIJ. ABCDEFGHIKLMN.
(3) See Tab. V. 9. 10. &c. tt