' 4 OF THE H E A R T, V E S S E L S, AND C I R C U L A T I O N F I S H E S .
h
Shall conclude with fome obfervations on their Brain, Nerves, and the Organs
of their Senfes (a).
C H A P T E R I.
A Defcription of the HEART, VESSELS, and CIRCULATION of
t h e BLOOD, in FISHES.
IN all the fiflies I have diffcaed there is but one heart, confiffing o f one auricle
and one ventricle ; and, from the latter, one artery is fent out, which is entirely
fpent on the gills. From the gills, therefore, the returning blood palTes to
all the other parts of the body, without the intervention of a fécond heart, as in
man
So far is generally known: but the whole courfe of the blood has not been traced
with fufficient accuracy; fo that feveral curious and intereftingcircumftanceshave
efcaped notice.
I lhall therefore give a particular defcription of the circulating veffels of the
Hate, with which I have found the Pifces of Zhmxus agree in every material circumftance
; and I iliall begin with tire branchial artery.
T . I.
A T the beginning of the branchial artery there are three femilunar valves (I),
the middle parts of which, analogous to the Corpufcula Morgagni, are much
thicker than in man, and illuftrate the ufe of thefe organs in him, as they very
evidently aJLil in preventing.,^ return of the blood when the artery is in a£iion.
BetwMn tnclc valves of die ventricle in the iiate, there
is a cylindrical canal intcrpofed (i/), the coats'-of which have the fame mnfcolar
texture as the ventricle itfelf; and their contraflion co-operating with that of
the ventricle, we are thence led, more evidently than in maiij to pcrccivc the analogy
between the ilructure of the arteries and that of the heart.
The coronary arteries of the heart do not take their rife from the branchial
artery, which, like the pulmonary in man, is fpent enrircly on their lungs or gills,
by five pairs of great branches in the ikate, and by four in the Pifces of LinniEus
(e).
When
(a) To be more reaiiily uiiderftooil, I ftiali generally apply die terms fart, back, vppir, under, inner, euter, in llie fame
manner as is done in fpeaking of die liunmn body; or, I ihaii fuppofe die iifli to be phccd crcdl widi its liend .ippermofl-
Rut in defcribing the ear and other parts of the head itfelf, I ihall fuppofe the nfli in its natural fituatio.i, as the brain and
organs of the feiifcs have not tlie fame direaion as in man, wiih refpe^ to the trunk of die body.
(A) If lungs, well as gills, arc found in the Petromyzon and Diodon of Linnxus, it is prob;iblc there may be two ventricles
in their heart.
In tlie Sepia, which has been generally confidcrcd as a fiih, but which is widi
nsus, I have, many years ago, difcovered three hearts; of which, and of other c
animal, the reader will find an .ucount fubjoined to this Work.
propriety reckoned a wormhyl.in-
B particulars in the ftrudurc of diis
(<•) See Tab. II. 40. (rf) Tub. I. Fig. 4. (.) Tab. I. Fig. 4- and Tab, XXVI. Fig.
'S
When we take a fuperficial view of the fmall bulk of the gills; when we i-C''
fleit, that an organ for reftoring momentum to tlie blood, fimilar to the leftven^
tricle of our heart, is wanting in fiihes; and when, further, we confider the extraordinaiy
fize of the red particles of their blood; we might be tempted, at firft
fight, to fuppofe that their branchial artery did not divide into very minute
branches, or that it rcfembled tlie rete mirablle of the carotid artery of the herbivorous
quadruped. But a nearer view will undeceive us. For I have found,
that, by the elegant fubdivifions and folds of the membrane of the gills, their furface,
in a large fkate, is nearly equal to the whole external furface of tlie human
body.
Thus in cach fide of the body of a ikate tliere are four double gills, or gills
with two fides each, and one fingle gill; or there are in all eighteen fides or farfaces
on which the branchial artery is fpread out. On each of thefe fides there
are about 50 divifions, or doublings of the membrane of the gills Each
divifion has on each fide of it 160 fubdivifions, doublings, or folds of its membrane
; the length of each of which in a very large ikate is about one-eighth of an
inch, and its breadth about one-fixtccntli of an inch So that in the whole gills
there are 144,000 fubdivifions or folds, the two fides of cach of which are equal
to tlie 64th part of a fquare inch, or the furface of the whole gills in alargelkatc
is equal to 2,250 fquare inches, that is, to more than 15 fquare feet, which have
been fuppofed equal to the whole external furface of the human body. When,
after a good injection of the artery, a microfcope is applied, the whole extent of
the membrane of the gills is fcen covered with a beautiful net-work of exceedingl
y minute veiTels.
A s fiilies taint the water they refpire, in nearly the fame manner as we
taint the air we breathe, it appears that fome matter, either ufelcfs or hurtful, or
both, is carried off from their blood as it paiTes through their gills.
When diftilled oil of turpentine, coloured with vermilion, is injected with moderate
force into the branchial artery of a living- or recmrly Head ilcate, fome of
the colourlefs part of the oil exfndes upon the furface of the gills. But that
thefe are the only paflages by which tlie hurtful matter efcapes, feenis very
doubtful; as, from Dr Prieftley's experiments, it appears that the colour of the
craiTamentum of the blood can be changed by the atmofphere, though the craffamentum
be inclofed in an ox's bladder and covcrcd with ferum.
T . IL
WE iliall next trace the blood from the gills till it returns to the heart.
In the iippermoll gill (¿), which is fingle, there is but one confiderable
vein.
In each of tlic four double gills there are two principal veins, an upper and under,
joined together by a large tranfvcrfe canal.
The
(/) See Tab, I. Fig. I. 2. Tab. I. Fig. 3. (i) See Tab. I. Fig. 5. A B C. See. to W.