; I
IS OF THE H E A R T, V E S S E L S, AND
lobe of the liver; that organ being divided into three lobes. The two cavje,
at laft meet; and, after receiving the coronary veins, 37, of the heart, difcharge
themfelves through a round hole with rifing edges, into the back-part of the auricle
of the heart.
Within the external jugular veins, and at the termination of the internal jugulars,
I obferve a pair of valves like to thofe in the veins of the human fubject: At
the termination of the renal veins and large branches of the hepatic veins, we find
fingle membranes fixed by threads from their edges, refembling the valve of the
coronary vein of our heart; and at the termination of the other large veins, efpecially
near the heart, we not only find the orifices contracted, but doublings at
their edges, which have fo far the effeil of valves, even in the dead body, that wc
cannot fill completely all the veins by throwing an injedion in at one of their
branches; and, between the auricle and ventricle, valves are interpofed, which
perform their office with great accuracy.
S E C T . V.
AFTER tracing the blood in its courfe, I compared with each other the coats
o f the different veflels through which it paiTes.
When the large branches of the branchial arteries and veins, or the branches
of the aorta formed by the latter, were placed contiguous, they were found to agree
very nearly in thicknefs, elaiticity, and texture; nor could I perceive that the
fmall branches of the branchial veins differed from the fmall branches of the arteries,
or that any particular place could be pointed out, at which they might be
fuppofed to ceafe from performing the office of veins and to begin to do that of
arteries. But the coats of their branchial veins, or of the branches of their aorta,
feem to differ as much from the coats of the veins which compofe their vena portarum.
or ven« cav^, as the coats of our aorta differ from thofe of our venze
C H A P T E R il .
O b f e r v a t i o n s o n the CIRCULATION o f the BLOOD in FISHES.
FR O M reviewing the circulating fyftem, and refledingon the colour of the
parts of fiilies, the following conclufions arife.
I. As we obferve that the red particles of the blood are excluded from many
parts, as, in white fifties, from moft of their mufcular fibres, which we know have
in us numerous circulating arljcries difperfcd upon them, it is evident that in them
there
C I R C U L A T I O N F I S H E S .
there arc numberlefs colourlefs arteries; or we find here ftill clearer proof of the
cxiftence of fuch arteries, than arifes from the infpedion of the human body.
2. When we next confider, that from the circulating arteries of their mufcles^
liquors muft be fecreted into the interftices of the fibres, in order to prevent their
concretion, and to allow of their ready play upon each other, we fee proof that
fecerning branches maybe fent of f from colourlefs arteries ; .and that it is not neceffary,
on account of the want of impulfe A tergo in colourlefs arteries, that fecreting
veffels iliould come direftly from thofe that convey red blood, as Dr
Haller fuppofed (i).
3. When to this we add an obfervation I have repeatedly made on the blood
o f animals, to which I had given madder mixed with their food, to wit, that in
their ferum, which was deeply coloured, I could not, with the beft microfcopes,
perceive the colouring particles, and yet that the colour was excluded fi-om their
cornea and from their cartilages, we are led to perceive that the exiftence of defcending
feries of arteries has without juft reafon been called in queftion by Dr
Haller and others (J).
4. As there are few red parts in wliite fiilies, and yet their vcn® cav^ at their
heart, bear nearly the fame proportion to their aorta or branchial artery as in man,
•we muft conclude, that their colourlefs, as well as their red arteries, terminate in
their cavse.
Hence, by analogy, it may be allowable to infer, that our colourlefs arteries
do not end in our valvular lymphatic veins, but in our cava ; or that our cav^
correfpond with all the arteries which ferve for circulating the blood.
5. From the divifion of the branchial arteries into exceedingly minute branches
we fliould fuppofe, that the force of the ftroke of the heart upon the blood muft
be very much broken and loft before the blood gets into the branchial veins;
and, in faft, I faw no puliation in the branches of the aorta of a living Ikate.
Hence, in the firft place, we may infer, that the branchial veins are not made
thick and tough, merely to enable them to refift the TOV J ter^o.
6. As fo much ilrength and elafticity in tiic branchial veins are not lieceiTary
for merely reiilling the force of the blood, or that more itrength and elafticity
than we obferve in otu" pulmonary veins were not neceilary for receiving or for
merely condufting the blood to the other parts of tlje body, we muft fuppofe
that thefe thick, tough, and elaftic coats, are of a living and mufcular nature,
and that the progreftion of the blood through the reft of the body of the filh depends
much on their adivity.
7. We fliall ftill more readily admit that the mufcular power of the veffels
and particularly of tlie arteries, is neceffary for the progreftion of the blood'
when, proceeding aftepfarther, we obferve a third circle completed in their liver;
though, from tlie fituation of their gills, and tlie membranous nature of tlieir diaphragm,
the motion of the blood in their Uvcr is not affifted by their refpiration,
nor docs there fcem to be any other vicarious external impulfe fitted to forward
[I) SK HaUtr Prim. Lin. Phyfiolog. f KIT,
8. Apply-
IL