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34 E X P E R I M E N T S ON THE L Y M P H A T I C
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which is very curious, and the caufe difficult to aflign, the eifufion of thefe liquors
upon the furface of the ikin is made upon the upper or back part only of
the fiili, where the ikin is remarkably tough and fcabrous; whereas, on the
fuppofition of a rupture, the effuiion ihould have been chiefly or folely obfervuble
on the inferior part, where the ikin is thin, fmooth, and much more tender.
From the fmall force necelTary to be ufed in thofe injedions, from the
want of extravafation in the cellular fubftance, and from the regular diftribution
of the orifices from which the liquor is difcharged on the furface of the
ilvin, no perfon ufed to fuch experiments can entertain a doubt that thefe orifices
are the natural beginnings of the lymphatic veins.
The only perplexing circumftances that will occur to him are, how to account
for their being feen on the upper part only of tlie filli, and for their being
larger there than we would expe£l, when we confider the moft approved opinions
about the extreme fmallnefs of the mouths of abforbent veiTels.
Thefe difficulties, I apprehend, may be much leiTened, if not removed entirely,
by attending to the two following fa£ts.
Firit, within the echinus marinus efculentus of Linn^us, a large quantity of
falt-water is lodged between the inner fide of the iliell and outer fide of the alimentary
canal. On carefully examining the outer furface of the ihell, I have
difcovered above two thoufand veflels with orifices or mouths at their beginning,
vifible to the naked eye, and furroundcd with a hard fubftance, fo that they
greatly refemble the human puncta lachrymalia. Thefe veiTels, after penetrating
the fhell, divide upon membranes into a plexus of branches ; from the oppofite
fide of which other ducts are feat out, which terminate at the roots of the
teeth («).
There can be no doubt, therefore, that thefe veiTels with vifible orifices abforb
from the fea the falt-water which is depoiited within the ilicll.
Secondly I have found that the water which is contained in confiderable
quantity within the fkuli of drc-ikate^etween it and the furface of the brain,
contains a large proportion of fea-falt; for tfii' abforpuon of which the veiTels 1
have defcribed in the ikin fcem to be appropriated.
Upon the whole, I apprehend it may be concluded, that to the many arguments
which concur in iKowing that the lymphatic veins are a fyftem of abforbents,
we may now add the demonftration of fome of the orifices by which they
begin in fiihes.
5. It may be worth while to remark, that from the defcription given of the
abforbent veiTels of the echinus marinus, which belongs to the clafs of worms, we
receive proof, that the abforbent veiTels form a diftind fyftcm in the worms as
well as in the mammalia, birds, amphibia, and fiflies. Hence, too,itis highlyprobable
a fimilar fyilem will be difcovered in the clafs of infects, or will be found
to be univerfal in the animal kingdom.
6. As
0113 animal^ I gave 3 full account ia pjpct I read to tlic Pliilo-
('/) Of ttiefe abforbtiils and of ihe oüier parts of tili» very cu
fopLical Society of Edinburgh in 1761, whicU I liave »nneied t
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A B S Ö H B E N T V E S S E L S OF FISHES: 55
6. As no valves are found at the beginning or in the progrefs of the lymphatic
veiTels of fiflies, or in the abforbents of the echinus marinus, we find proof
of three very eflential particulars :
Firit, That the progreflive motion of the fluids. In their lymphatic veflfels
from their fmall to their large branches, does not depend on tlie external preffure
of the mufclcs, &c.; for, without valves, this would drive the fluids as
readily in the diredion from their trunks as towards them.
. Secondly, We arc of courfe- led to conclude, that the coats of the lymphatic
vefl"els, however thin they may feem, are truly mufcular.
Thirdly, Although thefe veiTels are in life conftantly, or at leail generally,
filled with fluids, and therefore equally itimulated in their whole extent; yet
we mufl: conceive that their a£tion, contrary to what we obfei-ve of the arteries,
muil begin at their orifices, and proceed from the fmall towards the large
branches.
7. The obfervations I made in a former chapter on the venous receptacles öf
the red blood, may be applied to thofe of the chyle and lymph. But, further,
as lymphatic glands are wanting in this clafs, it may feem in fome degree probable,
that thefe receptacles aflift in fupplying their place by the prefllare and
agitation to which the chyle and lymph are there expofed ; and that perhaps ar-'
terious fluids are fecreted from their coats, and mixed with the lymph.
8. Our reafon teaches us, that abforbent veflels muft exifl: in 'every the moil
minute part of the body: But when we view a well-inje6led preparation of the
iadeal or lymphatic veiTels of fiflies, we are flruck not only with the number of
their minute branches, but with the number of their anaitomofing canals ; many
of which enter the neighbouring lymphatics at right angles, inftead of being diredcd
towards the heart; by which means a net-work is produced, fo very intricate,
that, when we view a fmall part of it onV. - Jiflituic or impoiUble to
afcevtain what has been the nntural courie of the lymph (o).
B y the great number and unfavourable diredion of thefe anafl:omofing canals,
the flow of tlie lymph mufl be greatly retarded, to ferve purpofes which it is
diflicuk to afcertain or even to conjeture.
From obfei-ving them, and confidering their effed in retarding the flow of
the lymph towards the heart, we fee further proof, that general preiTure cannot
be a chief caufe of the progreflive morion of the lymph, but that each vefl'ei
muit contribute to its progrefs by a well-regulated adion.
Very numerous and large lympharics are difperfed upon the gills of the
fkate. To this, when we add that fiflies foon die when put into water from
which the air has been extraded, and yet that fuch water iS capable of wafliing
oiT exhaled matter from tlie gills, and of taking up phlogiiton readily (p), we
are led to the fuppofition, that the gills or lungs not only difcharge hurtful matter,
(0 SccT.b, XVIII. riff.2. Tab.XXl. fig,«. Tab. XXIII. fig. 2. and 4. (/.) See Cavallo on Air, p.48s-
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