84 O B S E R V A T I O N S ON THE
5. Can we imagine that our organs, varj'ing fo greatly in colour and confiftence,
are nourifhed by the nerves which feem uniform in their appearance
?
6. T o conclude with an argument againft this hypothefis which may be
coniidered as an argumentitm crucisy it is well known, that, if powder of madder
root is mixed with the food of a young annnal, the bones become red ;
or, if a bone has been broken, that the callus joining its parts will be red.
The ferum of the blood, in the firft place, is deeply tinged; but the red
colour.of the bones is not folely, nor even chiefly, owing to the coloured
ferum or blood circulating; for I have found, that, after injeiling water
into the vedels till thefe were emptied of the blood, and that the water
came out colourlefs, the tinge in the bones appeared equally deep, and
was, therefore, plainly owing to a great quantity of the red earth added
to the bones in the time of their growth. But this earth was not tranfmitted
by the nerves; for the colour of thefe, as I found, remained unchanged.
S E C T . V.
WHEN, on the other hand, we du'cft our attention to the offices of the
arteries, a variety of circumftances crowd upon us, which concur in Ihowing,
that, from their extremities, the nouriihment is direcUy fupplied.
1. The thoracic duf l conveys the chyle into the red veins, which terminate
in the right fide of the heart, fo that the chyle comes to be intimately
incorporated with the whole mafs of blood in its paffage through the lungs,
inllead of taking a particular dircflion towards the brain.
2. The chyle, in the general eourfe of circulation, is converted into blood,
the glutinous matter of which, called coagulable lymph, much refembles
the glaire of an egg, and feems equally fit for the growth or reparation of
the parts of an animal. Nay, fome late writers, who contend that nourifli-
N E R V O U S S Y S T E M . 85
Uient is conveyed by the nerves, fuppofe that they perform their office by
feparatii^ this gbten from the blood. So that the chicf or only remaining
queftion feems to be, Whether the extremities of the arteries or the nerves
are to be fuppofed to perform the fecretion of it, and nutrition of eourfe ?
Now, without repeating our extreme uncertainty as to the tubular nature
of the nerves, and the improbability that canals fo exceedingly minute
as thofe within the nerves muft be, and of fuch length, are delHncd for
tlie conveyance of glue, do we not find that this very matter is feparated
by the exhalant branches of the arteries of the peritoneum, pleurae, and other
ilmt facs, and univerfally, by the'branches of the arteries of the cellular
membrane ?
In like manner, is not the glaire of an egg, the nourifliment intended for
the chick, fecreted and prepared by the arteries of the Hen ?
3. The kinds of matter necefiary for the growth and nourifliment of our
feveral organs are fo various and difFei'ent in their nature, that it is altogether
incredible they can be furniihed by the nerves. Thus, water is needed
for the extenfion of the fore part of tlie eye, vifcid matter for the cryftalline
and vitreous humours, earth for the growth of the bones, &c.'
Whereas we can as eafdy conceive thefe to be furniilied by the arteries, as
that, in one place, they fhould furnifli faliva, in another the bile, in a
third the femen, and fo on.
4. In feveral inilances, an increafe of fecretion from the arteries produces
effefts which we very univerfally afcribe to an increafe of nutrition. Thus,
the weight of the humours of the eye increafes after birth : Or when a
perfon, reduced by a tedious fever, has grown plump again, we conceive
that a quantity of fat has been fecreted into the adipofe follicles. Nutrition
and fecretion by the arteries are, in fuch inilances, fynonymous terms.
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