82
I l t
O B S E R V A T I O N S ON THE
T. IV.
BUT, on this queilion, we can, I apprehend, not only dete£l the fallacy
of thofe arguments which have been ufed to ihow that the nourifliment is
conveyed by the nerves, but point out a variety of circumftances which are
inconfiftent with fuch an hypothelis ; and, on the other hand, we can, by
decifive experiments and arguments, prove, that the arteries diredly fecrete
and apply the nouriihment, without the intervention of the nerves.
Without iniifting farther on our uncertainty of the nature of the nervous
fluid, and, indeed, of its very exiftence, the following circumftances
are inconfiftent with the idea that a fluid conveyed by the nerves nou-
Uihes.
I. On comparing different animals we are far from finding any correfpondence
between the fize of their brain and rapidity of their growth, or
quantity of nouriihment they receive.
Thus, though an ox be fix times heavier than a man, his brain does not
weigh above a fourth part of that of a man. Does his brain, then, prepare
twenty-four times more nourifliment than a portion equal to it of the
brain of a man ?
Nay, in the fpace of two years, an ox comes to his full fize, fo that his
brain muft be fuppofed to tranfmit through the nerves daily two or three
pounds of flelh, bones, &c. whilft the much larger brain of the man has
not, in the fame time, added to his weight above a fiftieth part of the increafe
of the weight of the ox. Or, the brain of the ox, befide repairing
the wafte of a much larger body, is, by fuch an hypothefis, adding to his
weight, in any given period, two hundred times as much as an equal portion
of the brain of the man adds to his bulk.
Suppofe,
N E R V O U S S Y S T E M . 83
Suppofe, then, both animals to be fed on milk, we muft believe that thé
milk pafles two hundred times quicker through every medullary fibre of
the brain of the ox than through the medullary fibre of the brain of the
man.
2. In monfters, I have found the limbs very plump, thdugh the brain
was very fmall*. Nay, in fome monfters, the head has been wanting, yet
the limbs were as large and perfect as common. In other monfters with
one head and two bodies, I have found that the brain furniflied the nerves
of the head and fpinal marrow on the right fide of the monfter ; yet the
left fpinal marrow, at the top of which there was only a fmall medullary
knob, about the fize of a large pea, was as perfeft as tlie right one, and
that body and its limbs were as large and well nouriihed as thofe on the
right fide f . On the other hand, where there were two heads of the ordinary
fize, and only one body, the limbs were not remarkable for their
fize J.
3. We fee that organs, of which the nerves are fo fmall that we cannot
trace them by difleétion, as the bones, the placenta, &c. grow as quickly
as the otlier organs in which the nerves are large and numerous.
4. A year after I had cut acrofs the fciatic nerve of a living frog, I could
not perceive that limb fmaller than the other, yet it continued to be infenfible
and motionlefs. Nay, when I had broken the bones of the infenfible
hmb, or wounded the Ikin and fleih, I found that the callus formed, and
the wounds healed as readily as if the nerve had been enth'e. The event
•was tlie fame, -after dividing, tranfverfely, the lower or pofterior end of
the fpinal marrow of the frog.
5. Can
• Tab. VHI.
t Tab. XII. Fig. 3. 4.
1 Tab. VIU. • •
I