24 O B S E R V A T I O N S ON THE
C H A P T E R VIII.
Of the fuppofed Origin or Formation of Nerves.
E 1.
"fT^ROM the under parts of the brain and cerebellum the nerves of the
head and fpinal marrow arc commonly faid to originate, and when we
attend to the ideas of authors, we iliall find,
In the firjl place, that many write as if they conceived that the whole
mcdullaiy fubftance of the brain and cerebellum was employed in the formation
of the nerves of the head, and of the fpinal marrow*.
In tlie Jemid place, that, very generally, the brain and cerebellum are
conceived to be abfolutely neceiTary to the formation of nerves; and, indeed,
moft authors have confidered the brain as a gland, and the nerves as
the du£ts of that gland.
In the third place, the nerves are univerfally fuppofed to receive the whole
energy by which they operate, from the brain, cerebellum, and fpinal mar-
But every one of thefe opinions, on very important points, wiU, perhaps,
be found doubtful or erroneous when we confider them fully.
T. II.
WITH regard to the FIRFT opinion, when in any animal, in man fuppofe,
we
• Kaller, Pr. Lin. 356. Medulla omnU cerebri ct cerebelü per diverfa foramina cranio exit, ail locos
deftiaatos. Minores Mciculos ñervos vocant, majorenx medullara fpinalem.
N E R V O U S S Y S T E M .
We make an horizontal fedion of the brain, where its hemifpheres are joined
together by the corpus callofum, we divide a mafs of meduUary fubftance,
which is one hundred times, at leaft, as bulky as all the nerves of the head
and fpinal marrow conjoined. So that, unlefs we are to conceive that a
hundred medullary fibres of the brain concur in forming a fingle nervous
fibre, we muft conclude, that a great part of the machinery of the brain
ferves fome other purpofe.
Befides, it may be obferved, that many mediillary fibres of the brain appear,
from their direction, better calculated to conned the different fides
and diifercnt parts of the brain to each other, than to conned the brain to
the nerves. Thus, we obferve tranfverfe bundles of fibres in the corpus
callofum, the commiiTura cerebri anterior et pofterior, the tuber annulare,
and even in the meduUa oblongata
Further, it is obvious, and has been long and repeatedly obferved by
anatomifts, that fiflies, and other animals with very fmall brains, feel as a^
cutely, and cxercife their mufcles as violently, as the other clafles of animals
in which the brain is proportionally much larger.
In the laß place, the human brain fo greatly exceeds, in proportional fize,
the brain of the other animals of the fame clafs, that I have found the brain
of a large ox not to weigh more than one fourth part of the human brain,
whilft the weight of the ox was, probably, fix times greater than that of
the man; or the brain of the man was, in proportion to his weight, twentyfour
times heavier than that of the ox f : At the fame time, the nerves of
the mufcles of an ox are, in their fize, proportioned to the bulk of his
mufcles ; and thofe of the organs of his fenfes, as of the eye and nofe, arc
proportioned to the e.xtent of thefe organs; thus, the olfadory nerve of
an ox is many times larger than that of a man. Hence we are led to con-
G fidef
• See Tab. V. and Tab. Vll.
t See fimilar experiments made by Dr Kaller and others. El. Phyf.