t:: I
r
r .
38 O B S E R V A T I O N S ON THE
C H A P T E R X I I I .
Of the Appearance of the Nerves iii their Courfe, and
particularly of their Folds or Joints.
Perfon wh'o has read in authors that the nerves are the continuation
of tlie medullary fubftance of the brain, cerebellum, or fpinal marrow,
colledcd into cords, which confift of longitudinal fibres laid pai-allel
to each other, will be far from finding the appearance of them to correfpond
with his idea.
I have already obferved, that, with a very few exceptions, the nerves
are not white, Uke the medulla, but of a cineritious colour, and that they
are not to be confidered as pure meduUai-y cords.
I will now add, that when, in any of the four clafles of large animals,
we view the nerves carefully with the naked eye, or with a common magnifying
glafs, they appear to confift of a femipellucid fubftance, in wliich
a more white and opake fibrous looking matter feems to be difpofed in
tranfverfe and ferpentine lines
This appearance is fo remarkable, and fo evident in all the nerves, efpecially
in the fmall ones, where the external coats are thin, that, whoever
will examine it, muft be aftonilhed that no notice has been taken of it by
any author f .
When
' See Tab. XIH. Fig. 3 14.
\ The onJy perfon who, before me, had remarked, in a tranfient way, an appearance of ferpentine
or fpiral fibres in a nen'e, is the very ¡earned Dr Thomas Smith phyfician at Birmingham. In the
year 1767, whilit he was engaged in compofing a very excellent Inaugural DilTertation, which he publiilied,
De Ailione Miifcitlari, he told me he had feeii an appearance of fpiral fibres in the fciatic nerve
of
N E R V O U S SYSTEM. 39
When the nerve is fully relaxed, thefe ferpentine tranfverfe lines are beft
feen. When the nerve is moderately ftretched, they are much lefs evident.
When the nerve is greatly ftretched, beyond what it ever is in a living found
animal, it appears uniform in its colour and confiftence
Hence thefe lines are, in the JirJl place, to be confidered as folds or joints
in the nerve, and may be compared to the lines in the palm of the hand,
ferving to accommodate the nerve to the different ftates of flexion and extenfion.
To prove, ftill more clearly, that this is their chief ufe, I obferve, that
the tendons of all animals have a fimilar appearance, in their relaxed ftate,
but lofc that appearance when they are greatly ftretched f .
By attending to this appearance of the nerves, I have, with certainty,
, traced their fmall branches, when I could not, by any other mark, have
been able to diftinguifli them.
Thefe folds or joints appear in the nerves between the brain and foramina
in the cranium, or before the nerves reach the dura mater; and they
appear diftin£lly in the nerves, in their whole courfe, on the outfide of the
head: Nay, contrary to what might have been fuppofed, they are nearly as
numerous
of a young crow. He had formed no opinion of the nature or ufe of thefe fibres 5 and he fuppofed them
proper to that fpecies dur ing their growth.
The following year, I procured, in the evening, a young crow. I cut off its hind legs, and put
them into a bafon full of water till next morning: But I could not then obferve any fuch appearance
as Dr Smith had defcribed. I fuppofed, therefore, that there had been fome miftake in that matter;
and I did not obferve fuch an appearance in the nerves till many years afterwards, when I remarked it
firft in a fltate filh. By foaking a nerve in water this appearance is loft.
• See Tab. XIII. lug. i. 10. 11.
i- See Tab. XIII. Fig. i j .