48 O B S E R V A T I O N S ON THE
c h a p t e r XVIII.
Of the External Covering of the Trunks of the Nerves, and
of the Cords or Funiculi of which they confift.
| " ^ H E trunks of the nerves are known to be covered and defended by
a thick and ftrong membrane, which, in the optic nerve, is evidently
the continuation of the inner part of the dura mater. The holes in the
cranium, through which the other nerves of the head pafs, are lined with
the fame membrane, and the inner part of it adheres to the nerve when it
leaves the bone; the fpinal marrow is inclofed in a loofe {heath produced
from i t ; and the inner part of that llieath adheres to the beginning of the
fpinal nerves, and evidently covers them for fome way after they have
palTed out between the vertebrae. Hence tlie external covering of the
nerves has been confidered as the mere continuation of the dura mater.
But the very accurate Zinn has obferved, that the continuation of the
dura mater along the nerves in their courfe, is not to be traced fo evidently
as has generally been fuppofed; for, that the inner part of the dura mater,
which inclofes and adheres to the beginning of the nerves, is very thin,
and foon degenerates into the common cellular fubftance. He, therefore,
conilders the external covering of the nerves as the cellular fubftance condenfed.
But, although this opinion of Zinn is fo far juft, that the dura mater
cannot, fo evidently as is fuppofed, be demonftrated to form the whole of
the external coat of nerves, it may be obferved, that the external covering
of the nerves, in their courfe, is almoft as tough as the dura mater;
agrees much with that membrane in its colour and fibrous texture, and,
probably, in its properties ; and cannot, as Zinn reprefents, be readily diffolved
N E R V O U S S Y S T E M . 49
folved into cellular threads. So that it will occafion no material error in
phyfiology or pathology, whether wc confider the external coat of the
nerves in their courfe as the continuation of the dura mater, or as a cellular
fubftance condenfed, or as a coat proper to the nerves, created thick
and tough, merely to preferve them from laceration and other injuries.
Authors, whether they fuppofe this thick and tough coat to be derived
from the dura mater, or produced from cellular fubftance condenfed, have
reprefented it as giving a covering to the trunks only of the nerves, or to
branches detached from them, and have defcribed the fmaller cords, or funiculi
which compofe the trunks, as covered by the pia mater only But,
in by far the greater number of the nerves, and particularly in all thofe
which run ajnongft and terminate in the mufcles, I obferve, that the funiculi,
or fmaller cords, have a iimilar tough, denfe, fibrous coat, and within
it the thin vafcular pia mater.
C H A P-
• Kaller, El. Phyf. Tab. IV. L. lo. S. 5. § 3. p. 188. • Porro corum fumculorum quilibet membranula
ambicur, quam a pia mater effe, ipfa continuatio, in medulla fpinali cvidentiffima, demon.
= ftrat. Naturam etiam teneram fuam, atque vafculis pinftam, ea meninx in nervis retioet.'