62 O B S E R V A T I O N S ON THE N E R V O U S S Y S T E M . 63
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C H A P T E R XXI.
Of fome principal Nerves which have not been properly-
Traced by Authors.
I N this chapter I iliall defcribe fome principal nerves which have not
been properly traced by authors.
I. I begin-with obferving, that the branches of the firft, or olfa£tory
pair of nerves, are not fo extremely foft, and fo fuddenly difFufed upon the
membrane of the nofe, that it is impoffible to trace them diftindtly by diffedlion,
as Dr Zinn and Dr Haller have affirmed*; on the contrary, they
may, even in the human fubjed, be traced a great way within the nofe,
where their fmall branches are joined, and again feparated, fo as to form a
very elegant net-work -f.
In the iheep and ox thefe nerves are hollow, and a branch is fent off, not,
like the reft, from the bulb at the end of the trunk, but from the upper
part of the trunk, within the cranium, and hence it can be traced more
eafily than the other branches XWhether,
* Zinn Mem. de I'Acad. de Berlin 1755, torn. xi. p. 132. Quiconque en a fait ¡'experience, reconooitra
avec moi, que la moleffe coulante et muqueufe du nerf OlfaSoire, depuis qu'il a paffé la lame
aethmoide, empeche emierement qu'on puifle afEmier quelque choie de certain de fa diftribution, ou
de fon Anaftamofe avec les autres nerfs. ^Haller, El. Phyf. torn. iv. p. 200. Ita olfaftorius nervus,
nufquam equidem du ru s, in pituitaria membrana mollilBmus expanditur, ut ramos in liomine perfequi
nequeas.
t See Tab. XXIV. Fig. 1. and 2.
î See Tab. XSIV. Fig. 5. H.
Whether, in man, thefe nerves are hollow, and whether a iimilar nerve
is fent off from them, I have not had Icifure to examine fufficiently.
2. The nafal branch of the firft branch of the fifth pair carries along
with it a firm white external coat, and hence may, with ftill greater eafe,
be traced in man, as well as in the flieep or ox, to the upper, or fore part
of the feptum narium
3. The nafal branch of the fécond branch of the fifth pair can Hkewife
be very eafily traced within the nofe f ; and in the ox- and ilieep, it is evidently
fpent on the lower part of the feptum narium, as well as upon the
offa fpongiofa and fide of the nofe J.
4. The extent of the retina has been very differently defcribed by anatomifts.
Morgagni, my Father ||, and Zinn have aflirmed that it terminates
at the ciliary circle j whilft authors of equal authority, Euftachius,
Winflow, Ferrein, and Haller have reprefented the whole or a part of
it as extended to the lens, and even as giving a covering to that humour.
The opacity, however, of the retina, compared with the pellucidity of
the capfule of the lens, and the confufion, which we muft fuppofe, would
arife from light ftriking the retina before its rays are colledted into a focus,
render this latter defcription very queftionable ; and, in fad, not long after
I began to iludy anatomy, led by thefe contradidory accounts, to examine
this fubject with particular attention, I found that the retina ended
abruptly, refembling the edge of a tea-cup, fomewhat farther back in the
eye
• See Tab. XXIV. Fig. 3. 4.
+ See Tab. ÏXIV. Fig. 3.
• I See Tab. XXIV. Fig. 5. I. K. L. M. N. Fig. 6. K. P.
II Anat. of the Nerves, 1741, p. 41.
§ Zinn, de Oculo, 1755.
• • Haller, Pr. Lin. Phyf. 1764, p. 515.