render it impossible to trace them far, which 
lias made some authors deny them to be 
nerves ; bat when we break the circumfer- 
ence of the cribriform lamella, and then gently 
raise it, we may see the distribution of the 
nerves some way on the membrane of the 
nose, where they form a beautiful net-work, 
which has been exquisitely delineated by 
Scarpa, in his treatise De Olfactu et Au- 
ditu. 
The contrivance of defending these long 
soft nerves from being too much pressed by 
the anterior lobes of the brain, under which 
they lie, is singular ; because they have not 
only the prominent orbitar processes of the 
frontal bone to support the brain on each side, 
with the veins going into the longitudinal 
sinus, and other attachments bearing it up, 
but there is a groove formed in each lobe of 
the brain itself for them to lodge in. Their 
splitting into so many small branches before 
they enter the bones of the skull is likewise 
peculiar to them ; for generally the nerves 
come from the brain in aggregated filaments, 
and unite into cords, as they are going out 
at the holes of the bones. This contrivance 
is the best for answering the purpose they are 
designed for, of being the organs of smelling ; 
for had they been expanded upon the mem- 
brane of the nose into a medullary web, such 
as the optic forms, it would have been too 
sensible to bear the impressions of such ob- 
jects as are applied to the nose ; and a distri- 
bution in the more common way of a cord 
sending off branches, would not have been 
equal "enough for such an organ of sensa- 
tion. 
The second pair of nerves, the optic, rising 
from the thalami nervorum opticorum, make 
a large curve outward, and then run obliquely 
inward and forward, till they unite at the 
forepart of the sella Turcica ; these soon di- 
vide, and each runs obliquely forward and 
outward, to go out at its proper hole in the 
sphenoid bone, accompanied with the ocu- 
lar artery to be extended to the globe ot the 
eye, within which each is expanded into a 
very fine cup-like web, that lines all the in- 
side of the eye, to within a little distance ot 
the edge of the crystalline lens, and is uni- 
versallv known by the name of retina. 
Though the substance of this pair of nerves 
seems to be blended at the place where they 
are joined, yet observations of people whose 
optic nerves were not joined, and others 
who were blind of one eye from a fault in the 
optic nerve, or in those who had one of their 
eyes taken out, make it doubtful whether 
there is any such intimate union of substance ; 
the optic nerve of the affected side only being 
wasted, while the other was large and plump. 
In many fishes indeed, the doctrine of decus- 
sation is favoured; for their optic nerves 
plainly cross each other, without any union 
at the part where they are joined in men and 
most quadrupeds. 
Those people whose optic nerves were not 
joined, having neither seen objects double, 
nor turned their eyes different ways, is also 
a plain proof, that the conjunction of the 
optic nerves will not serve to account for 
either the uniform motions of our eyes, or 
our seeing objects single with two eyes, 
though it may be one cause of the remarkable 
sympathy of one eye with the other in many 
diseases. 
The retina of a recent eye without any 
ANATOMY. 
preparation, appears a very fine web, with 
some blood-vessels coming Irom its centre 
to be "distributed on it; but after a good in- 
jection of the arteries that run in tiie sub- 
stance of this nerve as is common to other 
nerves, it is with difficulty that we can ob- 
serve its nervous medullary substance. 1 he 
situation of these vessels in the central part 
of the optic nerve, the want of medullary 
fibres here, and the firmness of this nerve 
before it is expanded at its entry into the 
ball of the eye, may be the reason why we 
do not see such bodies, or parts of bodies, 
whose picture falls on this central part of the 
retina. An inflammation in ti.ose arteries ot 
the retina which several fevers and an oph- 
thalmia are generally attended with, may 
well account for the tenderness of the eyes, 
and inability to bear the light, which people 
have in these diseases. The over-distension 
of these vessels may likewise serve to account 
tor the black spots observed on bright-co- 
loured bodies especially, and for that smoky 
tog through which all objects are seen by 
people in some fevers. If these vessels lose 
their tone, and remain preternaturally dis- 
tended, no object affects our retina, though 
the eye externally appears sound; or this 
may be one cause" of amaurosis or gutta se- 
rena. From a partial distension oi these ves- 
sels, -or paralysis of part of the retina, the 
central part, or the circumference, or any 
other part of objects, may be lost to one or 
both eyes. 
The third rise from the anterior part of the 
processus annularis ; and piercing the dura 
mater a little before and to a side of the ends 
of the posterior clinoid process of the sphe- 
noid bone, run along the receptacula or ca- 
vernous sinuses, at the side of the ephippium, 
to get out at the foramina lacera ; after which 
each of them divides into branches, of which 
one, after forming a little ganglion, is distri- 
buted to the glebe of the eye ; the others are 
sent to the muscular rectus ot the palpebra, 
and to the attollens, adductor, deprimens, 
and obliquus minor, muscles of the eye-ball. 
These muscles being principal instruments in 
the motions of the eye-lid and eye-ball, tins 
nerve has therefore got the name of the motor 
oculi. We have frequently observed in convul- 
sions that the eye-lids widely opened, the cor- 
nea turned upward and outward, and the eye- 
balls sunk in the orbit, which well described 
the conjunct action of the muscles which this 
pair of nerves serves. The distension of a 
considerable branch of the carotid which 
passes over this nerve near its origin on each 
side, may possibly be the reason of the heavi- 
ness in the eye-lids and eyes after drinking 
hard or eating much. 
The fourth pair, which are the smallest 
nerves of any, derive their origin from the 
back part of the base of the testes; and then 
making a long course on the side of the an- 
nular protuberance, enter the dura mater a 
little further back, and more externally, than 
the third pair, to run also along the recepta- 
cula, to pass out at the foramina lacera, and to 
be entirely spent on the musculi trochleares, 
or superior oblique muscles of the eyes. These 
muscles being employed in performing the 
rotatory motions, and the advancement of the 
eye-balls forward, by which several of our 
passions are expressed ; the nerves that serve 
them have got the name of pathetici. Why 
these small nerves should be brought so far to 
101 
this muscle, when it could have been supplied 
easily by the motor oculi, we know not. 
The fifth pair are large nerves, rising from 
the annular processes, where the medullary 
processes of the cerebellum join in the ior- 
maticn of that tube to enter the dura mater, 
near the point of the petrous process ot t he 
temporal bones ; and then sinking close by 
the receptacula at the sides of the sella i ur~ 
cica, each becomes in appearance thicxeiy 
forms a distinct ganglion, and goes out of the 
skull in three great branches. 
The first branch of the fifth is the ophthal- 
mic, which runs through the foramen lacei inn 
to the orbit, having in its passage thither a 
connection with the sixth pair. It is after- 
ward distributed to the ball ot the eye with 
the third: to the nose, along with the olfac- 
tory, which the branch of the fifth that passes 
through the foramen orb.tarium _ internum 
joins, as was already mentioned in the de- 
scription of tiie liist pair, lifts ophthalmic 
branch likewise supplies the parts at the in- 
ternal canthns of the orbit, the glandula la- 
chrymalis, fat, membranes, muscles, and te- 
guments, of the eye-lids; its longest farthest 
extended branch passing through the fora- 
men superciliare of the os frontis, to be dis- 
tributed to the forehead. 
The small fibres which this first branch of 
the fifth and third pair of nerves send to the 
eye-ball, being situated on the optic nerves, 
and after piercing the sclerotic coat, running 
along the clinoid coat, on the outside of the 
reiina, in their course to the uvea or iris, 
may be a cause of the sympathy between the 
optic nerve and the uvea, by which we more 
readily acquire the habit ot contracting the 
iris, and thereby lessen the pupil, when too- 
strong light is excluded ; and, on the con- 
trary, enlarge the pupil when the light is too 
faint. This, with the sympathy which must 
arise from some of the nerves of the mem- 
brane of the nostrils being derived from the 
first branch of the. fifth pair of nerves, may 
also be the cause why an irritation of the re- . 
tina, by too strong light, may produce sneez- 
ing, as if a stimulus had been applied to the 
membrane of the nose itself; why pressing 
the internal canthus of the orbit sometimes- 
stops sneezing ; why irritation of the nose, on 
the oilier hand, causes the eye-lids to shut 
convulsively, and makes the tears to flow 
plentifully ; and why medicines put into the 
nose affect diseases of the eyes. 
The second branch of the fifth pair of 
nerves may be called maxillaris superior, 
from its serving principally the parts ot the 
upper jaw. It goes out at the round hole of 
the sphenoid bone, and sends immediately 
one branch into the channel on the top of 
the antrum maxillare ; the membrane of 
which, and the upper teeth, are supplied by 
it in its passage. As this branch is about to 
go out at the foramen orbitarium externum, 
it sends a nerve through the substance of the 
os maxillare to come out at Sterno’s duct, to 
be distributed to the fore part of the palate ; 
and what remains of it escaping at the exter- 
nal orbitar hole, divides into a great many 
branches that supply the cheek, upper lip, 
and nostril. The next considerable branch 
of the superior maxillary nerve, after giving 
branches which are reflected through the 
sixth hole of the sphenoid bone, to join the 
intercostal where it is passing through the 
skull with the carotid artery', and the portio 
