6 oo A N A T O M Y. 
ceffary conditions, fnch as a perfect clearnefs of the hu¬ 
mours ; a due mobility of the eye itfelf,. and its' parts; a 
fenfibility of rite pupil; and a retina neither too prurient 
nor too callous. 
Tire mind only receives a representation of the image of 
the objeift by the eye, imp relied on the retina, and transfer¬ 
red to the common fenfory or feat of the foul. Several 
circumftances relative to vifion are not determined by the 
inflrumentality of the eye, but are perceived by the mind 
from mere experience ; and fo me times the mind interprets 
the reprefentation to be very different from that which the 
eye gives to her. The magnitude of an objefl, for in- 
ftance, is not determined by the eye, but by the optical 
angle, which is formed by lines fuppofed to be drawn from 
the extremities of the objeiSt to the cornea. Hence near 
objects feem large, and tliofe at a diftance fmall. On this 
circumftance alfo the power of microfcopes depends, which 
magnify in proportion to the difference between the focal 
length of the magnifier and the diftance of dnftinft vifion. 
Objects, by the means of this inftrument, not only appear 
larger, but brighter and more diftiwft, and hende the mind 
thinks them nearer. 
Diftance vve cannot perceive; and if a blind man, who. 
never faw, fhould by any means be reftored to fight, he 
would imagine every thing lie faw to touch his eyes. liven 
we, who are accuftomed to judge by fight, make many fal¬ 
lacious conjeftures concerning the diftance of objects. 
We judge of the diftance of an object from the diminution 
of its known bulk; from its diminifhed brightnefs ; from 
the faintnefs of its image, by which we are lefs capable of 
diftinguifhing its parts; and, laftly, from the number ot 
bodies, wliofe diftance is known, that are interpofed be¬ 
tween us and the objeft. The vifible fituation of the 
parts of an object, is judged by the mind to be the fame 
with that which thefe parts naturally have in the object, 
nnd not the inverted pcfition in which they are painted 
upon the retina. The faculty of corredting this inverfion 
feems to be innate ; for new-bohi animals always fee ob¬ 
jects upriglit; and men who have been born with cata- 
paCts are obferved, upon couching tlie catarafts, to fee eve¬ 
ry thing in its natural fituation, without the ufe of any 
feeling, or previous experiences. The mind is often im¬ 
posed on by the continuance of the fenfation after it has 
been conveyed to the mind, by which means objects, al¬ 
though inftantly removed, continue to be feen for about a 
iecond. Hence proceeds the idea of a-fiery circle from 
the circumrotation of a lucid body; and hence alfo pro¬ 
ceeds the continuance of the Alining image of the fun, and 
fometimes of other bodies, after they have been viewed 
by the eye. 
Why do we fee only one object with two eyes ? became, 
when the impreflions of two objects are fimilar, the fenfa¬ 
tion becopaes fingle. Even without the concourfe of op¬ 
tic nerves, infefjs who have numerous eyes perceive ob¬ 
jects fingle. Hence the images of two objects excite only 
one fenfation, when they fall upon the fame point of the 
retina; but two fenfations arife from one objedt, when the 
images fall upon different parts of the retina, Whence 
proceed diurnal and nodturnal blindnefs ? The former is 
common to many nations living in the warmed climates, 
tinder the brighteft fun, and to old men. 1 he other hap¬ 
pens in inflamed eyes, and to young men of a hot tempe¬ 
rament, who are endowed with eyes vaftly fenfible. Whence 
do animals fee in the dark? From a large dilatable pupil, 
and tender retina; and a fhintng choroides, which reflects 
the light very ftrongly. Why are we blind when brought 
out of a ftrong light into a w.eak one? Becaufe the optic 
nerve, having fuffered the action of ftronger rays of light, 
is incapable of being moved by tliofe that are weaker, 
Whence have we a pain, by palling fuddenly from a dark 
place into the light ? Becaufe the pupil, being widely di¬ 
lated in the dark, fuddenly admits too great'a quantity of 
light before it can contraft; whereby the tender retina, 
which is eafily aftedted by a fmall light, feels, for a time, 
an impreflion too fiiarp and ftrong. \V hether do vve fee 
with one eye, or with both ? Mcft frequently with one, and 
more efpecialiy the right eye. But, when both are em¬ 
ployed together, vve fee more objedls, and more plainly; 
and we alfo diftinguifh more points of the fame object, and 
judge better of their diftances. 
OF THE NOSE. 
The parts of which the nofe is compofed, may be divi¬ 
ded in two different ways, viz. from their fituation, into 
external and internal pares; and, from their ftrudture, into 
hard and foft parts. The external parts are the root of 
the nofe, the, arch, the back or fpine of the nofe, the tides, 
the tip of the nofe, the alte, the external nares, and the part 
under the feptum. The internal parts are the internal 
nares, the feptum narium, the circumvolutions, the con¬ 
chas fuperiores, or offa fpohginofa fuperiora, conchae in- 
feriores, the pofterior openings of the internal nares, the 
firms frontales,, firms maxillares, finus fphenoidales, the 
ductus lacliryntales, and ductus palatini. The firm or hard 
parts are moftly bony, and the reft cartilaginous. The 
fort parts are the integuments, mufcles, facculus lachry¬ 
mal is-, membrana pituitaria, vefiels, nerves, &c. The in¬ 
ternal nares, or the two cavities of the nofe, comprehend 
the whole fpace between the external nares and pofterior 
openings, immediately above the arch of the palate, from 
whence thefe cavities reach upward as far as the lamina 
cribrofa of the os ethmoides, where they communicate for¬ 
ward with the finus frontales, and backward with the finus 
fphenoidales. Laterally, thefe cavities are bounded on 
the infide by the feptum narium; and, on the outfide, or 
that next the cheek, by the conchae or offa fponginofa, 
between which they communicate with the finus maxil- 
laris. The external nofe is covered by the common inte¬ 
guments, the fkin, epidermis, and fat. The parts which 
cover the tip of the nofe and alae narium, are pierced with 
the ducts of a great number of glandulae febacese. AH 
thefe bony and cartilaginous parts have likewife the com¬ 
mon periofteum or perichondrium. 
The membrana pituitaria is that which lines the whole 
internal nares, the offa fpongiofa, the fides of the feptum 
narium, and, by an uninterrupted continuation, the inner 
furface of the finus frontales and maxillares, and of the 
dudlus lachrymales, palatini, and fphenoidales. Jt is term¬ 
ed pituitaria, becaufe, through the greateft part of its large 
extent, it ferves to fe par ate from the arterial blood a mu¬ 
cilaginous lymph, called pituita by the ancients, which, in 
its natural ftate, is nearly liquid; but it is fubjedt to very- 
great changes, becoming fometimes glutinous, fometimes 
limpid, &c. neither is it feparated in equal quantities thro’ 
the whole membrane. The lachrymal facculus is an ob¬ 
long membranous bag, into which the ferous fluid is dif- 
charged from the eye, through the punfta lachrymalia, 
and from which the fame fluid paffes to the lower part of 
the internal nares. Theduftus inciforii, or nafo-palatini 
of Steno, are two canals which go from the bottom of the 
internal nares, crofs the arch of the palate, and open be¬ 
hind the firft or largeft dentes inciforii. In frefh fubjefts 
they are not fo apparent, efpecialiy in human fubjects; 
but in flieep and oxen they are eafily difcoverable. 
In animals which fmell acutely, the parts of the nofe 
are remarkably large; but the formation of the human 
head into a roundilh figure, ' has given to the organ of 
fmelling only a fmall extent of furface; but nature has 
made the internal parts of the nofe hollow, and varioufly 
complicated in a furprifing manner, to enlarge the faculty 
of fmelling, becaufe tailing noxious food might in many 
inftances be highly dangerous. We can by this fenfe, in¬ 
dependent of tafte, difeover the noxious quality of ali¬ 
ments, efpecialiy fuch as are putrid, and confequently 
peculiarly hurtful to the human frame; and at the fame 
time that it directs us to avoid what is dangerous, it ena¬ 
bles us to difeern-what is grateful and wholefome. This 
ufe of fmelling in choofing food, is more obfervable in 
brutes than in men ; yet men left to themfelves, and who 
are undebauched by a variety of feents; polfefs this faga- 
2 ciot*s 
