PHYSIOLOGY. 
the eye, and the iris dilates or becomes 
broader : hence the pupil is contracted, and 
the quantity of light admitted into the eye 
diminished. An opposite change takes 
place when we go from a strong into a 
weak light. These motions depend entirely 
on the mode in which light affects the re- 
tina ; for the iris is of itself insensible to 
luminous rays. The painful effect pi-odiiced 
on the retina by a strong light is obviated 
by the contracted state of the pupil ; while 
the opposite condition of that opening, in 
darkness, is designed to admit a sufficient 
quantity of rays, to produce a proper im- 
pression on the retina. 
The seeing of bodies erect, although their 
images are painted inverted on the retina, 
is thus explained. An object is said to be 
inverted in respect to others, which are 
erect : now all objects whatever are painted 
inverted on the retina, and all therefore 
correspond to each other in situation and 
connexion, just as if they had been repre- 
sented in their natural position. All con- 
fusion is, therefore, guarded against in tire 
mind; to which the image itself on the 
retina is not communicated, but only an 
impression caused by its formation. 
The motion of the iris contributes to dis- 
tinctness of vision by regulating tire quan- 
tity of light admitted into the eye : and 
there is anotlier provision tending to the 
same effect; viz. the absorption of any 
superfluous rays, which may have entered 
the eye by the black covering of the cho- 
roid coat. The utility of this dark pigment 
may be understood from observing the 
effects of its absence in the albino, where it 
causes a tenderness of the organ, and an im- 
patience of the light. 
Distinct vision also requires that the focus 
of the refracted rays should fall exactly on 
the retina, without falling short of it in the 
vitreous humor, or being elongated beyond 
it. The former fault constitutes near-sighted- 
ness (myopia); where the cornea and lens 
' are too convex, and the sight of remote ob- 
jects is imperfect. The latter defect is far- 
sightedness (presbyopia, as being common 
in old persons); where an opposite con- 
dition of the eye obtains, and near objects 
are seen imperfectly. 
As the eye possesses a very considerable 
range of power in seeing distinctly both 
near and distant objects, it must possess 
some powers of accommodation adapting it 
to these differences of distance. Various 
opinions have been held on this subject; 
ibiit none are supported by sufficiently 
direct and convincing arguments to com- 
mand universal assent. The changes in tlie 
condition of the pupil have some effect: 
it contracts when we look at a near object, 
to exclude those rays which would be too 
divergent for the powers of the eye ; and it. 
dilates in the opposite case, to take in the 
divergent rays. Besides this, different phy- 
siologists have admitted a power of motion 
in the crystalline, by which its convexity 
may be altered ; a movement of that bodjr 
backwards and forwards, in the eye, by the 
ciliary processes, so as to place it at diffe- 
rent distances from the retina ; a compres- 
sion of the globe by the four recti muscles, 
and consequent elongation and shortening 
of the optical axis. 
That the retina, in the very axis' of the 
eye is insensible, owing to the entrance of 
an artery at that part, is shewn by experi- 
ments, in which objects vanish when their 
images are brought on that point. 
Single vision, with two eyes, probably 
arises from habit ; for children seem to see 
double ; and the same affection, aften dis- 
oiises, has been conquered by use. 
The sight vi ould lead us into many errors 
concerning the distance and figure of ob- 
jects, were it not corrected and assisted by 
the touch. The person born blind, mentioned 
by Cheselden, tlioiight when he liad gained 
his sight, that all objects touched his eyes, 
A square tower, at a great distance, appears 
'round ; and lofty trees, in a distant pei - 
spective, seem no larger than small bushes 
tliat are nearer to ns. 
Hearing. The undulations of the atmos- 
phei’e, excited by the vibrations of sonorous 
bodies, are collected in the external ear 
arjtd auditory passage, as in a hearing 
trumpet, and are conveyed to the mem- 
brana tympani, which they cause to vibrate. 
The effect is transmitted through the small 
bones to the watery fluid tliat fids the inter- 
nal ear, in which the delicate filaments of 
the auditory nerve float ; and by this nerve 
the sensation is conveyed to the brain. 
Muscles attached to the small bones of the 
tympanum haye the power of stretching or 
relaxing tlie membrane; and probably 
thereby adapt the organ to various quanti- 
ties of sound, by diminishing acute, and 
augmenting the force of grave sounds, as 
the changes in the pupil of the eye accom- 
modate that organ to a greater or less num- 
ber of rays according to the effect they pro- 
duce. 
An entire state of themenibrana tympanj 
^ not essential to hearing ; for the sense re- 
