Principle of ophthalmoscope: Angle of incidence and angle of
reflection in same line, light close to one side of the eye, reflected
into it by a mirror, having a hole in the centre for eye of observer.
Opacities show a dense white in transparent media: if in front of lens
move with rolling of eye: if behind in opposite direction. To see
fundus must use biconvex lens. Emmetropic eye: myopic: hypermetropic.
Static refraction. Mydriatics: Atropine, homatropine, daturine,
duboisine, hyoscyamine.
In the healthy eye, the pupil and iris, and in cataract, even the opaque
anterior capsule of the lens, can be clearly seen. The reflection of the
pupil, however, is dark and no object back of the iris can be observed.
The reason of the difference is that the rays of light, entering through
the whole cornea, are reflected at the same angle at which they strike
the surface of the iris. The angle of incidence is the same as the angle
of reflection. In the hollow fundus of the eye, however, the light
entering through the narrow pupil, strikes the fundus at a point which
is hidden from the observer, behind the iris, and being reflected by the
concave fundus, in exactly the same line along which it entered, it
remains invisible. To illuminate the fundus of the eye, for the
observer, his line of vision must be made exactly the same as that in
which the pencil of light enters the fundus. This is best effected by
reflecting the light into the eye by the aid of a small plane or concave
mirror having a hole in the center through which the observer looks into
the pupil. The concave mirror gives the stronger illumination, but the
plane article is more easily manipulated and tends to cause less active
contractions of the pupil. This is the simplest form of ophthalmoscope.
For careful examination of the fundus of the eye, it is best to have the
subject in a dark chamber, with a single large flame of an oil lamp or
gas (electric light with an obscure globe may answer). The light is held
behind and on the same side as the eye to be examined, at the level of
the eye and the perforated mirror and the eye of the observer are kept
from 10 to 20 inches in front of the eye and also at the same level. For
the horse or ox under favorable conditions in a stall, the light of day
coming from a fansash over the door may serve the purpose. Nicholas
assures us that it may be accomplished even under the shadow of a shed
or a tree. In such a case it is better not to have too much glare of
light as the reflection from cornea and lens may prevent accurate
observation. A somewhat cloudy day may therefore prove advantageous.
In focusing the reflected light on the cornea, and then on the pupil and
lens, any opacities in these will be shown as a grayish nebular
reflection or a denser white according to their degree of opacity. The
opacities in the cornea or aqueous, in front of the axis of vision in
the lens move in the same direction and to the same degree as the eye
rolls, while opacities on the posterior capsule or in the vitreous, move
in a direction opposite to the motions of the eye, and to a degree
corresponding to their distance back of the lens. Thus if the eye looks
downward such opacities move upward; if it looks upward they move
downward; if it looks inward they move outward; and if it looks outward
they move inward.
To secure an image of the fundus of the eye, including the entrance of
the optic nerve (optic papilla), the tapetum, the pigmentary surface and
retina and vessels, accommodation must be made for the normal refraction
of the eye of the patient, and even for that of the observer.
In the =emmetropic= (normal) eye, the rays leave the surface of the
cornea parallel to each other and it may be possible for the observer to
secure a good image on his retina, without the aid of lenses. In the
=myopic= (short sighted) eye they assume a convergent course on leaving
the cornea, and to secure a satisfactory image a biconcave or
plano-concave lens must be interposed between the cornea of the patient
and the eye of the observer.
In the =hypermetropic= (long sighted) eye, the rays diverge in leaving
the cornea of the patient, and a convex lens must be interposed between
this and the eye of the observer, in order that the rays may be focused
on the eye of the observer.
To adapt the vision to the different eyes the modern ophthalmoscope is
furnished with a series of lenses concave and convex, any one of which
can be moved behind the hole in the mirror to suit the demands of the
particular case.
To make a satisfactory examination the pupil should be dilated as for
oblique focal illumination. A 1:200 solution of apomorphia may be
instilled into the eye (a drop or two) and in 20 to 25 minutes a
satisfactory dilatation will have been secured. The effect of the
homatropin will usually have disappeared in twenty-four hours.
Determination of Static Refraction.
This can be best done in the lower animals by determining the strength
of the lens required to render clear the image of its fundus. By knowing
the refracting power of the lens, we may ascertain what deviation from
the normal refraction there is in the eye under observation.
In making this test the mirror of the ophthalmoscope must be brought
closely to the eye of the patient—1 to 2 inches.
If in such a case and without the use of any lens a distinct image of
the fundus is obtained, and if this is rendered less distinct by
interposing the lowest convex lens in front of the eye of the observer,
the eye is =emmetropic=.
If the ophthalmoscopic mirror without a lens gives an indistinct vision
of the fundus, and if the image is rendered clear by interposing one of
the convex lenses, the eye is =hypermetropic=. The strength of the
convex lens, +1, +2 or +3, dioptrics will give the measure of the
hypermetropia.
If, on the contrary, the ophthalmoscopic mirror gives an indistinct
image of the fundus, which is rendered even more indistinct by the
interposition of a convex lens, but is cleared up and rendered definite
by a concave lens, the patient is myopic. The strength of the concave
lens used will give the degree of _myopia_, –1 dioptric, –2 dioptrics,
etc.
The tendency in the horse is constantly to slight long-sightedness, but
the deviation is rarely found to be serious either in this direction or
in that of astigmatism.
Mydriatics.
Dilation of the pupil by mydriatics (mydriasis dilation of the pupil) is
a most important means of diagnosis, and therefore a knowledge of the
action of the different mydriatics is essential. The mydriatics in
common use not only dilate the pupil, but also paralyze the ciliary body
and the power of accommodation in ratio with the strength of the
solution employed. This determines an adaptation of the eye to the
farthest point of vision and holds it there until the action of the
mydriatic passes off and normal power of accommodation is restored. In
short it renders the subject long sighted, during its action.
=Atropine= the alkaloid of atropa belladonna is the most generally
available and persistent of the mydriatics, and is in most common use.
It is usually employed as sulphate of atropine, though some prefer the
nitrate, the salicylate or the borate to obviate the danger of
atropinism. This form of poisoning may show in the occurrence of
conjunctivitis and in case of one attack the susceptibility to atropine
is greatly to be dreaded, so that it should never again be used on the
same subject. The real cause of atropinism is uncertain, it has been
variously ascribed to too great acidity or alkalinity, or to
micro-organisms growing in the solution. Hence the importance of using
the antiseptic salts of atropine, and of testing the solution to see
that it is exactly neutral before it is applied.
The strength of the solution of atropine is an important consideration.
Donders found that 1:120 of water produced a full effect, while Jaarsma
obtained the full effect in one hour from a drop of a solution of one to
twelve hundred of water. The action on carnivora (dogs and cats) is
equivalent to that on man, while on the herbivora (rabbit, horse, ox,
sheep) it is somewhat less, and on birds very slight indeed. On diseased
eyes a large amount may be required, and with synechia (adhesion of the
iris to the capsule of the lens) dilatation may be impossible. The full
effect may last 24 hours, and accommodation may remain very imperfect
for 11 days.
The direct action of atropine on the eye is shown in dilatation of the
pupil of the frog after the eye has been detached from all connection
with heart or brain, by excision. It acts also in the normal system
through reflex nervous action, since, after division of the sympathetic
trunk going to the eye, that eye does not dilate so much under atropia
as the opposite eye.
Atropine is usually employed by lodging a drop in the pouch of the
conjunctiva (inside the lower lid), and from this it makes its way into
the aqueous humor, for if that liquid is transferred to the conjunctiva
of another animal it causes dilatation. Puncture of the cornea with
evacuation of the aqueous humor lessens the action of the atropine.
Atropine dilatation is increased by following it with cocaine which
causes contraction of the iridian vessels, the antithesis of the
dilatation of the vessels which occurs when the cornea is perforated and
the pressure of the aqueous humor is removed.
Atropine is one of the most potent poisons and must be used with caution
especially in the carnivora and omnivora. The danger lies not alone in
the absorption from the conjunctiva, but also from the escape of the
liquid through the lachrymo-nasal duct, to the nose and later to the
actively absorbing mucosæ of the lungs and stomach.
The symptoms of general poisoning are: rapid pulse, vertigo, weakness of
posterior limbs, general prostration and thirst or dryness of the
throat.
=Homatropine= is an oily liquid produced by the action of muriatic acid
on the cyanate of atropine. With hydrobromic acid it forms a readily
crystallizable salt, the solution of which acts on the eye like
atropine, but more promptly and transiently. One drop of a solution of
one to one hundred and twenty, usually gives in twenty minutes, full
pupillary dilation and complete paralysis of accommodation which lasts
only for twenty-four hours. Add to this that there is little danger of
constitutional disturbance and poisoning, and homatropine must be
accepted as a more desirable agent than atropine. It is especially to be
preferred in cases of senility with shallow anterior chambers, and in
glaucoma, in which atropine tends to aggravate the lesion.
=Daturine=, the alkaloid of datura stramonium is a potent mydriatic,
causing pupillary dilatation in a solution of one to one hundred and
sixty thousand of water. It appears to be identical with atropine.
=Duboisine= the alkaloid of duboisia myoporoides is also a potent
mydriatic. Jaarsma found that a solution of the sulphate, of one to
three thousand, paralyzed accommodation for twenty-four hours. It acts
more promptly than atropine but is more poisonous.
=Hyoscyamin=, the alkaloid of hyoscyamus niger, is also strongly
mydriatic. One drop of an one to three hundred solution of the sulphate
paralyzed accommodation for from seventy-five to one hundred hours.
Risley found it to act more promptly than atropine, and to be less
dangerous than duboisine.