Glaucoma | Page 6

the discussion of
those practical aspects of glaucoma which are to claim your attention
through the papers and remarks of subsequent speakers.

Dr. Edward Jackson's Paper on Etiology and Classification of
Glaucoma
Discussion,
FRANCIS LANE, M.D.
Chicago.
Not one of the theories thus far propounded to explain the essential
cause of increased intra-ocular tension is satisfactory. Our present day
knowledge apparently ceases with a more or less incomplete
understanding of the mere circumstance under which increase of
tension in general depends.
The question of the source of the normal intra-ocular pressure must first
be solved before any discussion of a pathological increase can be
engaged in. This question primarily hinges on whether the
corneo-sclera is to be regarded as an unelastic capsule with a fixed
volume, or as a yielding envelope with an ever changing capacity.
This brings us at once to the consideration of that theory which
probably has held our attention for the longest period of time, i. e., the
volumetric theory. According to it, the normal intra-ocular tension

depends on the volume of fluids within the eyeball. Any variation in
the quantity of the contents gives rise to a change in the pressure,
therefore, the globe has been regarded as "an elastic capsule, whose
capacity, form, and internal pressure depend on the balance struck
between a constant inflow, or formation of aqueous, and a
proportionate outflow or resorption." (Henderson.)
Hill has satisfactorily demonstrated that, under physiological conditions,
the hydrostatic pressure within the eye and the skull is identical; it rises
and falls simultaneously; it is the same as the cerebral venous pressure;
it is constantly varying, depending directly on the general circulation.
Upon these findings Henderson based his opinion that the physiological
properties of the tunica fibrosa and the skull are identical, realizing at
the same time, that the rigidity of the corneo-sclera, because of its
fibrous nature, is not as firm as the cranium. In accepting this belief the
inference was that the cubic capacity of both coverings is fixed.
Applying these conclusions to the eye, it can be said that the pressure
of the fixed intra-ocular volume varies with the venous tension within
the bulb, which in turn is influenced by the general circulation. Such a
conception, while not strictly in accord with recognized physiological
teachings, proves that the normal intra-ocular pressure is not a question
of volume content, but that it is purely a question of pressure of a fixed
volume within an unyielding capsule. Dr. Jackson virtually puts aside
the volumetric theory with his statement, that "the balance of
intra-ocular pressure is not maintained by the slight distensibility of the
sclero-corneal coat." Further discussion on the inadequacy of the
volumetric theory need not detain us.
It is well to recall a few anatomical features because of their bearing on
the theories herein considered.
1. The angle of the anterior chamber is a true angle and not an annular
sinus.
2. The meshwork of the iris angle (ligamentum pectinatum), a cellular
structure at birth, undergoes a progressive and physiological fibrosis
with early subsequent sclerosis, until finally it becomes a fibrous
structure. The individual strands of this meshwork are more than two

times as large at advanced age as at birth, consequently the alveoli of
the meshwork becomes markedly reduced in size.
3. The spongy nature of this meshwork affords free access of aqueous
to the venous sinus of Schlemm, thence by tributaries into the
supra-choroidal space and anterior uveal venous system.
4. Fuchs's iris cripts afford direct access of aqueous to the veins of the
iris.
Furthermore, two simple principles are taught by physics: Fluids are
incompressible and they seek the lowest hydrostatic level. The
application of these perfectly obvious principles to the eyeball makes
the intra-ocular pressure the same as that within the elastic venous
walls, which is the lowest circulating pressure within the bulb.
To summarize: The aqueous has direct access to the anterior uveal
venous system; the physiological thickening of the strands of the
meshwork of the iris angle supplies a mechanical obstruction between
the anterior chamber and the venous sinus of Schlemm; intra-ocular
pressure stands at the same level as the intra-venous, consequently, the
hydrostatic pressure is the same on both sides of the iris angle
meshwork, because the canal of Schlemm is a secondary venous system;
lastly, the outflow of aqueous into the venous sinus is by diffusion, not
by filtration, because the pressure is the same on both sides of the
meshwork.
These facts and deductions have given rise to the present day
circulatory theory of intra-ocular pressure, so we now can approach the
predisposing and exciting factors which determine glaucoma.
The central fact to be borne in mind is, if the physiological pressure is
vascular in origin and nature, the pathological pressure must likewise
be derived from the