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 summa