452 



Messrs. L. Hill and M. Flack. 



[May 29, 



pressure than the arterial the capillaries are rendered bloodless. When the 

 aqueous is allowed to escape, blood is forced in in increased volume to fill up 

 the vacant space. The pressure within and without is no longer balanced, 

 the dilated capillaries leak, and plasma passes out. Just as in the salivary- 

 gland the secretory pressure rules the circulatory pressure and tenseness of 

 the gland, so the pressure of the ciliary processes rules the internal pressure 

 of the eyeball. When the cells swell by imbibition of fluid and secrete the 

 aqueous, droplet by droplet, there is less room for blood; hence the veins 

 are narrowed and the pressure rises. It is clear that the secretory pressure 

 produced by any cell in a vesicle of secretion may at any moment be greater 

 than the capillary-venous pressure, but little by little, as the secretion is poured 

 out, the two pressures, that of the aqueous and that of the blood, rise together. 

 An immeasurably small difference of secretory pressure suffices to give the 

 circulation of the aqueous. Absorption goes hand in hand with secretion, 

 keeping the intraocular secretory pressure uniform. When the arterial 

 pressure rises the intraocular pressure is raised, for the increased capillary- 

 venous pressure is immediately transmitted to the aqueous. It goes without 

 saying that a rise in the general arterial pressure would not necessarily 

 produce an alteration of intraocular pressure, if the intraocular arteries were 

 constricted at the same time. 



At any moment there can be within the eye less blood and more tissue 

 fluid, or more blood and less tissue fluid. Thus, within such encapsulated 

 organs as the marrow of bone, the brain, and the eye, the volume of blood 

 is not fixed. It can slowly change, increased by the transference of the 

 tissue fluid or substance into the blood, diminished by transference of the 

 plasma, fat, etc., to and turgescence of the tissues. The volume of blood 

 within such organs is not affected by sudden variations of arterial pressure, 

 for the expansion of the arteries is balanced by a corresponding shrinkage 

 in the volume of the veins. The Monroe-Kellie doctrine is true only so far 

 as regards these quick alterations. 



Ophthalmic surgeons inform us that when the aqueous has leaked away the 

 tension of the eyeball is not rapidly restored even though the wound is closed. 

 Parsons* says " Prolonged pressure (applied) externally gradually squeezes 

 out fluid from the eye, so that the pressure again becomes normal, but on 

 removing the external pressure the tension remains subnormal until increased 

 secretion produces compensation — a comparatively slow process. Hence after 

 prolonged use of a pressure bandage the intraocular pressure may remain 

 subnormal for some time." The secretion and vaso-dilatation are adjusted to 

 suit the conditions which pertain. In the one case it is required that the 

 * Loc. cit., vol. 3, p. 1056. 



