1905.] On the Production of Intraocular Fluid. 302 



In a previous paper we have shown that the intraocular pressure varies 

 directly as the blood pressure in the vessels of the eyeball. We must 

 therefore conclude that the rate of absorption of intraocular fluid is also 

 determined by the height of the blood pressure, and since the absorption 

 must keep pace exactly with the formation of this fluid, it follows that the 

 formation of the intraocular fluid must also be determined by the height of 

 the intraocular blood pressure. So far then the conditions which we laid 

 down as necessary to be fulfilled in order to justify the filtration theory of 

 the production of intraocular fluid have been fulfilled, and we might conclude 

 with Leber that the formation of this fluid is exactly analogous to that of 

 lymph, arid is determined by the difference of pressure between the blood in 

 the vessels and the fluid outside the vessels. There are, however, certain 

 difficulties in this assumption which have so far not been considered by 

 previous workers, but which must be met satisfactorily before we can 

 come to any definite conclusion on the subject. 



It has hitherto been assumed by Leber, Niesnamoff, and others, that a fluid 

 having the composition of intraocular fluid might be formed by a process of 

 filtration through the blood vessels of the ciliary processes under any 

 difference of pressure. In this assumption they have neglected the question 

 of the different proteid content of blood plasma and intraocular fluid. It was 

 shown by one of us (E. H. S.) that, in order to separate a proteid-free 

 transudate from a fluid such as blood serum, a certain amount of work had 

 to be done, and that for this separation a minimum difference of pressure on 

 the two sides of the filtering membrane of at least 28 mm. Hg was 

 necessary. The intraocular fluid has such a small content in proteid that 

 it may be regarded as analogous in all respects to the fluid which is supposed 

 to be separated by the glomeruli of the kidney. In order therefore that any 

 fluid shall be poured out in the eyeball, a minimum difference of 30 mm. Hg 

 must be present between intraocular pressure and capillary blood pressure. 

 If this pressure difference is not present, work must be done by the cells 

 forming the filtering membrane, and the formation of intraocular fluid must 

 be regarded in the light of a secretion rather than in that of a transudation. 

 A definite decision on this point could be reached if we had any means of 

 determining the blood pressure in the capillaries of the eyeball. A method for 

 this purpose has been devised by Niesnamoff, (7) and this observer states that 

 the normal intraocular capillary pressure is about 50 mm. of mercury. His 

 arguments, however, involve several fallacies. In his experiments he con- 

 nected a cannula, attached to a reservoir of salt solution, with the eyeball 

 of a living animal. He found that the fluid neither ran in nor out at 

 25 mm. Hg, which was therefore the intraocular pressure. He then 



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