446 



Messrs. L. Hill and M. Flack. 



[May 29, 



intracapillary pressure, and that it varies pari passu with that pressure." 

 Again, on pp. 1046-7 : " The capillary pressure in most parts of" the body is 

 normally about 15 to 20 mm. Hg (Starling). No means has yet been 

 devised for measuring the intraocular capillary pressure, since the veins 

 are so small, and anastomose so quickly and profusely after leaving the eye, 

 that it is impossible to measure the venous pressure." ... "In any case," he 

 continues, " it is clear that, with an external pressure — the normal intra- 

 ocular pressure — of 20 to 30 mm. Hg, the capillary pressure must be 

 considerably above this level, and we shall probably not be far wrong in 

 considering it equal to 40 or 50 mm. Hg." 



We maintain, on the other hand, that the physical conditions which 

 pertain in the eyeball are closely similar to those which pertain in the 

 cranio-vertebral cavity, and that the results obtained and the conclusions 

 drawn by one of us* in regard to the cerebro-spinal fluid and the cerebral 

 circulation hold good for the eye. Thomson Hendersonf alone has upheld 

 this point of view, and has expressed it with great clearness. " The eyeball 

 is contained by an unyielding capsule of fixed volume, and the circulation is 

 conducted through an elastic system of vessels. The intraocular pressure is 

 the lowest circulatory pressure in the closed sphere of the eyeball." In the 

 case of the intracranial contents, the pressure of the cerebro-spinal fluid, the 

 pressure in the cerebral veins, and the pressure required to overcome that of 

 the brain against the skull wall, and flatten it when it bulges into a trephine 

 hole, are found to be one and the same. 



The following passage shows that Parsons} rejects the application of these 

 findings to the eye : " We know that the outflow of blood from the eye by 

 the veins is usually a steady stream. It therefore follows that the venous 

 pressure must also be considerably above the average in other parts of the 

 body, since, if it were below the intraocular pressure, the veins would 

 collapse, and would only be reopened when the internal pressure rose above 

 that level. It has been thought that increase of intraocular pressure would 

 tend to close the venae vorticosae, but experimental investigation does not 

 bear out this view. The very oblique course of these vessels through the 

 sclerotic is further a protective mechanism against kinking, such as might 

 be produced by the action of the ciliary muscle in dragging forward the 

 equatorial region of the choroid. As long as the intraocular tension is 

 constant, the fluid contents of the globe must be constant, any outflow 

 of fluid, either blood or lymph, being equalised by a corresponding inflow of 



* Leonard Hill, loc. cit. 



t Thomson Henderson, ' Glaucoma.' Arnold, London, 1910. 

 % Loc. cit., vol. 3, p. 1047. 



