1912.] Relation between Capillary Pressure and Secretion. 445 



iris, and that the volume of blood so expelled must be made good either by 

 a secretion "* of aqueous or by expansion of other blood-vessels, e.g. in the 

 ciliary region. The latter probably takes place, and as the dilating iris 

 shrinks the veins which drain it dilate in the ciliary region. We have no 

 evidence to offer as to how the small changes of intraocular pressure occur 

 during the stimulation of the cervical sympathetic. During asphyxia we 

 obtained a rise of 12 mm. Hg in the eye and 20 mm. Hg in the brain. 

 The intracranial pressure is much lower than the intraocular and is 

 affected by any rise of the general venous pressure, while the intraocular is 

 not. Therefore, the intracranial pressure is raised during asphyxia both by 

 the rise in arterial and in the general venous pressure. On squeezing the heart 

 the intraocular pressure falls 15-20 mm. Hg, while the intracranial rises 

 5-7 mm. Hg. Here again the difference is due to the greater effect 

 of the venous rise on the brain. Compressing the abdomen causes a rise 

 of intraocular pressure (3-10 mm. Hg.) and of intracranial pressure 

 (5-15 mm. Hg). Here also the rise in venous pressure adds itself to the 

 arterial in the case of the brain and not in the case of the eye. 



If the heart be divided, there occurs a rapid initial fall of pressure, 

 followed by a slow fall. A well-recognised sign of death is the flaccid 

 cornea. The initial fall is due to the fall of arterial pressure ; the residual 

 pressure demonstrates the existence of an intraocular pressure which is not 

 circulatory but secretory in origin, a point of importance. The tense form 

 of the eyeball, and the perfection of the eye as an optical instrument, depend 

 primarily on the secretory activity of the ciliary processes. It would be 

 much to the disadvantage of an animal if the eye became flaccid at once on 

 stopping the arterial blood supply to the eye. The intraocular pressure is of 

 circulatory origin, and varies with the arterial pressure, but it is the secretory 

 power of the cells of the ciliary processes which determines the circulatory 

 pressure in the eyeball. 



Turning to the excellent historical survey given by J. Herbert Parsons* 

 of past work on this subject, we cannot find therein any clear conception of 

 the physical conditions which underlie the intraocular pressure and the 

 circulation in the eyeball. Leber, Parsons, Henderson, and Starling conceive 

 that the capillary pressure is much higher than the intraocular, e.g. in the 

 ciliary processes, and that the aqueous is a filtration product depending on 

 the difference of pressure on either side of the membrane. " What is of 

 prime importance," writes Parsons, f " is the fact that everything tends 

 to show that the secretion of aqueous is directly dependent upon the 



* Loc. cit, vol. 3, pp. 1040—1070. 

 + Loc. cit., vol. 3, p. 1048. 



