1912.] Relation between Capillary Pressure and Secretion. 449 



of Fontana are covered only by a layer of endothelium less than l/x thick ;* by 

 this, supported as it is by an equality of fluid pressure, aqueous on one side, 

 venous blood on the other, the normal absorption of the aqueous is controlled. 

 In the bloodless eye of the dead animal the spaces are converted into filtering 

 structures by forcing fluid into the anterior chamber. Unsupported on one 

 side, the delicate membranes are ruptured and the fluid drains away. Since 

 the pores of this filter are small, serum will naturally escape more slowly than 

 salt solution. Starling and E. E. Henderson found this to be the case. No 

 further refutation is needed of their conclusion that by their second method 

 the normal rate of secretion can be measured. 



The only convincing method of studying the normal secretion of the 

 aqueous is that introduced by Ehrlich,f who traced it in rabbits by a 

 subcutaneous injection of fluorescein. He states that the dye can be observed 

 flowing into the anterior chamber from behind and round the edge of the 

 iris, producing a characteristic median vertical line which he regards as the 

 resultant of fluid eddies. In the young animal the fluorescent line may 

 appear within five to seven minutes after the subcutaneous injection has been 

 made. In the old the phenomenon may fail or appear after a period twice or 

 thrice as long. The method shows that there is a current of aqueous entering 

 the anterior chamber, therefore the aqueous must circulate through the 

 chamber, and this more actively in the young. After the cornea has been 

 punctured and the aqueous let out of the eye, the dye enters rapidly, so that 

 within five to six minutes after the injection the anterior chamber appears 

 full of fluorescent green water. While as little as one part of fluorescein in 

 2,000,000 is visible against a dark background, the concentration of the dye 

 in the blood at any one time must be very low. Ehrlich draws the conclusion 

 that the rapidity with which the dye appears in the aqueous shows that a 

 process of active secretion is taking place, which concentrates the dye. 

 Further work has shown that, after intravenous injection, the dye may appear 

 in the aqueous within a minute, the ciliary processes becoming rapidly 

 stained. After a subcutaneous injection the colour of the aqueous is most 

 intense in three to four hours, fading away in six to 24 hours. Since 

 fluorescein injected into the vitreous remains for two or three weeks there can 

 be no rapid circulation of fluid in the vitreous chamber. Subconjunctival 

 injection of suprarenal extract not only prevents the appearance of Ehrlich's 

 line, but if the anterior chamber be punctured, it refills very slowly and with 

 a fluid containing little if any excess of protein (Wesseley). After the 

 application of eserine or pilocarpine to the eye, the fluorescein appears and 

 * Thomson Henderson. 



t Ehrlich, 'Deutsch. Med. Woehen.,' 1882, Nos. 2-4. 



2 I 2 



