38 



VISION 



comparison of ascorbate concentration in human plasma and aqueous 

 showed that ultrafiltration alone could not account for the composition 

 found. 



Aqueous Humor, Composition— In a noteworthy study, Doolittle and 

 Stone (1960) determined the composition and measured osmotic pressure of 

 the aqueous humor in Mustelus. Table 1 shows their major findings. Of 14 

 components common to plasma and aqueous, an excess in the aqueous of 

 ascorbic acid (3X), sulfate (1.4X), and bicarbonate (2.5X) was measured. 

 However, the total osmotic pressure of aqueous humor was about 25 

 milliosmoles lower than that of the plasma, due primarily to urea and tri- 

 methylamine oxide. This suggests that water passes into the eye against the 

 gradient.* Doolittle et al. thought that the pattern of formation of aqueous 

 humor in the shark was similar to that in mammals, citing asevidence ciliary 

 body anatomy, plasma : aqueous distribution ratios, occurrence of carbonic 

 anhydrase, and aqueous turnover rate (1% per min). 



Davson and Grant (1960) confirmed that the aqueous of Mustelus is 

 hyposmolar to the plasma, but the difference was somewhat lower than the 

 figure given by Doolittle and Stone. They also measured an intraocular pres- 

 sure of 7.8 mm Hg in Mustelus; the IOP of man is 15.5 mm Hg (Davson 

 1969). Stone et al. (1960) agreed that the aqueous of Mustelus was 



Table 1. Normal electrolyte composition of elasmobranch aqueous humor. 



*It should be noted that the finding of aqueous humor with an osmolarity lower than 

 that of plasma is difficult to understand, since a mechanism for passage of water into 

 the eye against the gradient has not been shown. 



