VISUAL SYSTEM: STATE OF THE ART 39 



hyposmolar to the plasma, and obtained experimental evidence that environ- 

 mental water does not pass through the cornea into the eye. This agrees with 

 the results of Edelhauser (1968) discussed previously. Finally, they confirmed 

 the presence of high levels of carbonic anhydrase in the ciliary body. 



Maren (1962a, 19626, 1973) and Maren et al. (1975) analyzed the aqueous 

 humor of Squalus. Differences in the properties of aqueous humor between 

 Squalus and Mustelus appeared to be significant (Table 1). As in mammals, 

 Maren found the aqueous of Squalus to be iso-osmotic to plasma. In nearly 

 all electrolytes the distribution ratios of Squalus exceeded those of Mustelus. 

 Differences in rate of aqueous production and carbonic anhydrase activity 

 were also reported. These differences have surprised several authors (Maren 

 1973, Jampol and Forrest 1972, and Zadunaisky 1973) because of the sup- 

 posed taxonomic closeness of these two species. In fact, these elasmobranchs 

 are phylogenetically dissimilar, being on ends of evolutionary lines that sepa- 

 rated as early as the Jurassic (Schaeffer 1967). Thus they are distinguished 

 at the ordinal level in a way similar to the primate and rabbit— species that 

 differ considerably in their aqueous mechanisms (Maren 1973). 



Aqueous Humor, Formation— Jampol and Forest (1972) investigated 

 the mechanism and site of aqueous formation in Squalus. In man, aqueous is 

 almost certainly secreted from the nonpigmented cuboidal epithelium of the 

 ciliary body (Davson 1969). The ciliary body of elasmobranchs is quite 

 prominent— especially compared with that of teleosts— and similar in many 

 respects to its mammalian counterpart, including the presence of nonpig- 

 mented cuboidal epithelium. Maren (19626) had already suggested an active 

 transport mechanism based upon the secretory appearance of cellular ele- 

 ments in the ciliary body, the presence of carbonic anhydrase, and the effect 

 of carbonic anhydrase inhibitors on production and composition of aqueous. 

 Doolittle and Stone (1960) also speculated on active processes, as described 

 previously. Jampol and Forrest analyzed ciliary tissue for the enzyme aden- 

 osine triphosphatase, which in the presence of Mg + , Na + , and K + catalyzes 

 the reaction 



ATP *? ADP + Pi. 



This reaction provides high-energy bonds to run "sodium pumps" in a wide 

 variety of tissues. The occurrence of Na-K-ATPase in the ciliary body was 

 demonstrated by Jampol and Forrest and is further indirect evidence of active 

 secretion of aqueous in the shark. The authors compared Na-K-ATPase activ- 

 ity in various ciliary structures of Squalus, finding the highest activity 

 (greater than 5X compared with the iris) in the ciliary body proper. Thus, it 

 seems likely that the formation of aqueous takes place in the ciliary body 

 and utilizes active transport. 



Aqueous Humor, Dynamics— Chemistry and dynamics of aqueous in 

 Squalus were further studied by Maren et al. (1975) to determine whether a 

 constant phylogenetic pattern underlies the physiology of aqueous formation 

 in vertebrates. Anatomic and enzymatic similarities between sharks and 



