40 VISION 



mammals had already been demonstrated, suggesting analogous functional 

 patterns. Maren et al. confirmed the aqueous volume at 0.25 ml with a 

 turnover rate of 0.4 ml/h, about half the mammalian value. The vitreous 

 volume was estimated at 3.0 ml. Movement of Na + and Cl~ and HCOo from 

 plasma to aqueous was followed: it appeared that accumulation of Na + and 

 Cl~ was diffusion limited, while HCO3 formation and accession was different 

 (i.e., much faster). 



Maren 's data on bicarbonate accumulation in the shark and Kinsey and 

 Reddy's (1959) data on the rabbit are similar. Ion exchange between aque- 

 ous and vitreous was much greater than between plasma and aqueous, sug- 

 gesting that the vitreous is an important ionic outflow pathway in this 

 animal. Maren et al. deny the existence of a canal of Schlemm or aqueous 

 veins in Squalus, thus increasing the importance of the vitreous pathway. 

 However, studies on Squalus by Tripathi (1974) confirmed the finding by 

 Rochon-Duvigneaud (1943) of a sinus of Fontana— the analog of Schlemm's 

 canal in lower vertebrates. Tripathi also presented evidence for a conven- 

 tional outflow pathway similar to that of higher mammals. 



The shallow anterior chamber of elasmobranchs is bounded posteriorly by 

 the iris, which is bowed forward by the protruding lens. Thus, the angle 

 formed by the meeting of the cornea and iris is narrow and may extend back 

 nearly to the ora serrata. At the narrowest part of the iridocorneal angle the 

 so-called annular ligament is absent. However, ventrally the angle is wide and 

 filled with a mesh work of collagen fibrils and microfibrillar elements cor- 

 responding to the annular ligament. This meshwork supports a large 

 endothelium-lined sinus probably similar in function to Schlemm's canal in 

 mammals. Endothelial cells on the inner wall of this sinus contain giant 

 vacuoles about 10 /xm in diameter, comparable to those seen in mammals. 

 Preliminary studies suggest that this endothelium plays a significant role in 

 bulk outflow of aqueous humor. The elasmobranch condition fits nicely into 

 Tripathi's (1971) theory of vacuolar, transcellular aqueous outflow. Tripathi 

 (1974) imagines the initial state of outflow as macropinnocytosis, the 

 endothelial cells ingesting then transporting aqueous from the annular liga- 

 ment into the sinus. 



The Ocular Lens 



The vertebrate pattern of development and structure of the ocular lens is 

 remarkably consistent (Clayton 1974). Two obvious variations are found: 

 the system of cellular sutures occurring in most vertebrates is absent in 

 cyclostomes and a few reptiles and, depending on habitat and mode of 

 accommodation, the lens is either spherical and inelastic or flattened and 

 resilient. The cornea, principal refractor in the aerial eye, is optically absent 

 underwater. Thus, most aquatic vertebrates, including elasmobranchs, 

 possess a voluminous lens. 



The elasmobranch lens is slightly compressed on the optical axis and thus 

 subspherical or lenticular in shape (Figure 2). It has a relatively simple 

 sutural complex consisting of a single line running vertically in the anterior 



