566 HIGHER FISHES 



distortion which may be best described as a flattening of the anterior 

 dorsal region (Fig. 102b, p. 255). 



The sclera is thickened not only fundally, to receive the optic pedicel, 

 but also at the muscle-insertions and in a zone surrounding the cornea. 

 It is usually thinnest at the equator. The sclera consists largely of a cup 

 of hyaline cartilage, which is often calcified. The cornea is thick peripher- 

 ally (and often opaque there, particularly dorsally and ventrally), thin- 

 ner centrally, and is strongly arched in contrast to the flat comeae of 

 other kinds of fishes. The cornea is claimed to have all of the layers 

 characteristic of the human, and even has a thick Bowman's membrane ; 

 but while a very thin, hard-looking cuticular membrane similar to a 

 Descemet's membrane is present on the inner surface, the writer can 

 make out no mesothelium whatever lying upon it iSqualus acanthias) . 

 The substantia propria is very neatly laminated, the fibers of each layer 

 becoming progressively thinner toward the center of the cornea. Much 

 of the thinning of the corneal center is accomplished by a dropping-out 

 of layers, however. The epithelium may possess several times as many 

 layers of cells as the human; but it is not cornified. 



The chorioid is heavily pigmented, and typical in structure except for 

 the inner one-fifth or so of its thickness, which in nearly all species is 

 modified to form the remarkable tapetum lucidum characteristic of the 

 group. Over a restricted area in the fundus, the chorioid is often mark- 

 edly thickened by the presence, on its scleral side, of a so-called 'supra- 

 chorioidea'. This may consist of connective tissue with some blood-supply 

 from large veins embedded in it, or it may consist largely of a tangle of 

 such veins (possibly, then, a modification of the cyclostome subscleral 

 sinus) . The suprachorioidea is lacking in those species in which the optic 

 pedicel is absent or is incomplete in extent, and the same reason seems 

 to cover both lacks : less room than usual in the orbit, owing to a par- 

 ticularly large eyeball. To accommodate a suprachorioidea, the sclera is 

 bowed outward, and the curvature of the retina is thus not disturbed. 

 Between suprachorioidea and sclera there are believed to be lymph 

 spaces, so that the chorioid and sclera are not conjoined firmly except 

 near the limbus. Two arteries enter the eyeball, one temporally (which 

 supplies the chorioid), the other ventrally (which runs forward through 

 the chorioid to supply the iris) ; and two main veins, one dorsal, one 

 ventral, leave it. Only the uvea is vascularized in the adult, though in 

 embryos a vessel has been found to enter the embryonic fissure about 

 midway of its length, thence sending branches forward and backward 



