THE CROCODILIAN EYE 613 



(B) Crocodilians 



The Eye as a Whole — In this small group of large reptiles the eyeball 

 bears the stigmata of a long-continued noctumality, which has affected 

 every part of the organ. The specializations of the adnexa are directed 

 toward the largely aquatic activities of the group (see pp. 421-2). The 

 globe is of 'nocturnal' size, its diameter reaching 20nim. in the alligator 

 and exceeding this value in larger types. The eye of the American alli- 

 gator (Alligator mississippiensis) is better known than that of any other 

 form; but nearly all the studies of it have been made by European 

 investigators. 



The sclera has retained the ancestral cartilaginous cup, but has lost 

 the annulus of ossicles. Their disappearance has permitted the circum- 

 corneal zone of the sclera to become convexly curved like the rest of the 

 fibrous tunic. The eyeball is consequently practically a sphere, though 

 a bit shortened axially. The cartilage reaches nearly to the ora terminalis, 

 which lies a little in front of the equator. The purely fibrous tissue 

 anterior to the cartilage is greatly thickened, but thins again before it 

 coalesces with the substantia propria of the thin cornea. 



The chorioid is thick and richly vascular behind the tapetum iv.i.), 

 thin and poor in vessels elsewhere. The broad ciliary body shows — even 

 more markedly — the same divergence of the base-plate (bearing the 

 ciliary processes) from the muscular lamina (clinging to the sclera) 

 which we noted in the turtles. The cleft thus formed at the periphery of 

 the anterior chamber is filled by a wedge-shaped (in section) mass of 

 loose connective tissue, the anteriormost strands of which run directly 

 from the cornea to the root of the iris to form a pectinate ligament.* 

 The much-branched canal of Schlemm does not, as in turtles, lie in this 

 meshwork tissue, but is completely embedded in the thick contiguous 

 sclera. 



''For an analogous situation, see Figure 191, p. 645, 



