468 THE EYE IN EVOLUTION 



The Iris. The deeper layers of the iris conform to the general 

 vertebrate type. Both layers of the retinal epithelium are heavily 

 pigmented except when a dilatator papillae muscle is present in which 

 case the anterior layer lacks its pigment except near the pupillary 

 border. The pupillary muscles are non-striated ; a spliincter is always 

 present, massed particularly near the pupillary margin, but sometimes 

 (in aquatic Placentals such as the otter, the Pinnipedes and the 

 Cetaceans, and in the pig) extending peripherally tlu-oughout the entire 

 width of the iris ; the dilatator is absent in the nocturnal representa- 

 tives of the lower species. 



The main (deeper) mesodermal layer of the iris corresponds with 

 that of other Vertebrates, being supplied by a circular artery (the 

 circulus arteriosus iridis major) derived from 

 the anastomosis of each of the two long 

 -P.R posterior ciliary arteries. This arterial circle is 



usually hidden behind the limbus but can some- 

 times be seen on the anterior surface of the iris, 

 as in the guinea-pig (Fig. 600) ; from it radial 



Fig. GUI.— Structure of vessels are given off to supply the sphincteric 

 THE Iris of Placen- ® . x i ^ x 



TALs. and subsphincteric plexuses, the blood being 



SM, DM, superficial drained away by a radial system of veins. 

 i:;is.'''^P/^?":hfTo Superficially to this, however, lies a layer 

 posterior retinal layers unique to Mammals — the anterior ynesodermal 



thetoiefHSTrritrof '«?'"■■ I" embryonic life this layer grows in 

 the optic vesicle. from the periphery in advance of the deeper 



layer of mesoderm and the retinal epithelium, 

 carrying with it a rich vascular supply to constitute the anterior portion 

 of the tunica vasculosa lentis. The central (pupillary) portion of this 

 layer is diaphanous and almost acellular and as development proceeds 

 it gradually atrophies, receding to a sinuous scalloped line peripheral 

 to the pupillary margin where tlie superficial radial vessels anasto- 

 mose to form a very imperfect circular arcade, the circulus arteriosus 

 iridis minor. The site of the lesser circle which marks the limits of the 

 superficial mesodermal layer is fortuitous, sometimes being close to the 

 pupil, sometimes far away ; it varies in different species, between 

 individuals of the same species, and in different parts of the iris in the 

 same individual, but the general plan of vascularization remains the 

 same (Mann, 1931) (Fig. 601). In most Placentals this layer is com- 

 pact and covered by a continuous layer of endothelium ; in some 

 Rodents (rabbit) and the higher Primates {Macacus, the gorilla and 

 man) it tends to atroj^hy so that an incomplete layer is formed with 

 the development of open crypts (Wolfrum, 1926 ; Vrabec, 1952). As 

 we baA e seen, from its periphery are given off strands of endothelial- 

 lined nnective tissue which traverse the angle of the anterior chamber 



