THE VASCULAR TUNIC OF THE EYE. 455 



sists of fibers and lamellae lined by endothelial cells, and bounds 

 certain intercommunicating spaces lying in the ligament, known as 

 the spaces of Fontana. The latter communicate on the one side 

 with the perivascular spaces of the sinus venosus sclerae (canal of 

 Schlemm), and on the other with the anterior chamber. 



The iris must be looked upon as a continuation of the choroid, 

 and is connected at its anterior peripheral portion with the ligamen- 

 tum pectinatum. The iris possesses the following layers, beginning 

 anteriorly : (i) the anterior endothelium; (2) the ground layer, or 

 stroma of iris, together with the sphincter muscle of the pupil ; and 

 (3) the two-layered, pigmented epithelium the pars iridica retinae, 

 of which the anterior is in part replaced by a peculiar muscle tissue, 

 developed from the ectoderm and forming the dilator of the pupil. 



The anterior endothelium is a single layer of irregularly polyg- 

 onal, nonpigmented cells, and is directly continuous with the 

 endothelium of the pectinate ligament. 



The ground-layer or stroma of iris consists anteriorly of a fine 

 reticulate tissue rich in cellular elements (reticulate layer). The 

 remaining strata which form the bulk of the ground-layer consti- 

 tute its vascular layer. The vessels are here peculiar in that they 

 are covered by coarse, circular, connective -tissue fibers forming vas- 

 cular sheaths. There is also an entire absence of muscular tissue 

 in the vessel walls. The nerves, too, are enveloped by a dense con- 

 nective tissue. In all eyes (except the albinotic) pigment is found 

 in the connective tissue. 



On the posterior inner surface of the ground-layer is a band of 

 smooth muscle-fibers encircling the pupil the sphincter muscle of 

 the pupil. Posterior to this and in intimate relation with the layer 

 of pigmented epithelium covering the posterior surface of the iris is a 

 layer of spindle-shaped cells having a radial arrangement and contain- 

 ing pigment. Closer microscopic inspection reveals the fact that in all 

 probability these elements represent muscular tissue. Here, there- 

 fore, we have to deal with a dilator muscle of the pupil. There has 

 been much discussion as to the existence and structure of this muscle. 

 Recent investigations (Szili) indicate that it is developed from the outer 

 layer of the secondary optic vesicle. 



The posterior epithelium is the direct continuation of the two 

 epithelial layers of the ciliary body, and represents the anterior por- 

 tion of the secondary optic vesicle, the two layers being continuous 

 at the margin of the pupil. In the iris both layers of cells, so far as 

 they exist, are pigmented. 



The arteries of the choroid are derived from the short posterior 

 ciliary, the long ciliary, and the anterior ciliary arteries. The short 

 posterior ciliary arteries penetrate the sclera in the vicinity of the optic 

 nerve, where they anastomose with branches from the retinal vessels, 

 and spread through the choroid, where they form the choriocapillary 

 layer. The long posterior ciliary arteries (a mesial and a lateral) 

 penetrate the sclera and course forward between choroid and sclera to 

 the ciliary body, forming there the circulus arteriosus iridis major; they 



