202 VISCERAL ANATOMY. 



Uvea, or Tapetum, a layer of purple pigment-cells on the posterior surface 

 of the iris, continuous with the pigment layer of the ciliary processes. 



Sphincter Pupillce, a layer of circular muscular fibres around the pupillary 

 margin, supplied by the 3d nerve. 



Dilator Pupilltz, radiating muscular fibres from the pupillary margin toward 

 Sie ciliary border, supplied by fibres of the sympathetic, from the ciliary 

 ganglion. [See p. 137.] 



Membrana Puptilaris, a vascular membrane which covers the pupil in 

 the foetus, disappearing about the eighth month, occasionally permanent. 

 It is nourished by the Hyaloid Artery, through the Canal of Stilling (de- 

 scribed under the VITREOUS HUMOR). 



Arteries, are derived from the long and anterior Ciliary, forming the Cir~ 

 culus Major Iridis at the ciliary border, and the Circulus Arteriosus 

 Iridis Minor near the pupillary margin. 



Veins, empty into those of the ciliary processes and the anterior ciliary veins 



Sinus Circularis Iridis, Canal of Schlemm, or Canal of Fontana, is 

 minute canal at the internal corneo-sclerotic junction, extending arounc 

 the circumference of the attached border of the iris. It is lined by endo 

 thelium and encloses a venous plexus, receiving veins from the sclerotic 

 and the ciliary plexus. It communicates with the chamber of the aqueous 

 humor and with the anterior ciliary veins. 



Nerves of the Iris, are derived from the 3d, 5th and the sympathetic, 

 through the long and short ciliary nerves. The 3d supplies the circular 

 fibres, the sympathetic the radiating ones, the 5th being nerves of common 

 sensation. 



THE RETINA. 



What is the Retina ? It is the innermost or nervous tunic of the eye, 

 a delicate, grayish, transparent membrane, about ^ of an inch thick at the 

 fundus, 2^ inch at the anterior margin. It is formed by a membranous ex- 

 pansion of the optic nerve elements, and extends from the termination of that 

 nerve nearly as far forwards as the ciliary processes, terminating in a jagged 

 margin, the Ora Serrata, though its fibrous stroma is continued as the Pars 

 Ciliaris Retina over the ciliary body. The Retina presents for examination 

 the following points, viz., the 



Macula Lutea,or Yellow Spot, situated on the retina, exactly in the visual 

 axis ; is an elevated oval spot where vision U very acute, the retina being 

 very thin and full of nerve elements closely packed together at the ex- 

 pense of its connective tissue. No rods, no nerve-fibre layer here, but 

 :ne cones and ganglion-cells are very numerous. 



Fovea *Centralis, a depression at the centre of the macula lutea, T | T to -fa 

 inch in diameter, in which the sense of vision is most acute. 



