VISCERAL ANATOMY. 163 



nally. It is continued in front as the cornea, and is pierced 

 behind for the optic and ciliary nerves and central artery. 



The Cornea is the anterior one-sixth of the sclera. It is 

 shaped like a watch-glass, convex in front, and transparent. It 

 is composed of a middle proper structure of connective tissue, an 

 anterior layer from the conjunctiva, and a posterior, elastic in 

 structure, the membrane of Descemet. 



The Choroid lies between the sclera and retina, and is a thin, 

 brownish, vascular membrane. 



Its coats from without inward (Strangeways) are 



1. Membrana jusca, of connective tissue, vessels and nerves. 



2. Vascular layer, containing many veins, the venae vorticosae. 



3. Tunica Ruyschiana, of capillary vessels from ciliary arteries. 

 (The pigment layer of the retina was formerly included as a 



layer of the choroid.) 



The Ciliary Muscle is a band of circular unstriped fibres 

 running from the junction of the cornea and sclera to the iris. 

 Its function is connected with accommodation. 



The Ciliary Processes are folds of the choroid anteriorly, 

 about sixty to eighty in number and y 1 ^ inch in length. 



The Iris is a circular curtain, a continuation of the choroid 

 anteriorly, yellowish-brown in color, with a central opening, the 

 pupil. 

 Structure. 



(i) Anterior epithelial layer, continuous with the posterior cor- 

 neal layer; (2) middle muscular, of circular and radiated untriped 

 muscular fibres, supplied by the ciliary ganglion; (3) posterior 

 epithelial or uvea, with numerous pigment cells. 



The Retina, or internal tunic, is the expansion of the optic 

 nerve ending in a number of folds anteriorly, the zone of Zinn. 

 Its layers from without inward are 



1. Pigmentary, of hexagonal, pigment cells, formerly ascribed 

 to choroid. 



2. Layer of rods and cones. 



3. External granular layer. 



4. Intermediate layer. 



5. Inner granular layer. 



6. Molecular layer. 



7. Ganglionic layer. 



8. Optic nerve fibres. 



