ORGANS OF SPECIAL SENSE. 345 



Meridians are the lines upon the surface where these merid- 

 ional planes cut it. 



THE CAPSULE OF TENON, tunica vaginalis oculi, is a fascia 

 between the eyeball and the walls of the orbit, isolating the eye- 

 ball and allowing free movement. It is continuous in front with 

 the subconjunctival connective tissue, and behind with the dura 

 mater, through the sphenoidal fissure and optic foramen, and 

 consists of two layers a visceral layer investing the posterior 

 portion of the eyeball, and a parietal layer lining the cushion of 

 fat on which the eye rests. 



The inner aspect is lined with flattened endothelial cells, and 

 incloses a lymph space communicating with subdural and sub- 

 arachnoidean lymph spaces of the optic nerve-sheath. It sup- 

 ports the lachrymal gland, is strengthened by numerous fibrous 

 bands, and is pierced by the ocular muscles, inclosing them in 

 imperfect sheaths. 



The capsule of Bonnet is the name given to the portion 

 posterior to the passage of the tendons, and 



The capsule of Tenon is then applied to the anterior socket- 

 like half. 



THE EYEBALL is composed of a large, opaque segment of a 

 sphere, forming about five-sixths of the globe, for the protection 

 of its contents, and a smaller transparent segment of a sphere, 

 implanted upon and continuous with it in front. The optic 

 nerves enter the eyeballs to their nasal side, in the direction of 

 the axes of the orbit. 



It is composed of three tunics or coats: 



1. Sclerotic and cornea, 



2. Choroid, iris, and ciliary processes, 



3. Retina; 



And three humors, or refracting media : 



1. Aqueous humor, 



2. Crystalline lens (and capsule), 



3. Vitreous humor, or body. 



THE SCLEROTIC COAT, so called from its extreme hardness, 

 is thicker behind (one twenty-fifth of an inch) than in front 

 (one-sixtieth of an inch), and presents two surfaces for study 

 the external and internal. 



The external surface is smooth and white, and has attached 

 to it the various muscles of the eye. 



The internal surface is grooved for the passage of the ciliary 

 nerves, and connected to the choroid by a fine areolar layer the 

 lamina fusca. 



