334 THE EYE. 



the optic nerve, which, is derived from the dura mater. In 

 the normal state it possesses little sensibility, though when 

 attacked with inflammation, it gives the most intense pain. 

 Its nerves and vessels come from the ciliary branches. 



Choroid coat, (Fig. 98, B.) This membrane forms the, sec- 

 ond tunic of the eye. It is situated beneath the sclerotic, 

 and connected to it by cellular tissue, vessels and nerves. 

 It rests upon, but does not adhere to the retina. It extends 

 from the optic nerve behind, to the ciliary ligament in front, 

 which latter corresponds to the place of junction between 

 the sclerotic and cornea. In a word, it lines and is coex- 

 tensive with the inner surface of the sclerotic. 



Its posterior surface is pierced by the optic nerve. Its 

 anterior portion presents a large opening for the reception 

 of the iris. Its color is of a dark brown on the external 

 surface, while its inner surface is of a deep-black. Its struc- 

 ture is soft and extremely vascular. It has been divided 

 into three layers, an external or venous, a middle or arte- 

 rial, and an internal or pigmentary. Such a division is 

 very justly regarded as of little utility, since it is purely 

 artificial. This coat is, in fact, but one membrane highly 

 organized and vascular. It is believed to equal in vascu- 

 larity any of the mucous membranes, and has upon its 

 outer surface a number of large veins, which from their 

 peculiar arrangement are called vasa vorticosa, and con- 

 stitute what is called the first layer ; beneath this is seen a 

 beautiful net-work of arterial capillaries, called after Kuysch, 

 the tunica Euyschiana. It is composed of the ciliary arte- 

 ries and nerves, which penetrate the choroid from behind 

 in great numbers, round the optic nerve and supply this 

 tissue. The longer ciliary arteries, with the nerves, pass 

 on to the ciliary ligament, and through it to the iris, to 

 which organ the nerves seem chiefly destined. Beneath 

 this arterial layer, the microscope reveals a delicate mem- 

 brane, forming the internal lamina of the choroid, composed 

 of nucleated hexagonal cells, disposed in several lamina, 

 and containing the black pigment. This is the pigment- 

 ary membrane. 



