534 -ffiSTHESIOLOGY. 



other has become opaque, and from its great elasticity, its use 

 is supposed to be that of preserving the correct curvature of the 

 flaccid cornea propria. 



The proper cornea is continuous with the sclerotic coat, and 

 the elastic one with the ciliary muscle. 



The Second Tunic of the eyeball is formed by the choroid 

 coat and iris. 



The Choroid coat is a thin vascular membrane, of a brownish 

 or black colour. Externally it is connected to the sclerotic by 

 means of a fine areolar tissue, the membrana fusca, minute blood- 

 vessels, and nerves; internally it is in simple contact with the 

 retina ; posteriorly it is pierced for the passage, of the optic nerve ; 

 and anteriorly, at the junction of the sclerotic and cornea, it is 

 connected with a white circular ring, the ciliary muscle, which is 

 continuous with the circumference of the iris. 



The choroid membrane is composed of three layers ; external, 



middle, and internal. The external layer consists principally of 



veins, vence vorticosoe, arranged in peculiar curves, all of which 



converge to four or five trunks, which pierce the middle of the 



sclerotic ; the largef branches of the short ciliary arteries mingle 



with the veins, and the numerous pigment 



I cells, to which the outer colour of the choroid 



jB Bl^I^^B^ is due, are interspersed among the vessels. 



^BP^^a Bst This layer is connected with the ciliary 



^flnM^^S^^ muscle. The middle layer is formed by 



I^W^bPE9 the ciliary arteries, which ramify into a fine 



capillary plexus, formerly known as the 



tunica Ruyschiana, which is reflected in- 



FiG 208 wards, and is continuous with the ciliary 



Pigment cells, a. The proccsses. The internal or pigmentary 



Pigment granules conceal- i^y^^ ig composod of lam in SB of hexagOnal 



ing the nucleus, b, The ^ . . •, , i • i 



nndcus visible. cells, contamiug black pigment granules, 



and arranged in a tesselated form. On 

 the posterior wall the black pigment is absent, causing a peculiar 

 bluish lustre, which, owing to its brilliancy, has received the 

 name of the tapetum lucidum. 



The ciliary Tnuscle is a white ring of fibres, which forms the 

 bond of union between the external and middle tunics of the eye, 

 and serves to connect the cornea and sclerotic with the external 

 layer of the choroid and with the iris. A minute vascular canal, the 

 ciliary canal, exists between this ligament and the sclerotic coat. 



V9 



