53'4 JESTHESIOLOGY. 



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 vorticosce, arranged in peculiar curves, all of which 

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

 sclerotic ; the larger branches of the short ciliary arteries mingle 

 with the veins, and the numerous pigment 

 cells, to which the outer colour of the choroid 

 is due, are interspersed among the vessels. 

 This layer is connected with the ciliary 

 muscle. The middle layer is formed by 

 the ciliary arteries, which ramify into a fine 

 capillary plexus, formerly known as the 

 tunica Ruyschiana, which is reflected in- 

 wards, and is continuous with the ciliary 



IG. 20S. ^ 



pigment cells, a, The processes. The internal or pigmentary 

 pigment granules conceal- ^ is composed of laminae of hexagonal 



ing the nucleus ; 6, The _ 



nucleus visible. cells, containing 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 muscle 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. 



