212 THE ORGANS OF SPECIAL SENSE 



junction, but it is only visible when the vessels become 

 engorged with blood, and is seen best in scleritis. 



The Cornea consists mainly of modified fibrous tissue, and 

 it is directly continuous with the sclera. It is perfectly trans- 

 lucent in order that light may pass through it to reach the 

 retina, and it is therefore devoid of blood-vessels. Numerous 

 lymph-spaces lie in the meshes of its fibrous tissue, and the 

 cornea depends for its nutrition on the lymph which they con- 

 tain. Ulcers of the cornea derive their blood-vessels from the 

 anastomotic ring above referred to, and, after they heal, they 

 are liable to produce small areas of opacity. 



The corneal lymph drains away into a circular canal, termed 

 the sinus venosus sclerce (canal of Schlemm), which lies in the 

 corneo-scleral junction. This canal helps to drain away the 

 aqueous humor from the anterior chamber of the eye (p. 216). 



The Chorioid is the vascular coat of the eye. It is placed 

 within the sclera, to which it is attached by some pigment- 

 containing connective tissue, but it does not extend quite so 

 far forwards as the corneo-scleral junction. It contains the 

 blood-vessels which supply the various coats of the eye, and it 

 is separated from the retina internally by a translucent basal 

 membrane. 



Congenital deficiencies may occur in the chorioid and are 

 usually associated with similar deficiencies in the retina. The 

 condition, which is known as coloboma, is commonly found in 

 the lower and medial quadrant of the fundus. Owing to the 

 absence of the retinal and chorioid coats, the sclera is seen 

 over the affected area on ophthalmoscopic examination, and it 

 appears as a clearly outlined patch, pearly-white in colour. 



Inflammation of the chorioid causes localised swellings, 

 which lie deep to the retinal blood-vessels (p. 217), and can 

 therefore be distinguished from inflammatory areas in the 

 retina. The presence of chorioiditis in the region of the macula 

 is of great importance, as the pressure and the inflammatory 

 exudation usually give rise to permanent effects, and, on this 

 account, the prognosis should always be very guarded. 



