CHAP, iir.] SIGHT. 23 



underlying pigment epithelium of the retina, and so from the 

 retina. Although the retina possesses, as we have said, vessels of 

 its own, these as we shall see are largely confined to the anterior 

 or inner layers of the retina adjoining the vitreous humour ; the 

 posterior or outer layers of the retina, and the adjoining pigment 

 epithelium, are nourished by the blood vessels of the choroid. 

 Hence the great development of the chorio-capillary layer ; and 

 the rest of the choroid, if we exclude the muscular and nervous 

 elements of which we will speak presently, serves chiefly as a 

 means to carry blood to and from the chorio-capillary plexuses, and 

 as a bed for the lymphatic channels rendered necessary by the 

 amount of lymph which must be continually furnished by such a 

 close-set capillary network. 



The pigment in the choroid may be regarded as serving in 

 addition a dioptric purpose, absorbing the rays of light which have 

 passed into it through the retina ; but this cannot be of any great 

 importance, since as we shall see such an absorption is chiefly 

 carried out by the pigment epithelium of the retina. 



In some animals part of the surface of the choroid when the 

 eye is looked into shews various colours. The colouring is one 

 not of pigments but of iridescence like the colouring of Newton's 

 rings and thin films. It is due to the interference of light in 

 a special layer, called the tapetum, intervening between the 

 chorio-capillary membrane and the body of the choroid. In 

 herbivora the interference of light causing iridescence is brought 

 about by the peculiar arrangement of fine bundles of fibrillated 

 connective-tissue ; in carnivora the tapetum is composed of cells 

 loaded with minute crystals and the interference is caused by the 

 crystals. 



174. The Ciliary Processes. In front of the ora serrata, 

 at which line the retina proper ceases, the choroid changes in 

 character, being here thrown into the radiating plaits called the 

 ciliary processes. These like the choroid, of which they are in fact 

 a continuation, consist of blood vessels imbedded in a connective- 

 tissue groundwork ; but bundles of ordinary fibrillated tissue re- 

 place to a large extent the peculiar elastic lamellae, and the capillary 

 networks, though abundant, do not form a special close-set layer 

 like the chorio-capillary membrane, but are more equally diffused 

 through the bodies of the processes. The cells scattered through- 

 out the connective-tissue bear pigment, especially in dark eyes. 



The membrane of Bruch, or basal membrane, is continued over 

 the processes, and here, of-ten sculptured, rests on a layer of epithe- 

 lium cells which do not maintain the features of the pigment 

 epithelium of the retina, for these cease at the ora serrata, but 

 are plain cubical cells simple in character and loaded, except 

 in albino eyes, with black pigment. This pigment layer rests in 

 turn on a layer of columnar cells transparent and free from 

 pigment, into which the complex retina is suddenly transformed at 



