CHAP, in.] SIGHT. 837 



into nervous elements but soon become loaded with pigment, and 

 the greater part of it is known in the adult eye as the pigment 

 epithelium of the retina, which, as we shall see, is in close func- 

 tional connection with the nervous elements of the retina proper. 

 The fibres of the optic nerve, as they are developed in the stalk 

 of the retinal cup, become connected with the elements of the 

 inner or retinal wall only of the cup ; they pierce the outer wall 

 of pigment epithelium, making no connections with the cells of 

 that outer wall. 



The retina then, in which by the action of light visual impulses 

 are generated, is in reality a part of the brain, removed to some 

 distance from the rest of the brain but remaining connected with 

 it by means of the tract of white matter which we call the optic 

 nerve ; and, as we shall see, the retina is in structure similar to 

 parts of the grey matter of the brain. The optic nerve is not like 

 other nerves an outgrowth from the central nervous system, but 

 like the olfactory tract a commissure of white matter between 

 two parts of the brain, namely, between the outlying retina and 

 the internally placed corpus geniculatum, pulvinar, and corpus 

 quadrigeminum. We shall find accordingly that in structure it 

 differs from ordinary cranial or spinal nerves. 



Into the mouth of the retinal cup there is thrust a rounded 

 mass of epithelium, an involution from the superficial epiblast ; 

 this becomes the lens. The hollow of the retinal cup is occupied, 

 as we have said, by mesoblast ; this ultimately becomes modified 

 into the vitreous humour. The mesoblastic tissue siirrounding 

 the cup is developed into an investment of two coats ; an inner, 

 somewhat loose and tender, vascular and in part muscular coat, 

 which on the one hand serves to nourish the retina, and on the 

 other hand carries out certain movements of the dioptric appa- 

 ratus, and an outer, firmer and denser coat, which affords protec- 

 tion to the whole of the structures within. The inner vascular 

 coat, which may be compared to the pia mater, is called the cho- 

 roid (Fig. 139 Oh.), and in the front part of the eye, at about 

 the level of the lens, is thrown into a number of radiating folds 

 or plaits, the ciliary processes O.P. The outer coat, which may 

 be compared to the dura mater, is called the sclerotic (Fig. 139 

 ScL). Over the greater part of the eyeball the two coats are in 

 apposition, or separated only by narrow lymphatic spaces, which 

 may be compared with the subarachnoid spaces, but towards the 

 front they diverge ; the choroid is bent inwards towards the cen- 

 tral axis of the eye to form the diaphragm called the iris (Fig. 

 139 /.), while the sclerotic is continued forwards to form, beneath 

 the epidermis into which the superficial epiblast is developed, the 

 basis of the cornea (Fig. 139 (7.). At the angle of divergence 

 of the two coats is developed a small mass of muscular fibres, the 

 ciliary muscle of which we shall speak in detail presently. 



The inner or front wall of the retinal cup becomes as we have 



