4 STRUCTURE OF THE EYE. [BOOK in. 



The hind or outer wall remains thin, and continues to consist of a 

 single layer of epithelium, the cells of which are never developed 

 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 

 functional connection with the nervous elements of the retina 

 proper. At the time when the epithelial cells of the stalk of 

 the retinal cup are developed into the fibres of the optic nerve, 

 these 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 ( 674) 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 surrounding the 

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

 what 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 apparatus, and an 

 outer, firmer and denser coat, which affords protection to the 

 whole of the structures within. The inner vascular coat, which 

 may be compared to the pia mater, is called the choroid (Fig. 134 

 Ch.), 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 C.P. The outer coat, which may be compared to 

 the dura mater, is called the sclerotic (Fig. 134 Scl). Over the 

 greater part of the eyeball the two coats are in apposition, or 

 separated only by narrow lymphatic spaces, which may be com- 

 pared with the subarachnoid spaces, but towards the front they 

 diverge ; the choroid is bent inwards towards the central axis of 

 the eye to form the diaphragm called the iris (Fig. 134 /.), while 

 the sclerotic is continued forwards to form, beneath the epidermis 

 into which the superficial epiblast is developed, the basis of the 

 cornea (Fig. 134 C.}. At the angle of divergence of the two 



