726 THE EYE. 



water, alcohol, and other fluids. The outer surface is rough or slightly 

 flocculent when the choroid is detached, and is in contact with the pig- 

 mentary layer ; and from it a more or less complete stratum may be raised 

 with care in a perfectly fresh eye. This layer, at first called membrane of 

 Jacob from its discoverer, is now generally recognised as the columnar layer. 

 The inner surface of the retina is smooth, and is merely applied to the 

 vitreous body within it : on it the following objects may be seen. In the 

 axis of the ball is a yellow spot macula lutea (limbus luteus, Sommerriug), 

 which is somewhat elliptical in shape, and about -^th of an inch in diameter : 

 in its centre is a slight hollow, fovea centralis, and, as the retina is thinner 



Fig. 475. 



Fig. 475. SECTION THROUGH THE MIDDLE OP THE OPTIC NERVE AND THE TUNICS OP THE 

 EYE AT THE PLACE OP ITS PASSAGE THROUGH THEM (from Kolliker after Ecker). f 



The drawing was taken from a chromic acid preparation : a, arteria centralis retinae ; 

 6, fasciculi of optic nerve fibres with neurilemuia ; c, sheath of the optic nerve, passing 

 into </, the sclerotic coat ; c", outermost pigmental layer of the choroid or membraua 

 fusca ; d, choroid and inner pigment-layer ; e, /, columnar layer of the retina ; g, the 

 two granular layers ; A, layer of nerve-cells ; i, layer of nerve-fibres ; &, colliculus or 

 eminence at the entrance of the optic nerve ; I, lamina cribrosa. 



here than elsewhere, the pigmentary layer of the choroid is visible through 

 it, giving rise to the appearance of a hole through the tunic. About T ^th of 

 an inch inside the yellow spot is the round disc, porus opticus, where the 

 optic nerve expands, and in its centre the point from which the vessels of 

 the retina branch. At this place the nervous substance is slightly elevated 

 so as to form an eminence (colliculus nervi optici). 



STRUCTURE. The retina, when examined microscopically in vertical 

 sections, exhibits a series of dissimilar strata, together with structures not 

 confined to one stratum. (1st) Externally is the columnar layer ; (2nd), 

 in the middle is the granular layer, comprising the external nuclear, the 

 iuternuclear, the internal nuclear, aud the molecular layers ; and (3rd) 

 internally is the nervous layer, consisting of three strata, one of nerve-cells, 

 another of nerve-fibres the ramifications of the optic nerve, and, on the 

 inner surface of this last, a limiting membrane. (4th) Traversing the strata 

 from the columnar layer to the limiting membrane, are placed vertical fibrils 

 of varying kinds at different depths, and not fully ascertained to be con- 

 tinuous, the radiating fibres of Miiller. (5th) Blood-vessels distributed 

 in the retina, are placed chiefly towards the inner surface. 



