THE EETINA. 35 



what flattened in form, are partly imbedded in grooves on the inner surface of the 

 sclerotic, and communicate occasionally with each other before supplying the cornea 

 and entering the ciliary muscle. When the sclerotic is carefully separated from 

 the subjacent structures, these nerves are seen lying on the surface of the choroid, 



Fig. 44. DISTRIBUTION OF NERVES IN THE IRIS 

 (Kblliker). 50 DIAMETERS. 



The preparation was from the eye of an albino 

 rabbit ; a, smaller branches of the ciliary nerves 

 advancing from the choroid ; b, loops of union 

 between them at the margin of the iris ; c, 

 arches of union in the iris ; c', finer plexus in 

 the inner part ; e, sphincter pupil! te muscle. 



into which they send branches, and 

 in which they form a gangliated plexus 

 amongst the blood-vessels, the groups 



of ganglion-cells being often applied to the walls of the vessels. Within the 

 ciliary muscle the nerves also subdivide minutely, forming here another plexus, 

 which contains a number of medullated fibres, and the cells of which are smaller. 

 A few recurrent branches appear to pass back from it into the choroid coat, but 

 the greater number pass on to the iris (fig. 44, #, a). In the iris the nerves 

 follow the course of the blood-vessels, dividing into branches, which communicate 

 with one another as far as the pupil, forming a close plexus of fine non-medullated 

 fibres. Their ultimate destination is probably mainly the muscular tissue of the 

 iris and of its vessels. 



THE RETINA. 



The retina is a delicate membrane, which contains the expanded termination of 

 the optic nerve. It lies within the choroid coat, and rests on the hyaloid membrane 

 of the vitreous humour. It extends forwards nearly to the outer edge of the ciliary 

 processes of the choroid, where it ends in an indented border, named ora serrata 

 (fig. 45). From this border there is continued onwards a thin layer of a different 

 structure and containing no nerve-fibres, the pars ciliaris retince, which reaches as 

 far as the tips of the ciliary processes, and there gives place to the double layer of 

 pigment known as the urea, which is continued on to the posterior surface of the 

 iris (pars iridica retince). The thickness of the retina diminishes from behind 

 forwards, from 0*5 mm. near the yellow spot to 0*1 mm. at the ora serrata. In 

 the fresh eye it is translucent and of a light pink colour, but of a purple-red 

 colour if kept in the dark for a little while before removal. Under the influence 

 of sunlight it is quickly bleached and after death it soon becomes opaque. The 

 colouring matter was discovered by Boll and has been specially investigated by 

 Kiihne, who has given it the name of rhodopsin, It is absent at the yellow spot 

 and central fovea, and also close to the ora serrata. The outer surface of the 

 retina is covered with a layer of hexagonal pigment-cells which send fine offsets 

 between the elements of the next retinal layer : the rhodopsin becomes developed in 

 this layer by the agency of these pigment-cells. When the choroid is detached 

 these offsets are ruptured and the pigment-cells come away with it, so that this layer 

 was formerly described as part of the choroid coat. 



The inner surface of the retina is smooth : on it the following objects may be 

 seen. In the axis of the ball is a yellow spot macula lutea (Umbiis luteus, 

 Sommerring) which is somewhat elliptical in shape (fig. 45), and about 1 to 2 mm. 

 in diameter : in the centre of this, again, is a slight hollow, fovea centralis, 0'2 mm. 

 to 0-4 mm. in diameter (fig. 45), and, as the retina is thinner here than elsewhere, 



D 2 



