666 DISSECTION OF THE EYE. 



pyriform in shape, with the base turned to the cone, but are free from 

 cross stripes. 



The intervening molecular portion (rf) has a striated appearance, from 

 the passage of the fibres of Miiller through it. 



The fibres of Muller (fig. 234) are extremely fine threads, which pierce 

 the substance of the retina, supporting and uniting the several layers, as with 

 a framework of areolar tissue ; they reacli from Jacob's layer to the inner 

 surface. As they pass through the granular layer they are connected with 

 very fine threads amongst both set of granules (c and e) ; and in the inner 

 set (*?) each possesses an oval nucleated cell. At the inner surface of the 

 retina they are dilated and joined to each other, so as to construct a bound- 

 ing stratum (/) (membrana limitans interim) ; and at the outer part they 

 are united into another boundary line at the bases of the cones (membrana 

 limitans externa). 



Structure of the yellow spot. The strata of the retina undergo modifi- 

 cations in the yellow spot, the chief of which are the following : externally, 

 the rods of Jacob's layer ( a ) are absent, and the cones become less bulged 

 and longer towards the centre. Internally, the nerve cells (g) of the layer 

 ( 3 ) are six to eight deep, but the nerve fibres (h) gradually cease at the 

 edge of the central fovea. The intermediate strata thin gradually towards 

 the fovea centralis, where they are so blended as to render difficult their 

 identification. 



The yellow color is due to pigment diffused through the substance, ex- 

 cept through Jacob's membrane and the outer set of cells (c) of the granu- 

 lar layer. Water removes it easily. 



Capillary vessels occupy the yellow spot, whilst the larger branches pass 

 around on each side. 



The pigmentary layer (fig. 227) covers the outer surface of the retina, 

 and is connected thereto by fine processes. 1 It consists of a single stratum 

 of six-sided nucleated cells, with granular contents (fig. 230, B ) which are 

 applied to each other by the edges. Each possesses a nucleus towards the 

 outer surface, and much pigment towards the inner. From the inner 

 surface of the cells project very fine threads, which penetrate between 

 the rods and cones of Jacob's layer. The pigmentary layer is constituted 

 thus as far as the ora serrata of the retina ; but in front of that line, viz., 

 on the ciliary portion of the retina, the cells are rounded and full of pig- 

 ment, and are continuous with the pigment on the ciliary processes and 

 back of the iris. 



Vessels of the retina. The central artery of the retina, accompanied by 

 its vein, enters the eyeball through the optic nerve (p. 50). Inside the 

 retina the vessel divides into four or five branches ; these pierce the 

 stratum of fibres (fig. 234, h), and end in a network of capillaries amongst 

 the ganglion cells (<y), and in the molecular material outside them (/). 

 In the yellow spot only capillaries exist. 



In the foetus a branch of the artery passes through the centre of the 

 vitreous mass to supply the lens capsule. 



VITREOUS BODY. A transparent mass fills the greater part of the 

 space within the coats of the eyeball (fig. 227, A), which has been named 

 vitreous body, from its resemblance to glass ; it consists of a clear aqueous 



1 In the dissection of the retina tin's pigmentary layer is removed with the 

 choroid through the rupture of those pro.-esses ; and it has therefore been com- 

 monly described as part of the choroid coat. 



