Il8 ESSENTIALS OF PHYSIOLOGY. 



layer that of a cone bipolar arborises in the inner molecular layer in 

 relation with the dendrons of a ganglionic cell. In the outer part of 

 the inner nuclear layer are horizontal cells, the dendrons and axons of 

 which arborise in the outer molecular layer, so that these cells have an 

 associational function. In the inner part of the layer are cells which 

 have no axons and are therefore called amacrine cells. The dendrons 

 of these cells arborise in the inner molecular layer in relation with the 

 dendrons of the ganglionic cells. 



(6), (7), and (8) The cells of the ganglionic layer are arranged in a 

 single row in most parts of the retina. Each has branched dendrons 

 which extend into the inner molecular layer, where they terminate at 

 different levels in relation with the terminal arborisations of the axons 

 of the cone bipolars. The axons of the ganglionic cells are continued 

 as non-medullated nerve fibres in the stratum opticum. Some of the 

 fibres in the stratum opticum have a centrifugal course and run into 

 the retina to terminate by arborisation in the inner nuclear layer. 



Three parts of the retina require a special description the macula 

 lutea, the place of exit of the optic nerve, and the ora serrata. 



The macula lutea is the part of the retina concerned with distinct 

 vision. When an object is "looked at" its image is formed on the 

 macula, or more particularly on the fovea centralis, a small depression 

 in the centre of the macula. At the fovea there are no rods, and the 

 cones are longer than in the remainder of the retina. The cones and 

 their nuclei are the only retinal structures present in the fovea, and the 

 cone fibres are inclined away from the fovea towards the inner nuclear 

 layer. In the peripheral region of the macula the ganglionic layer is 

 several cells deep, and cones are more numerous than rods. The 

 proportion of cones diminishes from the macula to the periphery of the 

 retina, and near the ora serrata very few cones are present. The 

 macula is situated slightly to the lateral side of the posterior pole of the 

 eyeball. 



The fibres of the stratum opticum converge to a point about 

 3 mm. to the nasal side of the macula lutea to form the optic nerve, 

 which passes back through a gap in the choroid coat and a perforated 

 part of the sclera known as the lamina cribrosa. The place of exit of 

 the nerve as seen from the interior of the eyeball is a sharply defined 

 pale area, nearly circular in outline, and is called the optic disc. At 

 the disc, all the retinal layers are absent except the stratum opticum. 

 the fibres of which acquire a myelin sheath as they emerge from the 

 eyeball. 



The layers of the retina cease abruptly at the ora serrata, and are 

 represented in the ciliary region by two layers of cells, the deeper, 



