goo 



A MANUAL OF PHYSIOLOGY 



granular or nuclear layer, containing many fusiform (bipolar) 

 ' granule ' cells which send out axons into the fourth, and dendrites into 

 the sixth, or outer molecular layer, and are thus connected with the 

 ganglion cells of the third layer on the one hand, and with the termi- 

 nations of the rod and cone fibres of the seventh or outer nuclear layer 

 on the other. The arborizations of the axons of these bipolar cells 

 are situate at different levels in the internal molecular layer. The 

 bipolar cells connected with the rod fibres send their axons right 

 through the internal molecular layer to arborize around the bodies 

 of the ganglion cells, whereas the axons of the bipolar cells con- 

 nected with the cone fibres ramify about the middle of the layer 

 (Fig. 383). The seventh stratum receives its name from the large 

 number of nuclei which it contains. These belong to structures 

 continuous with the rods and cones of the ninth layer, which is 



divided from the seventh by the 

 external limiting membrane. 

 Each rod is prolonged into the 

 external nuclear layer as a fine 

 fibre, which has on its course a 

 swelling containing a nucleus, 

 and terminates (in mammals) 

 in a fine knob in the external 

 molecular layer among the den- 

 drites of the bipolar cells. Each 

 cone of the rod and cone layer 

 is directly prolonged into a 

 nucleated enlargement in the 

 external nuclear layer. From 

 this enlargement a fibre (cone 

 fibre), of considerably greater 

 calibre (in mammals) than the 

 rod fibre, passes into the ex- 

 ternal molecular layer, where 

 it forms an arborization, which 

 comes into relation with the 

 arborization of the dendrites of 

 a bipolar cell. At the fovea 

 centralis the rods are entirely 



FIG. 384. RETINAL BLOODVESSELS 

 (HENLE). 



The arteria centralis is seen issuing 

 from the optic disc and branching over 

 the retina. The shaded area in the 

 middle of the figure represents the 



yellow spot with the fovea centralis in its 

 centre. 



absent, and the other layers of 

 the retina greatly thinned ; 

 over the optic disc neither rods 



nor cones are present. The disc is pierced by the retinal blood- 

 vessels (Fig. 384). 



External to the rods and cones is a sheet of pigmented epithelial 

 cells of hexagonal shape, belonging to the choroid, but remaining 

 attached to the retina when the latter is separated, and therefore 

 often reckoned as its most external layer. 



A little behind the cornea and anterior to the retina is the lens, 

 enclosed in a capsule, and attached to the choroid by the suspensory 

 ligament, or zonule of Zinn. The ins hangs down in front of the 

 lens like a diaphragm, with a central hole, the pupil. Incorporated 

 in the stroma or framework of the iris are two arrangements of 

 smooth muscular fibres, which confer on it the power of adjusting 

 the size of the pupil. One of these the sphincter pupillae con- 

 sists of a well-defined band of concentric fibres surrounding the 

 margin of the pupil. The other the dilator pupillse is less sharply 



