THE RETINA 



833 



layer of the retina. The cone-granules are more pyriform in shape, non-striated 

 and lie in close opposition to the external limiting membrane and the bases of 

 the cones. Their inner processes are continued into the outer zone of the outer 

 molecular layer, where they terminate in a prominent varicosity. 



The outermost layer of the return consists of the rods and cones. The former 

 are elongated cylindrical in shape and measure about 0.06 mm. in length and 0.002 

 mm. in width. The latter are shorter and thicker and measure 0.035 mm. in 

 length and 0.006 mm. in breadth. Both present an outer and an inner limb, the 

 former being imbedded in the neighboring pigment layer. The processes derived 

 from the inner limbs of the rods, pass into the 

 nuclei of the outer nuclear layer. A central fiber 

 extends from here into the outer molecular laj'er, 

 where it ends in a knob-like structure. The pro- 

 cesses from the inner limbs of the cones traverse 

 the external nuclear layer and terminate in a broad 

 expanse in the outer molecular layer. At this point 

 connection is made with the bipolar cells of the 

 inner nuclear layer by means of short fibers. l But, 

 since the retina contains many millions of rods 

 and cones, while the optic nerve embraces only 

 about 48,000 nerve fibers, these peripheral axones 

 must gradually become confluent. 2 



The cells of the pigment layer measure 12 to 

 18/x in diameter, but decrease in size near the 

 yellow spot. Their outer surfaces are smooth, 

 and while their outer portions are practically free 

 from pigment, their inner marginal zones are packed 

 with it and present filamentous prolongations ex- 

 tending inward along the limbs of the rods and 

 cones. The former are almost completely sur- 

 rounded by this pigmentous material. 



While we shall have occasion to refer 

 to these structural details later on in con- 

 nection with the theories pertaining to 

 vision, attention is called to the fact that 

 the retina is something more than a simple 

 sense organ; in fact, its complex structure 

 would lead us to believe that it is as truly 

 a subdivision of the brain as is the cerebral 

 hemisphere itself. For the same reason, it 

 may be concluded that the optic nerve is 

 not a simple nerve but a true tract of the 

 cerebrum. In this system the rods and BRAKE. 

 cones form the receptors, and hence, con- 

 stitute its neurones of the first order, while the cells composing the 

 internal granular layer, are its neurones of the second order and those 

 of the zone of ganglion cell, its neurones of the third order. The latter 

 convey the impulses to the ventral aspect of the brain by way of the 

 optic nerve, whence they are relayed to: (a) the cortical center for 

 vision through the thalamic radiation, (6) the roof of the colliculus 



J Ladd and Woodworth., Elements of physiol. Psychology, New York, 1911. 

 2 Salzer, Sitzungsber., Wiener Akad., Ixxxi, 1880, 3. 



53 



FIG. 437. I, A ROD; II, A 

 CONE OF MAMMALIAN RETINA; 

 h, EXTERNAL LIMITING MEM- 

 (G. Greeff.) 



