THE DIENCEPHALON AND THE OPTIC NERVE 



225 



the forebrain. By a folding of its walls in the reverse direction, i. e., by invag- 

 ination, the optic vesicle becomes transformed into the optic cup (Fig. 15) ; and 

 the cavity of the vesicle becomes reduced to a mere slit between the two layers 

 forming the wall of the cup. The inner of these two layers develops into the 

 nervous portion of the retina; and nerve-fibers arising in it grow back to the brain 

 along the course of the optic stalk, which still connects the optic cup with the 

 forebrain. This mode of development serves to explain why the structure of 

 the retina resembles that of the brain more than it does that of other sense 

 organs, and why the optic nerve-fibers, like those of the fiber tracts of the cen- 

 tral nervous system, are devoid of neurilemma sheaths. These fibers take origin 

 from the ganglion cells of the retina, the structure of which must be briefly con- 

 sidered at this point. 



Optic nerve 



Optic chiasma 



GanglioniciStratum oplicum ^- 



neurons \ Ganglionic layer " 



( Inner molecular layer 



Bipolar } Inner nuclear layer 

 neurons I 



^ Outer molecular layer 



Rod and f uter nuckar layer 



cone < Ex. limiting membrane 



neurons \Layerofrodsandcones 



Optic tract 



Lateral geniculate body 

 Medial geniculate body 

 i * Pulvinar 



Superior colliculus 



Fig. 160. Schematic representation of the retina and the connections established by the optic 



nerve-fibers. 



The retina presents for consideration three layers of superimposed nervous 

 elements: (1) the visual cells, (2) the bipolar cells, and (3) the ganglion cells 

 (Fig. 160). These, with some horizontally arranged association neurons and 

 supporting elements, form the nervous portion of the retina and are derived 

 from the inner layer of the optic cup. The pigmented stratum of the retina is 

 derived from the outer layer of the cup. 



The visual cells are bipolar elements, whose perikarya are located in the 

 outer nuclear layer (Fig. 160). Each presents an external process in the form of 

 a rod or cone, so differentiated as to respond to photic stimulation and thus to 

 serve as a visual receptor. The other process terminates in the outer molecular 

 layer in relation to processes from the bipolar cells. These latter elements have 

 their perikarya in the inner nuclear layer and branches in the inner and outer 

 molecular layers. The ganglion cells send their dendrites into the inner molec- 

 ular layer, where they are related to the inner branches of the bipolar cells; 

 15 



