SENSE OF SIGHT. 



549 



Porus Opticus (Fig. 328). About 3 mm. to the nasal side of 

 the macula lutea, and about 1 mm. below its level, is the optic disk, 

 the entrance of the optic nerve ; inasmuch as vision is absent at 



Layer of nerve- 

 fibers. 



Ganglion-cell layer. -- 



Inner molecular 

 layer. 



Inner nuclear layer. 



Outer molecular -- 

 layer. 



Outer nuclear layer. < 



Ext. limiting mem- 

 brane. 

 Inner segment of 



rod. 

 Inner segment of 



cone. 



Outer segment of 



cone. 

 Outer segment of 



rod. 



FIG. 330. Section of the human retina; X 700 (Bohm and Davidoff). 



this point, it is called the blind spot. At its center, through the 

 porus opticus, enters the arteria centralis retince. As the retina is 

 somewhat elevated at the blind spot, this portion of it has also re- 

 ceived the name of colliculus nervi optici. 



. Lamina cribrosa. 



Pigment-layer, 

 and cones. 



er nuclea 



er roolecu 



Layer of ner 



ayer. 

 ar 'aver, 

 layer, 

 ar 'layer, 

 e- fibers. 



Blood-vessels. Physiologic excavation. 



FIG. 331. Section through point of entrance of human optic nerve ; X 40 (Bohm and 



Davidoff). 



The arteria centralis retinae is a branch of the ophthalmid 

 artery, and, after piercing the porus opticus, it divides into four or 

 five branches which run between the hyaloid membrane and the 



