834 



THE SENSE OF SIGHT 



of the midbrain for purposes of reflex action, and (c) the pulvinar and 

 lateral geniculate bodies of the region of the thalamus. These connec- 

 tions convert the optic nerve into a correlation path which is 

 comparable to the lemniscus system. 



The Blind Spot. 1 The retina derives its nerve supply from the 

 optic nerve which pierces the eyeball in the optic disc or porus opticus. 

 Beginning at this point, its fibers spread out fan-like across the entire 



A 



FIG. 438. SECTION THROUGH THE PLACE OF ENTRANCE OF THE OPTIC NERVE (B'>, 

 TOGETHER WITH THE OPHTHALMOSCOPIC VIEW OF THE Disc (A), TO SHOW THE CORRE- 

 SPONDING PARTS OF THE Two. (Fuchs, after Jaeger.) 



cd, Lines of correspondence; 6, depression in center of disc; r, retina; ch, choroid; 

 si,so, inner and outer parts of the sclerotic coat, s; ci, a ciliary artery cut longitudinally; 

 a,v, central artery and vein; sd, subdural space; sa, subarachnoid space; du, dural 

 sheath; or, arachnoidal sheath of nerve; p, pial sheath; n, nerve-bundles; se, septa 

 between them. 



expanse of this membrane. The circumference of this disc is slightly 

 elevated, while its center shows a depression from which the blood- 

 vessels pass radially outward toward the ora serrata. Its diameter 

 measures about 1.8 mm. When looked at from in front, it appears 

 as a whitish circle surrounded by a dark ring, the latter indicating the 

 line where the pigmented choroid begins. Inasmuch as this entire 

 area is composed solely of nerve fibers, blood-vessels and reticular tissue 



1 The blind spot of the eye was discovered by Mariotte in 1668 (M6m., Acad. 

 de Paris, 1669). 



