TEXT-BOOK OF PHYSIOLOGY. 



tufts of the visual cell axon, the latter with the dendrites of the gang- 

 lion cell. The retjna may be regarded therefore as the peripheral 

 end-organ in which the opjic nerve originates. From their origin the 

 axons turn backward, atthe sameTIrnlTconverging to form a distinct 

 bundle which passes through the chorioid coat and sclera. After 

 emerging from the eyeball the nerve-bundle (the optic nerve) passes 

 backward as far as the sella turcica, traversing in its course the or- 

 bit cavity and the optic foramen. At the sella turcica there is a 

 union and partial decussation in man and other mammals of the 

 two nerves, forming the **$& rfr^Sflfc 



Decussation of the Optic Nerves. The 

 results of various methods of research would seem 

 to indicate that the fibers from the nasalthird of 

 the retina of the left eyp cross in the chiasrrfto 

 unite with the fibers "from* the temporal two.- thirds 

 of the retina of the righ^__e^e. In a similar 

 manner the fibers from the nasal third of the 

 retina of the right eye cross in the chiasm and 

 unite with the fibers from the temporal two-thirds 

 of the retina of the left eye. Posterior to the 

 chiasm the crossed and uncrossed fibers form 

 the so-called optic tracts, which after winding 

 around^ the crura cerebri enter the optic basal 

 ganglia. Transection of the optic nerve shows 

 that it Is composed of an enormous number of 

 non-medullated nerve-fibers, estimated by Salzer 

 at from 450,000 to 800/000, enclosed in a sheath 

 of the dura mater, tn the central portion of the 

 there is seen a distinct~bundle of fibers, 



FIG. 240. RETINAL 

 CELLS, s', z'. 

 Visual cells with 

 their peripheral 

 terminations, s, 

 Rods. 2. Cones. 

 b. Bipolar cells. 

 g. Ganglion cells 

 from which arise 

 the axons of the 

 optic nerve. 



nerve 



_ 



triangular in shape, that come from the region of 

 the macula lutea. At the chiasm this bundle of 

 fibers undergoes a partial decussation similar to 

 that of the fibers coming from the more peripheral 

 portions of the retina. In the left optic tract, 

 therefore, fibers from four different regions are to 

 be found: viz., the two-thirds of the temporal side of the left retina, 

 the temporal half of the left macula, the nasal third of the right 

 retina, and the nasal half of the right macula. Corresponding fibers 

 are to be found in the right optic tract. As the optic tract passes 

 aroundJ&e crus cerebri it divides into a lateral, or outer, and a mesial 

 or inner bundle, which then terminate in the optic basal ganglia* 

 The fibers of the lateral bundle are tracea'S^JUSWrTne external 

 geniculate body (the pre-geniculum), the pulvinar of the optic 

 thalamus, and the anterior quadrigeminal body (the pregeminum). 

 These are in all probability the true visual fibers. The fibers of 



