THE ENCEPHALIC OR CRANIAL NERVES 



627 



layers of cells which collectively constitute the retina. Though the retina is 

 said to consist of ten or eleven layers, it may be reduced practically to three, 

 viz. (Fig. 260): 



1. The layer of visual cells. 



2. The layer of bipolar cells. 



3. The layer of ganglionic cells. 



The visual cells present peripherally modified dendrites, known as the 

 rods^and cones; centrally they give off an axon which after a short course 

 terminates in an end-tuft. The bipolar cells also possess dendrites and an 

 axon; the former interlace with the end-tufts of the visual cell axon, the latter 

 with the dendrites of the ganglion cell. The retina may 

 be regarded therefore as the peripheral end-organ in 

 which the optic nerve originates. From their origin the 

 axons turn backward, at the same time converging to 

 form a distinct bundle which passes through the cho- 

 rioid 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 orbit 

 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 optic chiasm. 1 



The Decussation of the Optic Nerves. The extent 

 to which the fibers from each eye decussate at the chiasm 

 is a subject of dispute, but the results of various methods 

 of research would seem to indicate that the fibers from 

 the nasal third of the retina of the left eye cross in the 

 chiasm, to unite with the fibers from the temporal two- 

 thirds of the retina of the right eye. In a similar man- 

 ner 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 (Fig. 261). 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. Tran- 

 section 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. 



The visual fibers comprising the optic nerve may be physiologically 

 divided into two clases, (a) those coming from the peripheral portion of the 

 retina, and (b) those coming from that central area known as the macula 

 lutea. The retinal fibers are by far the more abundant, and make up the 

 major portion of the nerve; the macular fibers are less abundant. An ex- 

 amination of a cross-section of the optic nerve shows the presence of a wedge- 

 shaped tract occupying the center of the nerve which is regarded as 



1 Though the foregoing is the usual method of stating the origin and course of the optic nerve, 

 nevertheless morphologically the true optic nerve lies wholly within the retina and is composed 

 of the visual cells there found. The remainder of the visual system from and including the 

 ganglion cells of the retina to the optic basal ganglia, is the optic tract, there being no anatomic 

 or physiologic distinction between the optic nerve so called and the optic tract. Both are out- 

 growths from the brain and hence possess properties which differentiate them from other cranial 

 nerves. 



FIG. 260. RETI- 

 NAL CELLS, s', z'. 

 Visual cells with 

 their peripheral ter- 

 minations, s. Rods. 

 z. Cones, b. Bi- 

 polar cells, g. Gan- 

 glion cells from which 

 arise the axons of the 

 optic nerve. 



