23S LESLIE B. AREY 



By tlie use of (lolgi methods niul by juethods of primary and 

 secondary degeneration, centrifugal fil)(u-s oi-iginating in the verte- 

 brate brain and extending to the retina can be demonstrated. 

 The following quotation from Johnston ('06, p. 265) summarizes 

 the general results gained from studies of this kind: 



In fishes in which one eye has long l)een lost the optic tract of the 

 opposite side degenerates with the exception of these efferent fibers, which 

 persist and are stained by the Weigert method. In mammals, following 

 section of the optic tract there occurs secondary degeneration of cells 

 in the anterior quadrigeminum, and in the dorsal part of the genicu- 

 latnm laterale and pulvinar. These fi.ndings in mammals agree with 

 those in fishes by the Golgi and degeneration methods, where the cen- 

 trifugal filiers arise from the tectum opticum and geniculatum (Catois, 

 '02) . The significance of these fibers is not understood but their pres- 

 ence in all vertebrates seems to show that they have some constant 

 function. 



A further anatomical difficulty is presented by the fact that a 

 connection between optic nerve fibers and the pigment cells has 

 never been demonstrated, for even the erfetont fibers shown by 

 Cajal ('94) and others have their endiii.;s near the internal 

 nuclear layer. Garten ('07, p. 85) suggColcJ that an actual con- 

 nection might not be necessary: '' Natiirfich liesse sich behaupten, 

 die centrifugalen Opticusfasern rufen in der Stabchenzapfen- 

 schicht einen Erreginigsvorgang hervor, der sich 'per contigui- 

 tatem' dem Pigmentepithel mitteilt." In view of the tentative 

 conclusion reached by me concerning the balanced action of the 

 'retino-motor' and the inhibitory nerve fibers, it is difficult to 

 state what anatomical conditions would be imposed by such a 

 system. Such matters of detail need not stand in the way of 

 the fact of fundamental importance, which is the existence of 

 demonstrably functional efferent optic nerve fibers. 



The significance of these efferent nervous elements in the light 

 of the theory of nerve components may appear to demand no 

 serious consideration, since the majority of neurologists prefer 

 not to homologize the optic 'nerve' with true cranial nerves. 

 Neither the afferent nor the efferent fibers of the optic nerve are 

 comparable to the components of a true peripheral nerve, for both 

 lie within the primary optic apparatus, which is itself a differ- 



