MIDBRAIN AND THALAMUS OF NECTURUS 241 



2. The optic tracts 



The fibers of the optic nerves of Necturus are entirely unmye- 

 Hnated and can therefore readily be distinguished from the 

 myelinated fibers of the postoptic commissm'e complex with 

 which they are mingled in the chiasma ridge. Moreover they 

 are readily impregnated in preparations prepared by the 

 methods of Golgi and Ram6n y Cajal, so that one would expect 

 to be able to gain a tolerably complete knowledge of their central 

 connections. The relations about the optic chiasma are, how- 

 ever, peculiarly difficult and many points remain obscure. 



As has long been known, the so-called optic nerve of adult 

 Necturus retains the embryonic character of a hollow epithelial 

 tube whose lumen communicates with the ventral part of the 

 pre-optic recess. The number of fibers in the nerve is relatively 

 small and as the nerve approaches the optic chiasma these 

 fibers are accumulated chiefly in the caudo-ventral wall of the 

 epithelial tube (fig. 57). All of these fibers decussate in the 

 optic chiasma. 



In the frog and some other lower vertebrates, connections of 

 the optic nerve have been described with other parts of the brain 

 than the optic tectum of the midbrain. Wlassak ('93) de- 

 scribes three optic tracts in the frog, the axial, marginal, and 

 basal bundles. The marginal bundle is the chief tract and 

 maintains a superficial position until it terminates in the more 

 superficial layers of the stratum medullare superficiale of the 

 tectum. The axial bundle is of coarser fibers, which decussate 

 in the chiasma farther dorsally than the other optic tracts. 

 They pass across the face of the thalamus at a deeper level than 

 those of the marginal bundle, traversing both the corpus genic- 

 ulatum thalami and its nucleus anterior superior (see below, 

 p. 243), and terminate in the deeper layers of the stratum medul- 

 lare superficiale of the tectum. These fibers appear to be those 

 described by Bellonci ('88) as fibrae ansulatae. The basal 

 bundle decussates farthest ventral and caudal in the chiasma 

 and its fibers pass backward to a ventral nucleus in the cerebral 

 peduncle lying rostrally of the oculomotor nucleus. 



