HISTOLOGICAL ELEMENTS RETINA NECTURUS 411 
B. Optic nerve 
Estimates of the number of optic nerve fibers, have been con- 
fined chiefly to the optic nerve of man. Kuhnt (’75) found 200 
fibers in the diameter of the optic nerve of a new-born child, 
and 220 to 240 in that of a man forty years old, giving totals of 
approximately 31,400 and 40,000 respectively for the entire cross- 
sections. Krause (’76) put the number at 1,000,000 at least, 
but later (80) reduced it to 400,000 including both large and 
small fibers, and an equal number of very small fibers. Salzer 
(’80) also worked on the optic nerve of man and from three nerves 
obtained an average of 437,745 fibers. 
The only important contribution, to which I have had access, 
relating to the number of optic nerve fibers in amphibians is a 
short paper by Lauber (’02), who counted 450 fibers in a cross- 
section of the optic nerve of Cryptobranchus japonicus. 
Among invertebrates Parker (’95) determined that seven 
fibers were connected with each ommatidium in the crayfish eye. 
This gave a total of 8085 retinal fibers in the case of a young 
crayfish and 16,625 in an old one. Proximal to the fourth optic 
ganglion these totals were reduced to 2021 fibers in the former 
and 4156 in the latter. 
It is a generally credited theory that the majority of the optic 
nerve fibers originate in the retina and pass centripetally along 
the optic stalk to the brain, and that a smaller number arise in 
the brain and pass centrifugally to the retina. This view has 
the support of such investigators as His (’90), Ramén y Cajal 
(91) and Robinson (96). On the other hand Balfour (’81) 
did not hold this view, believing rather that the fibers of the 
optic nerve were derived from a differentiation of the epithelial 
cells of which the nerve was at first formed. 
The axis-cylinders of the optic nerve in a majority of verte- 
brates are small and medullated. In some amphibians, viz., 
urodeles, the optic nerve was described by Osborn (’88) as ‘greatly 
reduced,’ and in some examples of Necturus there was stated to 
exist, in the adult condition a persistent lumen which opened into 
the T-like expansion of the third ventricle of the brain. Kings- 
