HISTOLOGICAL ELEMENTS RETINA NECTURUS 435 
the surrounding tissues. Over-blackened margins and the appar- 
ent fusion of occasional fibers did not prove to be insurmountable 
difficulties, for by focusing, the separate fibers could usually 
be distinguished, so that I believe the number of fibers counted 
in a cross section is very close to the actual number present. 
In addition to variations in diameter in a given nerve, there 
are also individual variations which are considerable. Measure- 
ments near the chiasma are fairly uniform, but at the distal end 
of the nerve I found a range in area of 62.9 per cent. The number 
of fibers here correlates in some degree with the cross-sectional 
areas of the nerves. Thus in table 15, nerve no. 2, with a cross- 
sectional area of 0.019 sq. mm. has an average of 1659 fibers, 
and nerve no. 5, with an area of 0.027 sq. mm., has an average of 
2613 fibers. 
~The number of fibers in other nerves than the optic nerve of 
Necturus have been counted or estimated in some instances. 
From Salzer’s (80) calculations there are between 60,000 and 
70,000 fibers per square millimeter in the human optic nerve; 
Birge (’82) has counted 3550 fibers per square millimeter in the 
7th spinal nerve of the frog, and 14,133 in the 10th; Hatai (03) 
found a variation in the number of fibers in the spinal nerves 
of the white rat according to the position of the plane of transec- 
tion, there being more in the proximal than in the distal planes, 
which is the reverse of the condition in the optic nerve of Nec- 
turus; Donaldson and Bolton (’91) record an average of 11,900 
fibers to every square millimeter in the dorsal roots of the spinal 
nerves of man; Dunn (’00) has found that the total number of 
fibers innervating the hind foot of the frog is between 5000 and 
6000; and on page 430 I have shown that in the optic nerve of 
Necturus there are about 100,000 fibers per square millimeter. 
Thus we see that, although in general the visual apparatus 
of Necturus is described as degenerate, and the animal’s habits 
have apparently brought about a reduced functional activity 
of the visual cells, the optic nerve appears to have more nerve 
fibers per unit area than any other nerve so far studied. 
