430 SAMUEL C. PALMER 
of the supporting tissue, all the largest fibers, and all the prom- 
inent markings of the optic nerve carefully sketched. The draw- 
ing was then marked off with coérdinating lines and the rest of 
the optic nerve fibers were drawn in place and counted. By 
practice in counting I was able to make out and record the fibers 
in the sections with accuracy and speed. 
I have counted the fibers in three of the nerves given in table 
15 twice each. The results of the second counts were in each 
case so close to the first counts that I did not think it necessary 
to continue the process for every nerve; thus, in nerve no. | (fig. 12) 
I counted 2001 fibers on the first trial and 2003 on the second; 
in nerve no. 2, 1657 fibers on the first count and 1661 on the second; 
TABLE 15 
Areas of cross-sections of optic nerves and numbers of nerve fibers counted at different 
planes of transection 
CALCULATED AREA IN SQUARE MILLI- 
TOTAL NUMBER OF FIBERS IN 
DESIGNATION OF ian rape a ate rid Oar CROSS-SECTIONS 
NERVES 
Close to eye Close to chiasma Close to eye | Close to chiasma 
— = |. —— a ———+ 
1 0.017 2002 
2 0.019 ~ | 1659 
3 0.019 0.008 1780 $53 
4 0.026 0.009 1858 1071 
| 
5 0.027 2613 | 
| | 
and in nerve no. 5, 2616 fibers on the first count and 2611 on the 
second. The number of fibers appearing in table 15 for these 
optic nerves are averages obtained from the two counts. The 
complete results of my enumerations, together with the areas 
of the cross-sections of the optic nerve, are given in table 15. 
The average area near the eye was 0.0216 sq. mm. and contained 
by actual count an average of 1982.4 fibers or about 92,000 per 
square millimeter; the average of the cross-sections near the 
chiasma was 0.0085 sq. mm. with an average of 962 fibers, or 
approximately 113,000 per square millimeter. Although the 
proximal portion of the optic nerve is smaller in diameter than 
the distal part, it is, on the other hand, seen to be richer in fibers 
per unit area. 
