54 JOURNAL OF COMPARATIVE NEUROLOGY. 
I. Introduction. 
This investigation was undertaken in order to ascertain 
the number of medullated nerve-fibers in the dorsal roots of the 
spinal nerves of man. It was thought that such an enumera- 
tion would assist us in determining whether we may postulate 
separate nerve fibers for each of the forms of dermal sensation, 
since it would permit us to calculate the average area of the 
skin innervated by a single nerve-fiber. 
TT. Htstorical Statement. 
Attempts have been made to determine the value of the 
roots of the spinal and cerebral nerves as pathways for nerve 
impulses both by measuring the areas of their cross-sections 
and by estimating or counting the number of nerve fibers 
in them. 
Limiting our attention to these nerves in man we find that 
the area of the cross-sections of N. opticus alone has been de- 
termined by SALzER (1880)', W. Krause (1880)’, and for the 
Nn. opticus, oculomotorius, and trochlearis by DonaLpson and 
Botton (1891). 
The determination of the number of nerve fibers was made 
by H. Rosentuat (1845)* for all the cerebral nerves except N. 
olfactorius, opticus, and acusticus, by estimations based on the 
number of fibers counted in a few squares of the ocular-mi- 
crometer. TERGAST (1872) made a determination of the num- 
ber of nerve fibers in the N. abducens, but makes no mention 
of his method. Kuunr (1879)° counted the number of nerve- 
fibers in a row representing the diameter of the N. opticus, and 
estimated the entire number by the formula 1° X 3.1416. 
Krause (1876 and 1880)* made determinations for the N. 
opticus, and (1880)" for N. oculomotorius, but does not give 
his method of estimation. 
The area of the cross-section of the roots of the spinal 
nerves has been determined by KOLLIKER (1850)" and by 
STILLING (1859)". 
The only determination on record of the number of fibers 
