66 JoURNAL OF COMPARATIVE NEUROLOGY. 
were taken for rest, and after one hour of counting a longer 
rest was taken. On the average, about two hours a day were 
devoted to counting, though on several occasions the period 
was longer. 
The number of fibers counted per hour varied greatly. 
On three or four mornings, under the most favorable condi- 
tions, as many as 3,000 fibers were counted per hour. Under 
ordinary conditions, from 1,000 to 1,500 fibers per hour was 
the best that could be done. In very many instances less than 
500 per hour was found to be a severe task. 
In estimating the accuracy of this enumeration, three 
sources of error have to be considered. 
(a) Errors in the identification of the small nerve fibers. 
This has already been mentioned. The sources of error here 
are that either connective tissue corpuscles, or that fragments 
of broken medullary sheaths may be mistaken for nerve fibers. 
The characteristic sought for in doubtful cases of this kind is 
the ring-like appearance of the nerve fiber due to the greater 
transparency of its axis-cylinder. The only case where this 
test is not conclusive is where the fiber is cut so obliquely as to 
fail to show this ring-like aspect. But in these cases the 
elongated appearance of the fiber is quite different from the ap- 
pearance of a connective tissue corpuscle and markedly differ- 
ent from the sharp-angled fragments of broken medullary 
sheaths. Since in only a comparatively few instances are the 
fibers either cut obliquely or broken into fragments, I consider 
this source of error very small. 
(b) Errors in counting, due to the overlooking of some 
fibers, or to counting the same fibers twice. To determine the 
amount of this error I counted, preparatory to my enumera- 
tion, certain fascicles several times and found the results to vary 
less than 2%. It must be apparent that this error also in- 
cludes, at least to a large extent, the error from source (a). 
Still it is probable that the error from this source is a trifle 
greater than this preparatory test shows, because of greater 
fatigue at times, and poorer conditions, such as light, at other 
