70 JOURNAL OF COMPARATIVE NEUROLOGY. 
tive number may vary considerably even in the same root. It 
follows, in general, that the fascicle having a relatively large 
number of fibers per sq. mm., other things being equal, con- 
tains the fibers of smaller diameter. This holds true in a great 
number of instances, as can easily be seen from a microscopical 
examination of these fascicles. Still other factors beside the 
caliber of the nerve fiber enter in to determine this relative 
number. One of these factors is the amount of connective tis- 
sue present in the fascicles. This varies greatly in the different 
roots due, perhaps, to the fact that the roots were not sectioned 
at exactly the same point with regard to the dura mater. Thus, 
the cross-sections of the thoracic roots show far more connect- 
ive tissue than the cervical and lumbar roots. This, no doubt, 
is one reason why the lumbar roots, although they have a 
great number of large fibers, yet show such a large number of 
fibers per sq. mm. On the other hand, this is most probably 
also the reason why the thoracic roots, which contain many 
small fibers yet do not show a higher relative number per 
sq. mm. 
Another factor is the compactness of the tissue of the 
fascicles. That this is often determined by the connective tis- 
sue is evident; still in many cases the fibers may be seen to be 
much scattered without the presence of much connective tissue ° 
between them. 
Upon studying my results in order to determine whether 
there be any relation between the diameter of the fascicle and 
its number of nerve fibers per sq. mm., although no conclusive 
statement can be made respecting this point, yet it can be said, 
that in general the small fascicles have fibers of a small caliber. 
On the. basis of the results obtained by the measurement 
of the roots and the enumeration of its fibers I have con- 
structed Chart III in Fig. 1. This has been constructed in the 
same manner as Chart II. The percentage values for these 
curves are found in Table I“ and the figures on which these 
values are based in Table I°. 
