220 JOURNAL OF COMPARATIVE NEUROLOGY. 
smaller subject were used for the determination of both the 
number of fibers and area of skin, the area of skin for each 
cutaneous nerve would be somewhat smaller. 
V. THE BEARING OF THE AUTHOR’S ESTIMATE OF THE INNERVA- 
TION OF THE DERMAL SURFACE ON THE THEORY OF THE SPE- 
CIFIC ENERGIES OF NERVES. 
It might be of interest to consider the bearing this esti- 
mate of the innervation of the skin has upon different theories 
as to the number of classes of nerve fibers that mediate the 
dermal sensations. If we assume with WEBER (1846) that 
impulses giving rise to sensations of heat, cold, pressure, and 
pain, pass over the same afferent cutaneous nerve fiber, then 
the above calculations will hold true, viz., one cutaneous nerve 
fiber, on the.average, innervates 2.05 mm’. of the dermal 
surface, giving all these forms of sensation to that area. If, on 
the other hand, we assume with Foster (1891) that there are 
four classes of afferent cutaneous nerve fibers, then this esti- 
mate will have to be so changed that one cutaneous nerve fiber 
of each class will innervate, on the average, 42.05 mm7?., or 
8.2 mm’. of the dermal surface. To specify more in detail, 
one cutaneous nerve fiber of each class will have to innervate 
4X1.3 mm’*., or 5.2 mm”. of the dermal surface of the arm; and 
4X3.15 mm”, or 12.6 mm’. of that of thetrunk. If this theory 
be true, then a histological examination of the nerve termina- 
tions in the skin ought to show each cutaneous fiber of the dor- 
sal roots to innervate, on the average, an area of the skin of 
the trunk equal to 12.6 mm*. However, until we know the 
amount of branching of these fibers, not only in the skin but 
also in the peripheral trunks, we are unable to judge whether 
or not it is possible for one nerve fiber to innervate so large an 
area of the skin. 
VI. Summary. 
1. According to the estimate here made, about 79% of 
the medullated nerve fibers in the dorsal roots of the spinal 
nerves of both sides, or 1,032,730 fibers, go to innervate the 
dermal surface and about 21%, or 274,521, are afferent fibers 
