274 'Journal of Comparative Neurology and Psychology. 



It is also important to note that the number of medullated dorsal 

 root fibers is constantly increasing in the growing animal (Hatai 

 '02). In the second cervical root of the white rat at tw^elve days 

 of age the number of medullated fibers is less than that given in 

 Table II, which represents the number in rats seventy-two days 

 old. In two rats twelve days old the numberof medullated dorsal 

 root fibers was found to be 1608 and 1521, respectively. Hence, 

 when the nerve is cut in animals of this age, even fewer medullated 

 fibers are injured than would be the case in the adult animal. 



It should also be mentioned that the number of dorsal root fibers 

 may be taken as a fair indication of the number of aff^erent fibers 

 in the peripheral nerve, the "distal excess" (Hardesty '05) not 

 being very large in this nerve of the rat. Table III shows that 

 the ventral and dorsal rami of the nerve contain only 8 percent, 

 or 10 per cent, more fibers than are found in the ventral and dorsal 

 roots. It is, therefore, not misleading to use the number of dorsal 

 root fibers as an index of the afferent fibers in the peripheral nerve. 



TABLE III. 

 Showing the Distal Excess in the II C. Nerve of the Adult White Rat. 



The present studies were carried out on w^hite rats in which 

 the second cervical nerve of the right side had been divided. 

 Some animals, operated on when twelve days old, were allowed to 

 live for two months, others for four months; the nerve was also 

 cut in adult rats which were allowed to survive the operation four 

 months. A numerical analysis w^as then made of the spinal gan- 

 glion and of the ventral and dorsal roots of the injured nerves, in 

 order to determine to what extent degenerative changes had taken 

 place and what amount of repair, if any, had occurred. 



Technique.— \n operating upon white rats twelve days of age 

 it is necessary to work rapidly and to conserve, as far as possible, 

 the body temperature. The little rat was held in position in the 

 hands of an assistant with its neck slightly stretched and head 

 bent forward. An incision was made in the midline on the back 

 of the neck, with the atlas at its middle point, and carried down- 



