562 ANNALS NEW YORK ACADEMY OF SCIENCES 



central stump to grow out into the distal stump, before the larger fibers 

 can grow. Conversely, it demonstrates that large fibers of the central 

 stump send out small extensions into the distal stump, which then ma- 

 ture and take on the action potential characteristic of smaller fibers, 

 regardless of their origin. 



A consistent finding, not illustrated in figure 1, was that the conduc- 

 tion velocity of the distal fibers was less at greater distances from the 

 suture. 



Fiber Diameter Measurements 



The regenerating tibial, peroneal, and saphenous nerves which had 

 been excised and used for action potential experiments were fixed in 

 osmic acid and studied microscopically. The most obvious change in 

 the character of the regenerating nerves, as they were allowed to grow 

 for longer and longer periods, was the gradual increase in fiber diam- 

 eters. The outside diameters, including both axis cylinders and myelin 

 sheaths, were measured with a movable, ocular micrometer. To insure 

 random sampling, the fibers were measured in horizontal bands, with 

 approximately 500 fibers measured in each nerve. The results were 

 plotted along fiber distribution curves, as illustrated in figure 2. These 

 nine histograms were picked from a series of regenerating tibial nerves, 

 to show the diameter characteristics of the fibers in the distal segments 

 at different time intervals following suture. Since the diameters were 

 smaller, at greater distances from the suture, the portions of tibial 

 nerves studied in each of the histograms in figure 2 were taken from 

 similar levels, just beyond the upper branches to the gastrocnemius 

 muscles, 3 to 5 cm. distal to the sutures. 



The shift in distribution from left to right in the histograms of 

 FIGURE 2 shows rapid diameter growth between 33, 59, and 127 days 

 after transection and suture. At longer regeneration times of 207, 318, 

 and 420 days, the fibers continued to mature, but more slowly. At 

 544, 901, and 1363 post-operative days, there was negligible increase 

 in fiber diameter, but a complete recovery of the normal fiber size was 

 never attained. Even at 1363 post-operative days, the large group of 

 fibers between 9 and 20 micra had not appeared. Similar lack of com- 

 plete recovery of fiber size was found in the peroneal and saphenous 

 nerves. 



Particular attention was paid to the measurement of the largest 

 fibers in each nerve, since the maximum conduction velocity had al- 

 ready been determined accurately, and it was a reasonable assumption 

 that the largest fibers were responsible for the maximum conduction 

 velocity. The growth of the largest fibers is illustrated in figure 3 



