568 ANNALS NEW YORK ACADEMY OF SCIENCES 



the control in E shows much larger fibers throughout (figure 6) . Both 

 nerves were allowed to regenerate 337 days after suture. 



The maximum delay which caused no change in fiber growth could 

 not be determined accurately from these experiments, because the 

 regeneration time was not kept constant. However, in those experi- 

 ments with 253 and 440 days delay, approximately a year was allowed 

 for regeneration. These experiments were more conclusive, and indi- 

 cated that such delay periods restrict the diameter growth of the re- 

 generating fibers. Not only were the largest caliber fibers limited in 

 growth, but the whole fiber distribution curve was altered. 



The Effects of Cross- Suturing Nerves of Different Fiber Caliber 



On the basis of cross-suture experiments, Simpson and Young'* de- 

 scribed a restrictive influence on fiber diameter growth by very small 

 Schwann tubes. Using a similar approach, Hammond and Hinsey® 

 cross-sutured the hypoglossal nerve and the cervical sympathetic 

 trunk. The choice of these nerves was fortunate, since they contain 

 much different tube diameter distributions, and they are situated close 

 ^ together for easy manipulation for cross-suturing. 



Fiber diameter distributions of the normal hypoglossal nerves, meas- 

 ured from osmic preparations, showed large fibers, between 2.3 and 17 

 micra in outside diameter, with a unimodal peak between 6.5 and 8.5 

 micra. Calculation from 11 hypoglossal nerves showed the median 

 fiber diameter to be 7.7 micra, with only 3% of the fibers smaller than 

 4.5 micra. Similar observation on the cervical sympathetic trunk at 

 its rostral end showed relatively small fibers, between 1.2 and 8.4 

 micra in diameter, with a unimodal peak at 2.5 to 3.5 micra. The 

 median diameter calculated from 7 experiments was 3.3 micra, and 

 only 7% of the fibers were larger than 4.5 micra. 



In order to compare the effects of suture of a nerve with large fibers 

 into a nerve with smaller fibers and the effects of a control experiment 

 in which the nerve with large fibers was sutured into its own distal 

 stump, it was necessary to run simultaneous experiments, with the 

 nerves excised and measured at the same intervals after suture. There- 

 fore, in one set of experiments, the hypoglossal nerves were sectioned 

 and immediately joined to their own distal stumps. The fiber distribu- 

 tion in the distal stumps of these hypoglossal-to-hypoglossal sutures 

 was determined as illustrated in the top row of histograms in figure 7. 

 The progress of diameter growth of the fibers is indicated by a shift 

 of the curves from left to right, as the time of regeneration increased 

 to 216, 250, 300, and 365 days. This increase is similar to that found 



