ON THE SIGNIFICANCE OF THE CALIBER OF THE 

 PARTS OF THE NEURONE IN VERTEBRATES.^ 



J. B. JOHNSTON. 



University of Minnesota. 



The reflections published here were aroused by the study of the 

 peripheral nerves of the brook lamprey (see p. 569 of this Journal). 

 The main facts observed in that study which bear on the present 

 subject were: 



1. The great difi^erence in the cahber of fibers in nerve roots 

 or the peripheral nerve trunks. The largest motor fibers are 

 about 70 to 80 times as thick as the finest fibers in the dorsal nerve 

 roots. 



2. The great increase in thickness of the motor fibers between 

 their cells of origin and the muscles which they innervate. This 

 increase is greatest in the spinal nerves which supply the myo- 

 tomes. These fibers begin as cones of origin averaging about 2/z 

 in thickness, decrease to less than i/x and increase again before 

 leaving the spinal cord to about 3/^. They eventually reach a 

 diameter of 20 to 24.1J. as they enter the muscles. In one case an 

 increase in thickness from 9 to 19/z was noticed in a distance of 

 .3 mm.; in another case an increase from 4.4 to 8.8/z in the same 

 distance. A similar condition is seen in all the motor nerves, but 

 those to the branchial muscles show the least increase in caliber. 



3. The large motor fibers in the lamprey are equal in diameter 

 to the largest medullated fibers in man. The fibers in the lamprey 

 are non-medullated and the species studied is a slender fish not 

 more than 150 mm. in length. 



4. The ratio between the number of motor fibers and the 

 number of muscle fibers to be supplied. In the spinal nerves each 

 motor fiber must supply about one hundred muscle fibers. In the 

 cranial nerves supplying specialized muscles the ratio seems to 

 be nearly as large. 



' Neurological Studies, University of Minnesota, no. 3. 



