949 



any information about anything but the nunil)er of the medullated 

 nerve fibres under different circumstances. As far as the post- 

 embryonal growth in the number of axons is concerned, these 

 investigations, which in themselves are in many cases very fine 

 pieces of work, afford no information, but with iliis method one 

 can, of course, only obtain a knowledge of the number of medullated 

 axons, and the total result is that they indicate a gradual process 

 of myelinisation, which is even stated by Donaldson ^) : "The increase 

 in the number of myelinated fibres in the spinal root with advancing 

 age is due mainly to progressive myelination. Both roots at maturity 

 still contain functional fibres without myelin sheaths (Ranson '06)." 



During the piogress of the work of counting the numl)er of nerve 

 fibres in the roots of the spinal nerves in the animals I investigated, 

 it soon appeared that it was impossible to get reliable numbers as 

 to the conditions of the nerve fibres in specimens of different ages 

 by counting those in the roots of a single or a small number of 

 spinal nerves. For fairly great displacements and individual varia- 

 tions occur, and these prevent the values from being as good as 

 might be desired, if only a small number of nerves are taken 

 into consideration. On account of this and also in order to obtain 

 an insight into the distribution of the axons in the different regions 

 I have, in most animals, counted each spinal nerve on the same 

 side and in certain specimens on both sides. As I intend to give a 

 more complete and detailed account of these matters in a future 

 work, 1 merely include here some totals from a part of the calcu- 

 lations in question. 



By calculating the number of Jierve fibres in the same section of 

 the root and by using the method of least squares for the values, 

 it has been shown that the percentage of error in the dorsal roots 

 of very young animals (from four to ten days old) has not exceeded 

 ± 10, and that for other places it is, as a rule, about db 2. (In the 

 dorsal roots of the spinal nerves the nerve fibres are situated very 

 close together in young specimens, so that one has to work with 

 thin (5 ii) sections and strong light iii order to obtain exact results). 

 The totals of the dorsal and vejitral roots of the spinal nerves given 

 in the tables below are thus to be considered as exact within the 

 limits of the percentages mentioned. That there is thus a post- 

 embryonal increase in the number of nerve fibres in the dorsal and 

 ventral roots is shown with all the clearness that could be desired 



1) Donaldson, H. H. The rat, reference tables and data for the albino rat and 

 the Norway rat, 1915, Philadelphia. 



