THE NERVE ELEMENTS. 



157 



average of 3-4 /j,. In the adult the large fibres are 

 from 10-15 IJL in diameter. The enlargement in 

 diameter which is thus indicated is dependent on an 

 increase in both the medullary sheath and the axis 

 cylinder. The history of events is as follows : When 

 first formed the axis cylinder is small in diameter, but 

 soon increases. While this is transpiring the medullary 

 sheath is formed, and grows rapidly in thickness, after 

 which both constituents grow at such a rate that their 

 respective areas, as seen in a cross section of the fibre, 

 remain nearly equal to one another. The area of the 

 cross section of any mass of fibres may therefore increase 

 in two ways : either by the addition of new fibres which 

 grow into it, or by an increase in the diameter of those 

 already there present. This second method is shown 

 in Table 34 based on the observations of Birge on the 

 second spinal nerve of frogs where the enlargement of 

 the trunk is due to the increase in the size of the fibres, 

 the increase in their number not being progressive in 

 these instances. 1 



TABLE 34. SHOWING THE INCREASE IN THE DIAMETER OF THE 

 NERVE FIBRES COMPOSING THE SECOND SPINAL NERVE OF 

 FROGS, WEIGHING 23 AND 63 GRAMMES RESPECTIVELY, AS 

 COMPARED WITH ONE WEIGHING 1-5 GRAMMES. (Birge.} 



1 Birge, Arch. f. Anat. u. Physiol^ 1882. For an explanation 

 of the failure of these nerves to increase in number of fibres as 

 well as in diameter, the original paper should be consulted. 



