1 68 THE GROWTH OF THE BRAIN. 



it is to be remembered that a nerve grows not only 

 by the increase in the number of its fibres, but also 

 by the enlargement in diameter of the fibres at any 

 time present, and therefore the observations on number 

 should be supplemented by measurement of the area of 

 the nerves at different ages. 



It was further shown that the fully-developed large 

 nerve cells in the ventral horns of the frog's cord, each 

 corresponded to a single nerve fibre in the ventral roots, 

 and hence the results support the view that there is 

 a one-to-one relationship between them. As the number 

 of ventral fibres increases, the number of developed cells 

 in the cord also increases, and there is thus determined 

 for the entire cord of the frog growth relations similar 

 to those which Kaiser found in the cervical enlargement 

 of man. Some attention was also given to the dorsal 

 roots. Birge counted the fibres in these in the case of 

 two frogs, as shown in the Table 39. 



TABLE 39. SHOWING THE NUMBER OF FIBRES IN THE DORSAL 

 ROOTS OF Two FROGS HAVING DIFFERENT WEIGHTS. 

 (Birge.) 



NO. OF 

 WEIGHT. DORSAL ROOT FIBRES. 



23 grammes 7562 



63 10670 



The frogs are identical with the specimens of 23 and 

 63 grammes, the number of fibres in whose ventral roots 

 has been given in Table 38. In both cases the figures 

 show the dorsal roots to have the greater number of 

 fibres, and the rate of increase is 77 fibres for each 

 gramme of frog a rate which is 50 per cent, greater 

 than the average rate for the ventral roots. 



There is good reason to think that the fibres in the 

 dorsal roots, with perhaps a trifling exception, take their 

 origin from the cells of the spinal ganglia. These ceils 



