166 THE GROWTH OF THE BRAIN. 



difficult of enumeration. The early numerical com- 

 pleteness of this nerve is remarkable, and the explana- 

 tion of this may possibly depend on the fact that it is 

 a nerve with fixed physiological functions, the demands 

 upon it being similar at all ages. It is at the same time 

 conceivable that nerves whose functions become more 

 complicated with age might be, even in the same animal, 

 far less complete at birth. It is possible, too, that in 

 man the condition of the oculo-motor nerve is the same 

 as that found in the cat, yet it has just been shown that 

 in man the cervical enlargement of the spinal cord is 

 only half developed at this time. 



Still pursuing the questions of number, there are a 

 series of interesting facts, which have been determined 

 by Birge in the case of the frog. 1 He sought an answer 

 for two questions : first, does the number of nerve fibres 

 in the ventral roots of the spinal cord of the frog increase 

 with the size (age) of the individual ? and, second, does 

 the number of developed fibres in the ventral roots corre- 

 spond with the number of developed cells in the ventral 

 horns of the cord, thus presenting the relation to be 

 expected if each neuron were the outgrowth of a single 

 cell-body ? Both these questions were answered in the 

 affirmative. The examination was made upon seven 

 frogs, ranging in weight from one and a half to one 

 hundred and eleven grammes. The total number of 

 nerve fibres in the ventral roots of the smallest frog, 

 weighing 1*5 grammes, was 5,984, or 3,990 fibres for 

 each gramme of frog. In the heaviest frog, weighing 

 1 1 1 grammes, the total number of fibres in the ventral 

 roots was 11,468, or 103 fibres for each gramme of frog. 

 It is plain from this that on the whole as the frog grows 

 heavier its muscles and skin have a proportionally 

 smaller number of fibres controlling them. Taking the 

 1 Birge, Archiv. f.Anat. u. Physiol.^ 1882. 



