FUNCTION OF THE FIBRES. 385 



ing the identity of cell and fibre, in ultimate structure, and 

 of the identity of ganglion and tube, be admitted, can we 

 allow the old hypothesis of conduction to be more than a 

 metaphor. The notion of an actual conduction taking place, 

 analogous to the conduction of electricity, is extremely 

 doubtful to me. If the nerves are identical in elementary 

 structure with the ganglia, and consequently must participate 

 in the fimctions of the ganglia, they can no longer be re- 

 garded as the conducting-rods of the battery, but as essential 

 parts of it. In our present ignorance of the true process we 

 may continue to employ the metaphor of conduction, if we 

 understand by it simply the change whicli follows when a 

 nerve is affected ; and we may then gain some glimmering 

 of the special function of the fibres, and the meaning of then- 

 increase with old age. Nerve-tissue in its earliest stage is 

 wholly -without fibres ; as development advances, the fibres 

 multiply.* In old age the brain hardens from excess of 

 fibres, as the bones harden from excess of lime ; so that what 

 originally constituted a source of strength becomes a source 

 of weakness. Probably to this predominance of fibres may 

 be assigned the incapacity of acquiring new ideas in old age. 

 Intellectual vigour is often manifested by men of a very ad- 

 vanced age, but the vigour is shown in dealing with old trains 



* Not till the beginning of the fourth month of the human embryo are fibres 

 discoverablo in the spinal chord. — TiEDElfANN, Anatomii; du Cerrcau, yi. 12(3. 

 When the fibres first make their appearance in the brain, I know not, but in 

 the brains of a new-bora puppy and kitten I could find no trace of them. In- 

 deed, the naked eye showed that no differentiation had as yet taken place 

 between the grey and white matter ; and the Microscope, with a power of 750 

 linear, confirmed this impression, the structure of both grey and white matter 

 being wholly vesicular and granular. 



2 K 



