THE BEGINNINGS OF LIFE. 35 



3. Turning now to the third mode of vital activity 

 to that which manifests itself in the display of nervous 

 phenomena we shall find that these manifestations 

 are also closely dependent upon the integrity of certain 

 material structures, and that their appearance coincides 

 with an increase in the quantity of heat appreciable 

 in, or in the neighbourhood of these structures. 



The Nervous System is made up of nerve-cells and 

 nerve-fibres in various states of aggregation. The 

 nerve-cells are elements in which great molecular 

 changes are supposed to take place, attended by the 

 liberation of molecular motion, whilst the nerve-fibres 

 .are, for the most part, mere channels of communica- 

 tion along which this molecular motion is conducted. 

 The matter of which the nervous system is com- 

 posed was originally almost uniform in structure and 

 property; but, little by little, developmental differen- 

 tiations take place in the embryo, with which are 

 associated correlated differences in function. * As Mr. 

 Herbert Spencer says, all direct and indirect evidence 

 c justify us in concluding that the nervous system con- 

 sists of one kind of matter under different forms and 

 conditions. In the grey tissue this matter exists in 

 masses containing corpuscles^ which are soft and have 

 granules dispersed through them, and which, besides 

 being thus unstably composed, are placed so as to be 

 liable to .disturbance in the greatest degree. In the 

 white tissue this matter is collected together in ex- 

 tremely slender threads that are denser, that are uniform 



D 2 



