24 MENTAL EVOLUTION IN ANIMALS. 



CHAPTER II. 



The Structure and Functions of Nerve-Tissue. 



Having thus arrived at the best available Criterion of Mind 

 considered as an eject, we have now to pass on to the topic 

 which has already been propounded, viz., to a consideration 

 of the objective conditions under which known mind is in- 

 variably found to occur. 



Mind, then, so far as human experience extends, is only 

 certainly known to occur in association with living organisms, 

 and, still more particularly, in association with a peculiar 

 kind of tissue which does not occur in all organisms, and even 

 in those in which it does occur never constitutes more than 

 an exceedingly small percentage of their bulk. This peculiar 

 tissue, so sparingly distributed through the animal kingdom, 

 and presenting the unique characteristic of being associated 

 with mind, is, of course, the nervous tissue. It therefore 

 devolves upon us, first of all, to contemplate the structure 

 and the functions of this tissue, as far as it is needful for the 

 purposes of our subsequent discussion that these should be 

 clearly understood. 



Throughout the animal kingdom nerve-tissue is invariably 

 present in all species whose zoological position is not below 

 that of the Hydrozoa. The lowest animals in which it has 

 hitherto been detected are the Medusas, or jelly-fishes, and 

 from them upwards its occurrence is, as I have said, invari- 

 able. Wherever it does occur its fundamental structure is 

 very much the same, so that whether we meet with nerve- 

 tissue in a jelly-fish, an oyster, an insect, a bird, or a man, 

 we have no difficulty in recognizing its structural units as 

 everywhere more or less similar. These structural units are 

 microscopical cells and microscopical fibres. (Figs. 1, 2.) 



The fibres proceed to and from the cells, so serving to 



