i 4 4 AGE, GROWTH, AND DEATH 



two upper drawings are represented two isolated 

 nerve cells, to show their shape. They have been 

 coloured by a special process 1 so dark that the nucleus 

 which they contain in their interior is hidden from our 

 view ; it is of course none the less there. This dark 

 staining enables us to trace out the shape of these 

 cells very clearly, and you can see that instead of 

 being round and simple in form they have their 

 elongated processes stretching out to a very consid- 

 erable distance ; these processes serve to catch up 

 from remote places nervous impulses and carry them 

 into the body of the cell, and thus assist in the work 

 of nervous transmission. The elongation of these 

 threads is, as you see, adapted, like the elongation of 

 a wire, to long-distance communication. Here are 

 two other figures which represent nerve cells treated 

 by a different process, 2 and again artificially coloured. 

 But the colour in this case has attacked certain spots 

 in the protoplasm, consequently we see that the pro- 

 toplasm around the nucleus in both of these figures is 

 no longer simple and uniform, but contains these de- 

 posits of dark-coloured material. 3 Below are three 

 other nerve cells ; the one in the centre shows you 

 the accumulation, p, of pigmented matter in the 

 protoplasm ; again an index of change because the 

 previous uniformity has been replaced by diversity in 

 the composition of the various parts of the single cell. 



1 Carmine. 



2 Nissl's methylene blue method. 



3 The " tigroid markings" compare p. 54. 



