THE CELLULAR CHANGES OF AGE 49 



gradual progress and that in successive stages of the 

 individual we can find successive stages of cell change; 

 but it will suffice for our immediate purpose to consider 

 the results of differentiation as they are shown to us by 

 the study of the cells of the adult. I will have thrown 

 upon the screen for you a succession of pictures illus- 

 trating various adult structures. The first is, how- 

 ever, a part of a cross-section of the embryonic spinal 

 cord in which you can see that much of the simple char- 

 acter of the embryonic cells is still kept. All parts 

 of the spinal cord, as the picture shows, are very much 

 alike, and the nuclei of the cells composing the spinal 

 cord at this stage are all essentially similar in appear- 

 ance. What a contrast this forms with our next 

 picture, which shows us an isolated so-called motor 

 nerve cell from the adult spinal cord. It owes its 

 name motor to the fact that it produces a nerve fibre 

 by which motor impulses are conveyed from the 

 spinal cord to the muscles of the body. The cell has 

 numerous elongated branching processes stretching 

 out in various directions, but all leading back towards 

 the central body in which the nucleus is situated. 

 These are the processes which serve to carry in 

 the nervous impulses from the periphery towards the 

 centre of the cell, impulses which in large part, if not 

 exclusively, are gathered up from other nerve cells 

 which act on the motor element. At one point there 

 runs out a single process of a different character. It 

 is the true nerve fibre, and forms the axis, as it was 

 formerly termed, or axon, as it is at present more 



