44 BIOLOGY 



importance. For instance, a nerve fiber, even in the higher 

 animals, consists of a long bit of protoplasm extending from 

 the cell body; see page 169. The phenomenon of conductility 

 in this case is of great significance because it may carry an im- 

 pulse from the outer end of these nerves (the periphery) to the 

 cell body in the brain, or it may carry one that started within 

 the body rapidly outward to the periphery. This phenomenon 

 of conductility, therefore, forms the primary function of the 

 nerves. It is this function that makes it possible for a stimulus 

 applied to the outer part of the animal to be carried rapidly over 

 the animal so as to produce a response in other parts of the body. 

 4. Assimilation. All protoplasm has the property of taking in 

 food material, changing its chemical nature and converting it into 

 new protoplasm by assimilation; a process which may result in 

 growth. This process is probably always a constructive one; 

 i. e., it builds more complicated materials out of simpler ones. 

 Different kinds of protoplasm have this power developed to a 

 widely different extent. Some cells assimilate and grow with 

 great rapidity, with the result that they multiply rapidly; other 

 cells seem to have lost much of this power of assimilation in their 

 adult life, and are able only to replace the worn-out parts of 

 their own structure. In the higher animals, for example, the 

 cells are all capable of rapid assimilation, growth, and reproduc- 

 tion in youth, but many of them nearly or wholly lose this power 

 after the animal has reached adult life. The nerve cells in the 

 brain and spinal cord, for example, seem largely to have lost 

 this property of assimilation, for they are unable to grow after 

 they have once reached the adult form, although able to repair 

 their own wastes. Later in life, nearly all the cells in the body 

 lose this power, a condition characteristic of old age. Speaking 

 generally, this power of assimilation and growth is most active 

 at the very beginning of the life of a cell; it continues for a 

 period with a gradually declining vigor and finally comes to an 

 end, starting vigorously again as the result of the process of 

 reproduction. 



