FORMS OF NERVE-CELLS. 269 



make it possible definitely to infer the function of a cell 

 from its form. On the contrary, it is better, by experi- 

 ments with animals and experiences with invalids, to 

 determine step by step what functions belong to the 

 cells of a given region. Considering the complex and 

 yet very imperfectly known structure of- the central 

 nervous organs, it is not surprising that this task has 

 by no means yet been fully accomplished. As in the 

 present work we are not treating of the physiology of 

 the separate parts of the nervous system, but are only 

 concerned with the general characters of the elements 

 which constitute the nervous system, we must not 

 enter into details ; but we must be satisfied to show 

 what the nerve-cells in general are able to accomplish 

 and to give due prominence to the fact that each 

 separate nerve-cell is probably always able to accom- 

 plish only one definite thing. We will now run 

 through these capacities and show the facts which 

 serve as proof of these. 



5. The natural rise of excitement takes place 

 either voluntarily or involuntarily. We are always 

 able voluntarily to contract our muscles, though not 

 all of these, for many, especially the smooth forms, 

 are not subject to the will, but contract only as the 

 result of other causes. Sometimes, moreover, the want 

 of power to contract certain muscles is to be ascribed 

 only to want of use, as is shown by the fact that 

 some men are able voluntarily to contract the skin 

 of their scalps or their ear-muscles, though this is 

 impossible to most men, or is possible only in a 

 very restricted degree. Similarly, it is a matter of 

 use how far the will is able to effect a limited con- 

 traction of separate muscles or parts of a muscle. 



