CHAP. II.] THE STRUCTURE OF A NERVOUS SYSTEM. 27 



work of all simple modes of nervous activity. By an 

 indefinite multiplication of such combinations of nerve 

 units, variously arranged, stimuli or impressions (repre- 

 sented by molecular movements) are conducted from the 

 various sensitive surfaces or parts of the body to related 

 nerve centres, and are thence reflected so as to rouse the 

 activity of related muscles or glands. 



The groups of nerve cells above referred to, together with 

 some portions of their related fibres, are usually aggre- 

 gated so as to form distinct and separate nodules known 

 as * ganglia.' Those in connection with ingoing (or affe- 



Fio. 2. Small Sympathetic Ganglion (Human) with M-ultipolar Cells. Magnified 

 about 400 diameters. (Ley dig.) 



rent) fibres are commonly spoken of as ' sensory gan- 

 glia,' whilst those which lie at the roots of outgoing (or 

 efferent) nerves are known as ' motor ganglia.' 



Two or more sensory ganglia, or two or more motor 

 ganglia, may grow together into a single mass ; or what is 

 equally common, a sensory and its corresponding motor 

 ganglion, or two or more pairs of these, may fuse into a 

 single larger nodule, which maybe called a ' nerve centre.' 



