CHAP. II.] 



A NERVOUS SYSTEM. 



at all. They are to be regarded as single channels (however 

 devious their course may be), along each of which separate 

 stimulus-waves are capable of being transmitted. We can 



15 



FIG. 7. The Cervical Plexus, composed by interlacements of the last four cervical 

 (1, 2, 3, 4) and the first dorsal nerves (5). The various branches (6-21) are distributed 

 to the shoulder, arm, fore-arm, and hand. (Sappey after Hirschfeld.) 



speak here only of probability, as this is a subject neces- 

 sarily beyond the reach of actual observation. 



Nerve Cells vary much in size and shape the smallest 

 being about 3"- Vo* n whilst the larger may be -g-^th of 

 an inch or more in diameter. They are more or less 

 granular bodies, each of which contains a large nucleus, 

 and within this an unusually distinct 'nucleolus' (figs. 

 1 and 8). Near the nucleus a heap of yellowish or 

 orange coloured pigment granules may often be seen. The 

 substance of the cell is continued into two or many * pro- 



D 2 



