3o8 THE VITAL FUNCTIONS. 



tion, developement, and growth of each part, are 

 placed under the control of the nervous power.* 

 Although we are entirely ignorant of the na- 

 ture of the nervous power, we know that, when 

 employed in the vital functions, it acts through 

 the medium of a particular set of fibres, which 

 form part of the nervous system, and are classed, 

 therefore, among the nerves. The principal 

 filaments of this class of nerves compose what 

 is called the sympathetic nerve, from its being 

 regarded as the medium of extensive sympathies 

 among the organs ; but the whole assemblage of 

 these nerves is more commonly known by the 

 name of the ganglionic system, from the circum- 

 stance of their being connected with small masses 

 of nervous substance, termed ganglia, which are 

 placed in different parts of their course. Fig. 

 379, represents a ganglion (g), through which 

 the nerve (n), consisting at its origin of a number 

 of separate filaments (f), is seen to pass, before 

 it subdivides into branches (b). The numerous 

 communications and interchanges of filaments, 

 which subsequently take place at various parts, 

 forming what is called a plexus, are shown in 



* As the functions of plants are sufficiently simple to admit 

 of being conducted without the aid of muscular power, still less 

 do they require the assistance of the nervous energy ; both of 

 which properties are the peculiar attributes of animal vitality. 

 We accordingly find no traces either of nervous or of muscular 

 fibres in any of the vegetable structures. 



