Chap, i.] THE NERVOUS SYSTEM IN THE ZOOLOGICAL SERIES. 375 



fibro-cells. In ultimate analysis the great sympathetic can be 

 denominated nervous system of the vegetative muscles. Such at 

 least is its physiological characteristic. We shall see that his- 

 tologically it is pre-eminently composed of special nervous fibres, 

 like the office which is assigned to them. With these special 

 fibres are blended a small number of fibres similar to those of 

 the nervous system of the animal life. These last are probably 

 charged with the sensitive function. They gather the impressions 

 on the surface of the mucous membranes, and so forth. 



The sympathetic network is remarkable, morphologically, for 

 the very great number of ganglionary expansions which exist on 

 its plexus. Always in the vertebrates, it is connected with 

 the nervous centres, the spinal marrow, the brain, by numerous 

 roots. These roots throw themselves into the ganglions which 

 are connected with each other by cords, and whence set forth 

 other ganglionary cords, which go to form networks, complicated 

 plexus in the viscera. 



The sympathetic system is not much developed in fishes. It 

 seems even to be wholly absent in the more inferior fishes, and 

 to have its place supplied by simple intestinal ramifications. 



We shall have to return to the very interesting functions of 

 the great sympathetic ; but before going further it is needful to 

 expound with some detail the histological structure of the diverse 

 parts of the nervous system, and of the diverse types of nervous 

 systems, of which we have just given a general morphological 

 description. 



