104 ZOOLOGY. 



pheral. At their origin, the fibres composing the nerve are 

 called roots, and are grouped in the spinal nerves into anterior 

 and posterior roots. 



On the posterior root is a ganglion through which the 

 filaments pass, and then unite with the anterior root. On 

 certain of the cranial nerves, some of the roots also have 

 ganglions. Experiment has shown that the posterior roots 

 are nerves of sensation ; the anterior, nerves of motion : when 

 nerves unite with each other by what is called anastomosis* 

 (although they do not really unite, but merely exchange 

 fibres), there results what is called a plexus.")" Finally, in 

 the various organs the nerves seem to terminate by forming 

 wide plexuses or loops. 



192. Ganylionary System. This system which has 

 also been called the nervous system of organic life, the sys- 

 tem of the great sympathetic, &c. is composed of a certain 

 number of small masses of nervous matter, united to each 

 other by filaments of communication, and by the same means 

 to the nerves of the cerebro- spinal system. From these, 

 ganglions, nerves proceed to numerous vital organs, and are 

 supposed also to follow the course of all the great blood- 

 vessels. 



The ganglions composing this system are arranged in a 

 double column on each side the spine, anteriorly, from the 

 first vertebra to the last; but many others are scattered 

 through the viscera. They are not found in the limbs. A 

 broad distinction which exists between the cerebro-spinal 

 nerves and the ganglionary is, that the former proceed to the 

 skin, organs of sense, muscles, &c. ; the latter to the vital 

 organs, as the heart, intestines, liver, kidneys, &c. : the 

 former are nerves of relation ; the latter have more a reference 

 to nutrition. 



193. Nervous System of other Animals. In all ani- 

 mals, birds, fishes, and reptiles, the arrangements of the 

 nervous system are much as in man. 



But in the invertebrata it is not so ; in these *the cerebro- 

 spinal axis seems wanting, and all the nerves of the body 

 seem to reunite in certain ganglions more or less apart from 

 each other (Fig. 68). Finally, in the great division of the 

 zoophytes, there remains only a vestige of a rudimentary 



* It is not then a true anastomosis. 



t Plexus, from plecto (I intermingle) is a term applied also to the blood- 

 vessels, and means merely a sort of network. 



